Low disk space detector (#277)

* - Feature: Disk writes are now blocked early and return an insufficient storage error if free disk space falls below the `--low-diskspace-threshold` value
* Never keep an entry in m_ChunkBlocks that points to a nullptr

10 files changed, 314 insertions, 133 deletions

diff --git a/src/zenserver/cache/structuredcache.cpp b/src/zenserver/cache/structuredcache.cpp
index 3c829f0e8..9f2a448bb 100644
--- a/src/zenserver/cache/structuredcache.cpp
+++ b/src/zenserver/cache/structuredcache.cpp
@@ -18,6 +18,7 @@
 #include <zenhttp/httpserver.h>
 #include <zenhttp/httpshared.h>
 #include <zenhttp/httpstats.h>
+#include <zenstore/gc.h>
 #include <zenutil/cache/cache.h>
 #include <zenutil/cache/rpcrecording.h>
 
@@ -315,17 +316,19 @@ namespace {
 
 //////////////////////////////////////////////////////////////////////////
 
-HttpStructuredCacheService::HttpStructuredCacheService(ZenCacheStore&	  InCacheStore,
-													   CidStore&		  InCidStore,
-													   HttpStatsService&  StatsService,
-													   HttpStatusService& StatusService,
-													   UpstreamCache&	  UpstreamCache)
+HttpStructuredCacheService::HttpStructuredCacheService(ZenCacheStore&		   InCacheStore,
+													   CidStore&			   InCidStore,
+													   HttpStatsService&	   StatsService,
+													   HttpStatusService&	   StatusService,
+													   UpstreamCache&		   UpstreamCache,
+													   const DiskWriteBlocker* InDiskWriteBlocker)
 : m_Log(logging::Get("cache"))
 , m_CacheStore(InCacheStore)
 , m_StatsService(StatsService)
 , m_StatusService(StatusService)
 , m_CidStore(InCidStore)
 , m_UpstreamCache(UpstreamCache)
+, m_DiskWriteBlocker(InDiskWriteBlocker)
 {
 	m_StatsService.RegisterHandler("z$", *this);
 	m_StatusService.RegisterHandler("z$", *this);
@@ -983,7 +986,7 @@ HttpStructuredCacheService::HandleGetCacheRecord(HttpServerRequest& Request, con
 		bool		  Success		= false;
 		const bool	  PartialRecord = EnumHasAllFlags(PolicyFromUrl, CachePolicy::PartialRecord);
 		const bool	  QueryLocal	= EnumHasAllFlags(PolicyFromUrl, CachePolicy::QueryLocal);
-		const bool	  StoreLocal	= EnumHasAllFlags(PolicyFromUrl, CachePolicy::StoreLocal);
+		const bool	  StoreLocal	= EnumHasAllFlags(PolicyFromUrl, CachePolicy::StoreLocal) && AreDiskWritesAllowed();
 		const bool	  SkipData		= EnumHasAllFlags(PolicyFromUrl, CachePolicy::SkipData);
 		ZenCacheValue ClientResultValue;
 
@@ -1097,6 +1100,7 @@ HttpStructuredCacheService::HandleGetCacheRecord(HttpServerRequest& Request, con
 
 						for (const CbAttachment* Attachment : AttachmentsToStoreLocally)
 						{
+							ZEN_ASSERT_SLOW(StoreLocal);
 							CompressedBuffer	   Chunk = Attachment->AsCompressedBinary();
 							CidStore::InsertResult InsertResult =
 								m_CidStore.AddChunk(Chunk.GetCompressed().Flatten().AsIoBuffer(), Attachment->GetHash());
@@ -1188,6 +1192,10 @@ HttpStructuredCacheService::HandlePutCacheRecord(HttpServerRequest& Request, con
 	{
 		return Request.WriteResponse(HttpResponseCode::BadRequest);
 	}
+	if (!AreDiskWritesAllowed())
+	{
+		return Request.WriteResponse(HttpResponseCode::InsufficientStorage);
+	}
 
 	const HttpContentType ContentType = Request.RequestContentType();
 
@@ -1423,7 +1431,10 @@ HttpStructuredCacheService::HandleGetCacheChunk(HttpServerRequest& Request, cons
 				{
 					if (RawHash == Ref.ValueContentId)
 					{
-						m_CidStore.AddChunk(UpstreamResult.Value, RawHash);
+						if (AreDiskWritesAllowed())
+						{
+							m_CidStore.AddChunk(UpstreamResult.Value, RawHash);
+						}
 						Source = UpstreamResult.Source;
 					}
 					else
@@ -1492,6 +1503,10 @@ HttpStructuredCacheService::HandlePutCacheChunk(HttpServerRequest& Request, cons
 	{
 		return Request.WriteResponse(HttpResponseCode::BadRequest);
 	}
+	if (!AreDiskWritesAllowed())
+	{
+		return Request.WriteResponse(HttpResponseCode::InsufficientStorage);
+	}
 
 	Body.SetContentType(Request.RequestContentType());
 
@@ -1526,12 +1541,13 @@ HttpStructuredCacheService::HandlePutCacheChunk(HttpServerRequest& Request, cons
 	Request.WriteResponse(ResponseCode);
 }
 
-CbPackage
+HttpResponseCode
 HttpStructuredCacheService::HandleRpcRequest(const ZenContentType ContentType,
 											 IoBuffer&&			  Body,
 											 uint32_t&			  OutAcceptMagic,
 											 RpcAcceptOptions&	  OutAcceptFlags,
-											 int&				  OutTargetProcessId)
+											 int&				  OutTargetProcessId,
+											 CbPackage&			  OutResultPackage)
 {
 	CbPackage	 Package;
 	CbObjectView Object;
@@ -1554,25 +1570,37 @@ HttpStructuredCacheService::HandleRpcRequest(const ZenContentType ContentType,
 
 	if (Method == "PutCacheRecords"sv)
 	{
-		return HandleRpcPutCacheRecords(Package);
+		if (!AreDiskWritesAllowed())
+		{
+			return HttpResponseCode::InsufficientStorage;
+		}
+		OutResultPackage = HandleRpcPutCacheRecords(Package);
 	}
 	else if (Method == "GetCacheRecords"sv)
 	{
-		return HandleRpcGetCacheRecords(Object);
+		OutResultPackage = HandleRpcGetCacheRecords(Object);
 	}
 	else if (Method == "PutCacheValues"sv)
 	{
-		return HandleRpcPutCacheValues(Package);
+		if (!AreDiskWritesAllowed())
+		{
+			return HttpResponseCode::InsufficientStorage;
+		}
+		OutResultPackage = HandleRpcPutCacheValues(Package);
 	}
 	else if (Method == "GetCacheValues"sv)
 	{
-		return HandleRpcGetCacheValues(Object);
+		OutResultPackage = HandleRpcGetCacheValues(Object);
 	}
 	else if (Method == "GetCacheChunks"sv)
 	{
-		return HandleRpcGetCacheChunks(Object);
+		OutResultPackage = HandleRpcGetCacheChunks(Object);
 	}
-	return CbPackage{};
+	else
+	{
+		return HttpResponseCode::BadRequest;
+	}
+	return HttpResponseCode::OK;
 }
 
 void
@@ -1594,27 +1622,30 @@ HttpStructuredCacheService::ReplayRequestRecorder(cache::IRpcRequestReplayer& Re
 				uint32_t		 AcceptMagic = 0;
 				RpcAcceptOptions AcceptFlags = RpcAcceptOptions::kNone;
 				int				 TargetPid	 = 0;
-				CbPackage		 RpcResult	 = HandleRpcRequest(ContentType.first, std::move(Body), AcceptMagic, AcceptFlags, TargetPid);
-				if (AcceptMagic == kCbPkgMagic)
+				CbPackage		 RpcResult;
+				if (IsHttpSuccessCode(HandleRpcRequest(ContentType.first, std::move(Body), AcceptMagic, AcceptFlags, TargetPid, RpcResult)))
 				{
-					FormatFlags Flags = FormatFlags::kDefault;
-					if (EnumHasAllFlags(AcceptFlags, RpcAcceptOptions::kAllowLocalReferences))
+					if (AcceptMagic == kCbPkgMagic)
 					{
-						Flags |= FormatFlags::kAllowLocalReferences;
-						if (!EnumHasAnyFlags(AcceptFlags, RpcAcceptOptions::kAllowPartialLocalReferences))
+						FormatFlags Flags = FormatFlags::kDefault;
+						if (EnumHasAllFlags(AcceptFlags, RpcAcceptOptions::kAllowLocalReferences))
 						{
-							Flags |= FormatFlags::kDenyPartialLocalReferences;
+							Flags |= FormatFlags::kAllowLocalReferences;
+							if (!EnumHasAnyFlags(AcceptFlags, RpcAcceptOptions::kAllowPartialLocalReferences))
+							{
+								Flags |= FormatFlags::kDenyPartialLocalReferences;
+							}
 						}
+						CompositeBuffer RpcResponseBuffer = FormatPackageMessageBuffer(RpcResult, Flags, TargetPid);
+						ZEN_ASSERT(RpcResponseBuffer.GetSize() > 0);
+					}
+					else
+					{
+						BinaryWriter MemStream;
+						RpcResult.Save(MemStream);
+						IoBuffer RpcResponseBuffer(IoBuffer::Wrap, MemStream.GetData(), MemStream.GetSize());
+						ZEN_ASSERT(RpcResponseBuffer.Size() > 0);
 					}
-					CompositeBuffer RpcResponseBuffer = FormatPackageMessageBuffer(RpcResult, Flags, TargetPid);
-					ZEN_ASSERT(RpcResponseBuffer.GetSize() > 0);
-				}
-				else
-				{
-					BinaryWriter MemStream;
-					RpcResult.Save(MemStream);
-					IoBuffer RpcResponseBuffer(IoBuffer::Wrap, MemStream.GetData(), MemStream.GetSize());
-					ZEN_ASSERT(RpcResponseBuffer.Size() > 0);
 				}
 			}
 			JobLatch.CountDown();
@@ -1652,10 +1683,13 @@ HttpStructuredCacheService::HandleRpcRequest(HttpServerRequest& Request)
 						uint32_t		 AcceptMagic	 = 0;
 						RpcAcceptOptions AcceptFlags	 = RpcAcceptOptions::kNone;
 						int				 TargetProcessId = 0;
-						CbPackage RpcResult = HandleRpcRequest(ContentType, std::move(Body), AcceptMagic, AcceptFlags, TargetProcessId);
-						if (RpcResult.IsNull())
+						CbPackage		 RpcResult;
+
+						HttpResponseCode ResultCode =
+							HandleRpcRequest(ContentType, std::move(Body), AcceptMagic, AcceptFlags, TargetProcessId, RpcResult);
+						if (!IsHttpSuccessCode(ResultCode))
 						{
-							AsyncRequest.WriteResponse(HttpResponseCode::BadRequest);
+							AsyncRequest.WriteResponse(ResultCode);
 							return;
 						}
 						if (AcceptMagic == kCbPkgMagic)
@@ -1706,6 +1740,7 @@ CbPackage
 HttpStructuredCacheService::HandleRpcPutCacheRecords(const CbPackage& BatchRequest)
 {
 	ZEN_TRACE_CPU("Z$::RpcPutCacheRecords");
+
 	CbObjectView BatchObject = BatchRequest.GetObject();
 	ZEN_ASSERT(BatchObject["Method"sv].AsString() == "PutCacheRecords"sv);
 
@@ -2064,7 +2099,9 @@ HttpStructuredCacheService::HandleRpcGetCacheRecords(CbObjectView RpcRequest)
 				Request.RecordCacheValue = ObjectBuffer.GetBuffer().AsIoBuffer();
 				Request.RecordCacheValue.SetContentType(ZenContentType::kCbObject);
 				Request.RecordObject = ObjectBuffer;
-				if (EnumHasAllFlags(Request.DownstreamPolicy.GetRecordPolicy(), CachePolicy::StoreLocal))
+				bool StoreLocal =
+					EnumHasAllFlags(Request.DownstreamPolicy.GetRecordPolicy(), CachePolicy::StoreLocal) && AreDiskWritesAllowed();
+				if (StoreLocal)
 				{
 					m_CacheStore.Put(*Namespace, Key.Bucket, Key.Hash, {.Value = {Request.RecordCacheValue}});
 				}
@@ -2087,13 +2124,14 @@ HttpStructuredCacheService::HandleRpcGetCacheRecords(CbObjectView RpcRequest)
 				}
 				if (!EnumHasAllFlags(ValuePolicy, CachePolicy::SkipData) || EnumHasAllFlags(ValuePolicy, CachePolicy::StoreLocal))
 				{
+					bool StoreLocal = EnumHasAllFlags(ValuePolicy, CachePolicy::StoreLocal) && AreDiskWritesAllowed();
 					if (const CbAttachment* Attachment = Params.Package.FindAttachment(Value.ContentId))
 					{
 						if (CompressedBuffer Compressed = Attachment->AsCompressedBinary())
 						{
 							Request.Source = Params.Source;
 							Value.Exists   = true;
-							if (EnumHasAllFlags(ValuePolicy, CachePolicy::StoreLocal))
+							if (StoreLocal)
 							{
 								m_CidStore.AddChunk(Compressed.GetCompressed().Flatten().AsIoBuffer(), Attachment->GetHash());
 							}
@@ -2411,7 +2449,7 @@ HttpStructuredCacheService::HandleRpcGetCacheValues(CbObjectView RpcRequest)
 					Request.RawSize			   = Params.RawSize;
 					const bool HasData		   = IsCompressedBinary(Params.Value.GetContentType());
 					const bool SkipData		   = EnumHasAllFlags(Request.Policy, CachePolicy::SkipData);
-					const bool StoreData	   = EnumHasAllFlags(Request.Policy, CachePolicy::StoreLocal);
+					const bool StoreData	   = EnumHasAllFlags(Request.Policy, CachePolicy::StoreLocal) && AreDiskWritesAllowed();
 					const bool IsHit		   = SkipData || HasData;
 					if (IsHit)
 					{
@@ -2729,7 +2767,8 @@ HttpStructuredCacheService::GetLocalCacheRecords(std::string_view							Namespac
 				Record.CacheValue.SetContentType(ZenContentType::kCbObject);
 				Record.Source = Params.Source;
 
-				if (EnumHasAllFlags(Record.DownstreamPolicy, CachePolicy::StoreLocal))
+				bool StoreLocal = EnumHasAllFlags(Record.DownstreamPolicy, CachePolicy::StoreLocal) && AreDiskWritesAllowed();
+				if (StoreLocal)
 				{
 					m_CacheStore.Put(Namespace, Key.Bucket, Key.Hash, {.Value = Record.CacheValue});
 				}
@@ -2895,7 +2934,8 @@ HttpStructuredCacheService::GetUpstreamCacheChunks(std::string_view							 Names
 					return;
 				}
 
-				if (EnumHasAllFlags(Request.DownstreamPolicy, CachePolicy::StoreLocal))
+				bool StoreLocal = EnumHasAllFlags(Request.DownstreamPolicy, CachePolicy::StoreLocal) && AreDiskWritesAllowed();
+				if (StoreLocal)
 				{
 					if (Request.IsRecordRequest)
 					{
@@ -3054,6 +3094,12 @@ HttpStructuredCacheService::HandleStatusRequest(HttpServerRequest& Request)
 	Request.WriteResponse(HttpResponseCode::OK, Cbo.Save());
 }
 
+bool
+HttpStructuredCacheService::AreDiskWritesAllowed() const
+{
+	return (m_DiskWriteBlocker == nullptr || m_DiskWriteBlocker->AreDiskWritesAllowed());
+}
+
 #if ZEN_WITH_TESTS
 
 TEST_CASE("z$service.parse.relative.Uri")
diff --git a/src/zenserver/cache/structuredcache.h b/src/zenserver/cache/structuredcache.h
index 8309361f4..77a0aee6a 100644
--- a/src/zenserver/cache/structuredcache.h
+++ b/src/zenserver/cache/structuredcache.h
@@ -24,6 +24,7 @@ struct PutRequestData;
 class ScrubContext;
 class UpstreamCache;
 class ZenCacheStore;
+class DiskWriteBlocker;
 enum class CachePolicy : uint32_t;
 enum class RpcAcceptOptions : uint16_t;
 
@@ -67,18 +68,18 @@ namespace cache {
 class HttpStructuredCacheService : public HttpService, public IHttpStatsProvider, public IHttpStatusProvider
 {
 public:
-	HttpStructuredCacheService(ZenCacheStore&	  InCacheStore,
-							   CidStore&		  InCidStore,
-							   HttpStatsService&  StatsService,
-							   HttpStatusService& StatusService,
-							   UpstreamCache&	  UpstreamCache);
+	HttpStructuredCacheService(ZenCacheStore&		   InCacheStore,
+							   CidStore&			   InCidStore,
+							   HttpStatsService&	   StatsService,
+							   HttpStatusService&	   StatusService,
+							   UpstreamCache&		   UpstreamCache,
+							   const DiskWriteBlocker* InDiskWriteBlocker);
 	~HttpStructuredCacheService();
 
 	virtual const char* BaseUri() const override;
 	virtual void		HandleRequest(HttpServerRequest& Request) override;
-
-	void Flush();
-	void Scrub(ScrubContext& Ctx);
+	void				Flush();
+	void				Scrub(ScrubContext& Ctx);
 
 private:
 	struct CacheRef
@@ -111,16 +112,17 @@ private:
 	void HandleRpcRequest(HttpServerRequest& Request);
 	void HandleDetailsRequest(HttpServerRequest& Request);
 
-	CbPackage HandleRpcPutCacheRecords(const CbPackage& BatchRequest);
-	CbPackage HandleRpcGetCacheRecords(CbObjectView BatchRequest);
-	CbPackage HandleRpcPutCacheValues(const CbPackage& BatchRequest);
-	CbPackage HandleRpcGetCacheValues(CbObjectView BatchRequest);
-	CbPackage HandleRpcGetCacheChunks(CbObjectView BatchRequest);
-	CbPackage HandleRpcRequest(const ZenContentType ContentType,
-							   IoBuffer&&			Body,
-							   uint32_t&			OutAcceptMagic,
-							   RpcAcceptOptions&	OutAcceptFlags,
-							   int&					OutTargetProcessId);
+	CbPackage		 HandleRpcPutCacheRecords(const CbPackage& BatchRequest);
+	CbPackage		 HandleRpcGetCacheRecords(CbObjectView BatchRequest);
+	CbPackage		 HandleRpcPutCacheValues(const CbPackage& BatchRequest);
+	CbPackage		 HandleRpcGetCacheValues(CbObjectView BatchRequest);
+	CbPackage		 HandleRpcGetCacheChunks(CbObjectView BatchRequest);
+	HttpResponseCode HandleRpcRequest(const ZenContentType ContentType,
+									  IoBuffer&&		   Body,
+									  uint32_t&			   OutAcceptMagic,
+									  RpcAcceptOptions&	   OutAcceptFlags,
+									  int&				   OutTargetProcessId,
+									  CbPackage&		   OutPackage);
 
 	void		 HandleCacheRequest(HttpServerRequest& Request);
 	void		 HandleCacheNamespaceRequest(HttpServerRequest& Request, std::string_view Namespace);
@@ -156,6 +158,8 @@ private:
 	/** HandleRpcGetCacheChunks Helper: Send response message containing all chunk results. */
 	CbPackage WriteGetCacheChunksResponse(std::string_view Namespace, std::vector<cache::detail::ChunkRequest>& Requests);
 
+	bool AreDiskWritesAllowed() const;
+
 	spdlog::logger&			 Log() { return m_Log; }
 	spdlog::logger&			 m_Log;
 	ZenCacheStore&			 m_CacheStore;
@@ -167,6 +171,7 @@ private:
 	metrics::OperationTiming m_HttpRequests;
 	metrics::OperationTiming m_UpstreamGetRequestTiming;
 	CacheStats				 m_CacheStats;
+	const DiskWriteBlocker*	 m_DiskWriteBlocker = nullptr;
 
 	void ReplayRequestRecorder(cache::IRpcRequestReplayer& Replayer, uint32_t ThreadCount);
 
diff --git a/src/zenserver/config.cpp b/src/zenserver/config.cpp
index da0da2593..592de0f7f 100644
--- a/src/zenserver/config.cpp
+++ b/src/zenserver/config.cpp
@@ -506,7 +506,7 @@ ParseCliOptions(int argc, char* argv[], ZenServerOptions& ServerOptions)
 	options.add_option("gc",
 					   "",
 					   "disk-reserve-size",
-					   "Size of gc disk reserve in bytes. Default set to 268435456 (256 Mb).",
+					   "Size of gc disk reserve in bytes. Default set to 268435456 (256 Mb). Set to zero to disable.",
 					   cxxopts::value<uint64_t>(ServerOptions.GcConfig.DiskReserveSize)->default_value("268435456"),
 					   "");
 
@@ -519,6 +519,13 @@ ParseCliOptions(int argc, char* argv[], ZenServerOptions& ServerOptions)
 
 	options.add_option("gc",
 					   "",
+					   "gc-low-diskspace-threshold",
+					   "Minimum free space on disk to allow writes to disk. Default set to 268435456 (256 Mb). Set to zero to disable.",
+					   cxxopts::value<uint64_t>(ServerOptions.GcConfig.Cache.MinimumFreeDiskSpaceToAllowWrites)->default_value("268435456"),
+					   "");
+
+	options.add_option("gc",
+					   "",
 					   "gc-disksize-softlimit",
 					   "Garbage collection disk usage soft limit. Default set to 0 (Off).",
 					   cxxopts::value<uint64_t>(ServerOptions.GcConfig.Cache.DiskSizeSoftLimit)->default_value("0"),
@@ -867,9 +874,10 @@ ParseConfigFile(const std::filesystem::path& Path, ZenServerOptions& ServerOptio
 
 		if (sol::optional<sol::table> GcConfig = lua["gc"])
 		{
-			ServerOptions.GcConfig.MonitorIntervalSeconds = GcConfig.value().get_or("monitorintervalseconds", 30);
-			ServerOptions.GcConfig.IntervalSeconds		  = GcConfig.value().get_or("intervalseconds", 0);
-			ServerOptions.GcConfig.DiskReserveSize		  = GcConfig.value().get_or("diskreservesize", uint64_t(1u << 28));
+			ServerOptions.GcConfig.MonitorIntervalSeconds			 = GcConfig.value().get_or("monitorintervalseconds", 30);
+			ServerOptions.GcConfig.IntervalSeconds					 = GcConfig.value().get_or("intervalseconds", 0);
+			ServerOptions.GcConfig.DiskReserveSize					 = GcConfig.value().get_or("diskreservesize", uint64_t(1u << 28));
+			ServerOptions.GcConfig.MinimumFreeDiskSpaceToAllowWrites = GcConfig.value().get_or("lowdiskspacethreshold", 0);
 
 			if (sol::optional<sol::table> CacheGcConfig = GcConfig.value()["cache"])
 			{
diff --git a/src/zenserver/config.h b/src/zenserver/config.h
index 9559cae33..caf4adce7 100644
--- a/src/zenserver/config.h
+++ b/src/zenserver/config.h
@@ -69,11 +69,12 @@ struct ZenUpstreamCacheConfig
 
 struct ZenCacheEvictionPolicy
 {
-	uint64_t DiskSizeLimit		= ~uint64_t(0);
-	uint64_t MemorySizeLimit	= 1024 * 1024 * 1024;
-	int32_t	 MaxDurationSeconds = 24 * 60 * 60;
-	uint64_t DiskSizeSoftLimit	= 0;
-	bool	 Enabled			= true;
+	uint64_t DiskSizeLimit					   = ~uint64_t(0);
+	uint64_t MemorySizeLimit				   = 1024 * 1024 * 1024;
+	int32_t	 MaxDurationSeconds				   = 24 * 60 * 60;
+	uint64_t DiskSizeSoftLimit				   = 0;
+	uint64_t MinimumFreeDiskSpaceToAllowWrites = 256u * 1024u * 1024u;
+	bool	 Enabled						   = true;
 };
 
 struct ZenCasEvictionPolicy
@@ -88,11 +89,12 @@ struct ZenGcConfig
 {
 	ZenCasEvictionPolicy   Cas;
 	ZenCacheEvictionPolicy Cache;
-	int32_t				   MonitorIntervalSeconds = 30;
-	int32_t				   IntervalSeconds		  = 0;
-	bool				   CollectSmallObjects	  = true;
-	bool				   Enabled				  = true;
-	uint64_t			   DiskReserveSize		  = 1ul << 28;
+	int32_t				   MonitorIntervalSeconds			 = 30;
+	int32_t				   IntervalSeconds					 = 0;
+	bool				   CollectSmallObjects				 = true;
+	bool				   Enabled							 = true;
+	uint64_t			   DiskReserveSize					 = 1ul << 28;
+	uint64_t			   MinimumFreeDiskSpaceToAllowWrites = 1ul << 28;
 };
 
 struct ZenOpenIdProviderConfig
diff --git a/src/zenserver/projectstore/projectstore.cpp b/src/zenserver/projectstore/projectstore.cpp
index 80e3f27f1..9dd6ddcfc 100644
--- a/src/zenserver/projectstore/projectstore.cpp
+++ b/src/zenserver/projectstore/projectstore.cpp
@@ -1680,6 +1680,7 @@ ProjectStore::ProjectStore(CidStore& Store, std::filesystem::path BasePath, GcMa
 , m_Log(logging::Get("project"))
 , m_CidStore(Store)
 , m_ProjectBasePath(BasePath)
+, m_DiskWriteBlocker(Gc.GetDiskWriteBlocker())
 {
 	ZEN_INFO("initializing project store at '{}'", BasePath);
 	// m_Log.set_level(spdlog::level::debug);
@@ -2560,6 +2561,10 @@ ProjectStore::Rpc(HttpServerRequest&	 HttpReq,
 
 	if (Method == "import")
 	{
+		if (!AreDiskWritesAllowed())
+		{
+			return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+		}
 		std::pair<HttpResponseCode, std::string> Result = Import(*Project.Get(), *Oplog, Cb["params"sv].AsObjectView(), AuthManager);
 		if (Result.second.empty())
 		{
@@ -2602,6 +2607,10 @@ ProjectStore::Rpc(HttpServerRequest&	 HttpReq,
 	}
 	else if (Method == "putchunks")
 	{
+		if (!AreDiskWritesAllowed())
+		{
+			return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+		}
 		std::span<const CbAttachment> Attachments = Package.GetAttachments();
 		for (const CbAttachment& Attachment : Attachments)
 		{
@@ -2676,6 +2685,12 @@ ProjectStore::Import(ProjectStore::Project& Project, ProjectStore::Oplog& Oplog,
 	return ConvertResult(Result);
 }
 
+bool
+ProjectStore::AreDiskWritesAllowed() const
+{
+	return (m_DiskWriteBlocker == nullptr || m_DiskWriteBlocker->AreDiskWritesAllowed());
+}
+
 //////////////////////////////////////////////////////////////////////////
 
 HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects, HttpStatsService& StatsService, AuthMgr& AuthMgr)
@@ -2920,7 +2935,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 			uint64_t Offset = 0;
 			uint64_t Size	= ~(0ull);
 
-			auto QueryParms = Req.ServerRequest().GetQueryParams();
+			auto QueryParms = HttpReq.GetQueryParams();
 
 			if (auto OffsetParm = QueryParms.GetValue("offset"); OffsetParm.empty() == false)
 			{
@@ -2987,7 +3002,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 			HttpContentType AcceptType	= HttpReq.AcceptContentType();
 			HttpContentType RequestType = HttpReq.RequestContentType();
 
-			switch (Req.ServerRequest().RequestVerb())
+			switch (HttpReq.RequestVerb())
 			{
 				case HttpVerb::kGet:
 					{
@@ -3019,6 +3034,10 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 					}
 				case HttpVerb::kPost:
 					{
+						if (!m_ProjectStore->AreDiskWritesAllowed())
+						{
+							return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+						}
 						std::pair<HttpResponseCode, std::string> Result =
 							m_ProjectStore->PutChunk(ProjectId, OplogId, Cid, RequestType, HttpReq.ReadPayload());
 						if (Result.first == HttpResponseCode::OK || Result.first == HttpResponseCode::Created)
@@ -3107,6 +3126,11 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 		[this](HttpRouterRequest& Req) {
 			HttpServerRequest& HttpReq = Req.ServerRequest();
 
+			if (!m_ProjectStore->AreDiskWritesAllowed())
+			{
+				return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+			}
+
 			const auto& ProjectId = Req.GetCapture(1);
 			const auto& OplogId	  = Req.GetCapture(2);
 
@@ -3252,7 +3276,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 				if (std::optional<CbObject> MaybeOp = Oplog.GetOpByIndex(OpId.value()))
 				{
 					CbObject& Op = MaybeOp.value();
-					if (Req.ServerRequest().AcceptContentType() == ZenContentType::kCbPackage)
+					if (HttpReq.AcceptContentType() == ZenContentType::kCbPackage)
 					{
 						CbPackage Package;
 						Package.SetObject(Op);
@@ -3319,6 +3343,8 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 	m_Router.RegisterRoute(
 		"{project}/oplog/{log}",
 		[this](HttpRouterRequest& Req) {
+			HttpServerRequest& HttpReq = Req.ServerRequest();
+
 			const auto& ProjectId = Req.GetCapture(1);
 			const auto& OplogId	  = Req.GetCapture(2);
 
@@ -3326,22 +3352,22 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 
 			if (!Project)
 			{
-				return Req.ServerRequest().WriteResponse(HttpResponseCode::NotFound,
-														 HttpContentType::kText,
-														 fmt::format("project {} not found", ProjectId));
+				return HttpReq.WriteResponse(HttpResponseCode::NotFound,
+											 HttpContentType::kText,
+											 fmt::format("project {} not found", ProjectId));
 			}
 			Project->TouchProject();
 
-			switch (Req.ServerRequest().RequestVerb())
+			switch (HttpReq.RequestVerb())
 			{
 				case HttpVerb::kGet:
 					{
 						ProjectStore::Oplog* OplogIt = Project->OpenOplog(OplogId);
 						if (!OplogIt)
 						{
-							return Req.ServerRequest().WriteResponse(HttpResponseCode::NotFound,
-																	 HttpContentType::kText,
-																	 fmt::format("oplog {} not found in project {}", OplogId, ProjectId));
+							return HttpReq.WriteResponse(HttpResponseCode::NotFound,
+														 HttpContentType::kText,
+														 fmt::format("oplog {} not found in project {}", OplogId, ProjectId));
 						}
 						Project->TouchOplog(OplogId);
 
@@ -3352,14 +3378,18 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 						   << "markerpath"sv << Log.MarkerPath().c_str() << "totalsize"sv << Log.TotalSize() << "opcount"
 						   << Log.OplogCount() << "expired"sv << Project->IsExpired(GcClock::TimePoint::max(), Log);
 
-						Req.ServerRequest().WriteResponse(HttpResponseCode::OK, Cb.Save());
+						HttpReq.WriteResponse(HttpResponseCode::OK, Cb.Save());
 					}
 					break;
 
 				case HttpVerb::kPost:
 					{
+						if (!m_ProjectStore->AreDiskWritesAllowed())
+						{
+							return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+						}
 						std::filesystem::path OplogMarkerPath;
-						if (CbObject Params = Req.ServerRequest().ReadPayloadObject())
+						if (CbObject Params = HttpReq.ReadPayloadObject())
 						{
 							OplogMarkerPath = Params["gcpath"sv].AsString();
 						}
@@ -3370,19 +3400,19 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 							if (!Project->NewOplog(OplogId, OplogMarkerPath))
 							{
 								// TODO: indicate why the operation failed!
-								return Req.ServerRequest().WriteResponse(HttpResponseCode::InternalServerError);
+								return HttpReq.WriteResponse(HttpResponseCode::InternalServerError);
 							}
 							Project->TouchOplog(OplogId);
 
 							ZEN_INFO("established oplog '{}/{}', gc marker file at '{}'", ProjectId, OplogId, OplogMarkerPath);
 
-							return Req.ServerRequest().WriteResponse(HttpResponseCode::Created);
+							return HttpReq.WriteResponse(HttpResponseCode::Created);
 						}
 
 						// I guess this should ultimately be used to execute RPCs but for now, it
 						// does absolutely nothing
 
-						return Req.ServerRequest().WriteResponse(HttpResponseCode::BadRequest);
+						return HttpReq.WriteResponse(HttpResponseCode::BadRequest);
 					}
 					break;
 
@@ -3392,7 +3422,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 
 						Project->DeleteOplog(OplogId);
 
-						return Req.ServerRequest().WriteResponse(HttpResponseCode::OK);
+						return HttpReq.WriteResponse(HttpResponseCode::OK);
 					}
 					break;
 
@@ -3461,13 +3491,19 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 	m_Router.RegisterRoute(
 		"{project}",
 		[this](HttpRouterRequest& Req) {
-			const std::string ProjectId = Req.GetCapture(1);
+			HttpServerRequest& HttpReq	 = Req.ServerRequest();
+			const std::string  ProjectId = Req.GetCapture(1);
 
-			switch (Req.ServerRequest().RequestVerb())
+			switch (HttpReq.RequestVerb())
 			{
 				case HttpVerb::kPost:
 					{
-						IoBuffer		 Payload		 = Req.ServerRequest().ReadPayload();
+						if (!m_ProjectStore->AreDiskWritesAllowed())
+						{
+							return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+						}
+
+						IoBuffer		 Payload		 = HttpReq.ReadPayload();
 						CbObject		 Params			 = LoadCompactBinaryObject(Payload);
 						std::string_view Id				 = Params["id"sv].AsString();
 						std::string_view Root			 = Params["root"sv].AsString();
@@ -3487,7 +3523,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 								 ProjectFilePath,
 								 ProjectFilePath.empty() ? ", project will not be GCd due to empty project file path" : "");
 
-						Req.ServerRequest().WriteResponse(HttpResponseCode::Created);
+						HttpReq.WriteResponse(HttpResponseCode::Created);
 					}
 					break;
 
@@ -3496,9 +3532,9 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 						Ref<ProjectStore::Project> Project = m_ProjectStore->OpenProject(ProjectId);
 						if (!Project)
 						{
-							return Req.ServerRequest().WriteResponse(HttpResponseCode::NotFound,
-																	 HttpContentType::kText,
-																	 fmt::format("project {} not found", ProjectId));
+							return HttpReq.WriteResponse(HttpResponseCode::NotFound,
+														 HttpContentType::kText,
+														 fmt::format("project {} not found", ProjectId));
 						}
 						Project->TouchProject();
 
@@ -3520,7 +3556,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 						}
 						Response.EndArray();  // oplogs
 
-						Req.ServerRequest().WriteResponse(HttpResponseCode::OK, Response.Save());
+						HttpReq.WriteResponse(HttpResponseCode::OK, Response.Save());
 					}
 					break;
 
@@ -3529,20 +3565,20 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 						Ref<ProjectStore::Project> Project = m_ProjectStore->OpenProject(ProjectId);
 						if (!Project)
 						{
-							return Req.ServerRequest().WriteResponse(HttpResponseCode::NotFound,
-																	 HttpContentType::kText,
-																	 fmt::format("project {} not found", ProjectId));
+							return HttpReq.WriteResponse(HttpResponseCode::NotFound,
+														 HttpContentType::kText,
+														 fmt::format("project {} not found", ProjectId));
 						}
 
 						ZEN_INFO("deleting project '{}'", ProjectId);
 						if (!m_ProjectStore->DeleteProject(ProjectId))
 						{
-							return Req.ServerRequest().WriteResponse(HttpResponseCode::Locked,
-																	 HttpContentType::kText,
-																	 fmt::format("project {} is in use", ProjectId));
+							return HttpReq.WriteResponse(HttpResponseCode::Locked,
+														 HttpContentType::kText,
+														 fmt::format("project {} is in use", ProjectId));
 						}
 
-						return Req.ServerRequest().WriteResponse(HttpResponseCode::NoContent);
+						return HttpReq.WriteResponse(HttpResponseCode::NoContent);
 					}
 					break;
 
@@ -3556,14 +3592,20 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 	m_Router.RegisterRoute(
 		"{project}/oplog/{log}/save",
 		[this](HttpRouterRequest& Req) {
-			HttpServerRequest& HttpReq	 = Req.ServerRequest();
-			const auto&		   ProjectId = Req.GetCapture(1);
-			const auto&		   OplogId	 = Req.GetCapture(2);
+			HttpServerRequest& HttpReq = Req.ServerRequest();
+
+			if (!m_ProjectStore->AreDiskWritesAllowed())
+			{
+				return HttpReq.WriteResponse(HttpResponseCode::InsufficientStorage);
+			}
+
+			const auto& ProjectId = Req.GetCapture(1);
+			const auto& OplogId	  = Req.GetCapture(2);
 			if (HttpReq.RequestContentType() != HttpContentType::kCbObject)
 			{
 				return HttpReq.WriteResponse(HttpResponseCode::BadRequest, HttpContentType::kText, "Invalid content type");
 			}
-			IoBuffer Payload = Req.ServerRequest().ReadPayload();
+			IoBuffer Payload = HttpReq.ReadPayload();
 
 			CbObject								 Response;
 			std::pair<HttpResponseCode, std::string> Result = m_ProjectStore->WriteOplog(ProjectId, OplogId, std::move(Payload), Response);
@@ -3590,11 +3632,11 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 			{
 				return HttpReq.WriteResponse(HttpResponseCode::BadRequest, HttpContentType::kText, "Invalid accept content type");
 			}
-			IoBuffer Payload = Req.ServerRequest().ReadPayload();
+			IoBuffer Payload = HttpReq.ReadPayload();
 
 			CbObject								 Response;
 			std::pair<HttpResponseCode, std::string> Result =
-				m_ProjectStore->ReadOplog(ProjectId, OplogId, Req.ServerRequest().GetQueryParams(), Response);
+				m_ProjectStore->ReadOplog(ProjectId, OplogId, HttpReq.GetQueryParams(), Response);
 			if (Result.first == HttpResponseCode::OK)
 			{
 				return HttpReq.WriteResponse(HttpResponseCode::OK, Response);
@@ -3615,7 +3657,7 @@ HttpProjectService::HttpProjectService(CidStore& Store, ProjectStore* Projects,
 
 			const auto& ProjectId = Req.GetCapture(1);
 			const auto& OplogId	  = Req.GetCapture(2);
-			IoBuffer	Payload	  = Req.ServerRequest().ReadPayload();
+			IoBuffer	Payload	  = HttpReq.ReadPayload();
 
 			m_ProjectStore->Rpc(HttpReq, ProjectId, OplogId, std::move(Payload), m_AuthMgr);
 		},
diff --git a/src/zenserver/projectstore/projectstore.h b/src/zenserver/projectstore/projectstore.h
index 7b0c96205..b446de543 100644
--- a/src/zenserver/projectstore/projectstore.h
+++ b/src/zenserver/projectstore/projectstore.h
@@ -322,12 +322,15 @@ public:
 													CbObjectView&&		   Params,
 													AuthMgr&			   AuthManager);
 
+	bool AreDiskWritesAllowed() const;
+
 private:
 	spdlog::logger&						m_Log;
 	CidStore&							m_CidStore;
 	std::filesystem::path				m_ProjectBasePath;
 	mutable RwLock						m_ProjectsLock;
 	std::map<std::string, Ref<Project>> m_Projects;
+	const DiskWriteBlocker*				m_DiskWriteBlocker = nullptr;
 
 	std::filesystem::path BasePathForProject(std::string_view ProjectId);
 };
diff --git a/src/zenserver/zenserver.cpp b/src/zenserver/zenserver.cpp
index 75a49367c..c6ec4b82b 100644
--- a/src/zenserver/zenserver.cpp
+++ b/src/zenserver/zenserver.cpp
@@ -394,14 +394,16 @@ public:
 		}
 
 		ZEN_INFO("initializing GC, enabled '{}', interval {}s", ServerOptions.GcConfig.Enabled, ServerOptions.GcConfig.IntervalSeconds);
-		zen::GcSchedulerConfig GcConfig{.RootDirectory		 = m_DataRoot / "gc",
-										.MonitorInterval	 = std::chrono::seconds(ServerOptions.GcConfig.MonitorIntervalSeconds),
-										.Interval			 = std::chrono::seconds(ServerOptions.GcConfig.IntervalSeconds),
-										.MaxCacheDuration	 = std::chrono::seconds(ServerOptions.GcConfig.Cache.MaxDurationSeconds),
-										.CollectSmallObjects = ServerOptions.GcConfig.CollectSmallObjects,
-										.Enabled			 = ServerOptions.GcConfig.Enabled,
-										.DiskReserveSize	 = ServerOptions.GcConfig.DiskReserveSize,
-										.DiskSizeSoftLimit	 = ServerOptions.GcConfig.Cache.DiskSizeSoftLimit};
+		zen::GcSchedulerConfig GcConfig{
+			.RootDirectory					   = m_DataRoot / "gc",
+			.MonitorInterval				   = std::chrono::seconds(ServerOptions.GcConfig.MonitorIntervalSeconds),
+			.Interval						   = std::chrono::seconds(ServerOptions.GcConfig.IntervalSeconds),
+			.MaxCacheDuration				   = std::chrono::seconds(ServerOptions.GcConfig.Cache.MaxDurationSeconds),
+			.CollectSmallObjects			   = ServerOptions.GcConfig.CollectSmallObjects,
+			.Enabled						   = ServerOptions.GcConfig.Enabled,
+			.DiskReserveSize				   = ServerOptions.GcConfig.DiskReserveSize,
+			.DiskSizeSoftLimit				   = ServerOptions.GcConfig.Cache.DiskSizeSoftLimit,
+			.MinimumFreeDiskSpaceToAllowWrites = ServerOptions.GcConfig.Cache.MinimumFreeDiskSpaceToAllowWrites};
 		m_GcScheduler.Initialize(GcConfig);
 
 		return EffectiveBasePort;
@@ -845,8 +847,12 @@ ZenServer::InitializeStructuredCache(const ZenServerOptions& ServerOptions)
 		}
 	}
 
-	m_StructuredCacheService =
-		std::make_unique<HttpStructuredCacheService>(*m_CacheStore, *m_CidStore, m_StatsService, m_StatusService, *m_UpstreamCache);
+	m_StructuredCacheService = std::make_unique<HttpStructuredCacheService>(*m_CacheStore,
+																			*m_CidStore,
+																			m_StatsService,
+																			m_StatusService,
+																			*m_UpstreamCache,
+																			m_GcManager.GetDiskWriteBlocker());
 
 	m_Http->RegisterService(*m_StructuredCacheService);
 	m_Http->RegisterService(*m_UpstreamService);
diff --git a/src/zenstore/blockstore.cpp b/src/zenstore/blockstore.cpp
index 05bc69fcb..378d9fd52 100644
--- a/src/zenstore/blockstore.cpp
+++ b/src/zenstore/blockstore.cpp
@@ -472,7 +472,10 @@ BlockStore::ReclaimSpace(const ReclaimSnapshotState&			Snapshot,
 					 WriteBlockTimeUs += ElapsedUs;
 					 WriteBlockLongestTimeUs = std::max(ElapsedUs, WriteBlockLongestTimeUs);
 				 });
-				OldBlockFile			   = m_ChunkBlocks[BlockIndex];
+				if (auto It = m_ChunkBlocks.find(BlockIndex); It != m_ChunkBlocks.end())
+				{
+					OldBlockFile = It->second;
+				}
 			}
 
 			if (!OldBlockFile)
@@ -504,8 +507,9 @@ BlockStore::ReclaimSpace(const ReclaimSnapshotState&			Snapshot,
 					});
 					if (OldBlockFile)
 					{
-						m_ChunkBlocks[BlockIndex] = nullptr;
 						ZEN_DEBUG("marking cas block store file '{}' for delete, block #{}", OldBlockFile->GetPath(), BlockIndex);
+						ZEN_ASSERT(m_ChunkBlocks[BlockIndex] == OldBlockFile);
+						m_ChunkBlocks.erase(BlockIndex);
 						m_TotalSize.fetch_sub(OldBlockFile->FileSize(), std::memory_order::relaxed);
 						OldBlockFile->MarkAsDeleteOnClose();
 					}
@@ -582,7 +586,9 @@ BlockStore::ReclaimSpace(const ReclaimSnapshotState&			Snapshot,
 								ReadBlockTimeUs += ElapsedUs;
 								ReadBlockLongestTimeUs = std::max(ElapsedUs, ReadBlockLongestTimeUs);
 							});
+							ZEN_ASSERT(m_ChunkBlocks[NextBlockIndex] == NewBlockFile);
 							m_ChunkBlocks.erase(NextBlockIndex);
+							NewBlockFile->MarkAsDeleteOnClose();
 							return;
 						}
 
@@ -627,8 +633,9 @@ BlockStore::ReclaimSpace(const ReclaimSnapshotState&			Snapshot,
 					ReadBlockTimeUs += ElapsedUs;
 					ReadBlockLongestTimeUs = std::max(ElapsedUs, ReadBlockLongestTimeUs);
 				});
-				m_ChunkBlocks[BlockIndex]	   = nullptr;
 				ZEN_DEBUG("marking cas block store file '{}' for delete, block #{}", OldBlockFile->GetPath(), BlockIndex);
+				ZEN_ASSERT(m_ChunkBlocks[BlockIndex] == OldBlockFile);
+				m_ChunkBlocks.erase(BlockIndex);
 				m_TotalSize.fetch_sub(OldBlockFile->FileSize(), std::memory_order::relaxed);
 				OldBlockFile->MarkAsDeleteOnClose();
 			}
@@ -704,8 +711,8 @@ BlockStore::IterateChunks(const std::vector<BlockStoreLocation>& ChunkLocations,
 		size_t					  ChunkIndex	= LocationIndexes[LocationIndexOffset];
 		const BlockStoreLocation& FirstLocation = ChunkLocations[ChunkIndex];
 
-		const Ref<BlockStoreFile>& BlockFile = m_ChunkBlocks[FirstLocation.BlockIndex];
-		if (!BlockFile)
+		auto FindBlockIt = m_ChunkBlocks.find(FirstLocation.BlockIndex);
+		if (FindBlockIt == m_ChunkBlocks.end())
 		{
 			while (ChunkLocations[ChunkIndex].BlockIndex == FirstLocation.BlockIndex)
 			{
@@ -719,6 +726,9 @@ BlockStore::IterateChunks(const std::vector<BlockStoreLocation>& ChunkLocations,
 			}
 			continue;
 		}
+		const Ref<BlockStoreFile>& BlockFile = FindBlockIt->second;
+		ZEN_ASSERT(BlockFile);
+
 		size_t BlockSize  = BlockFile->FileSize();
 		size_t RangeCount = GetNextRange(LocationIndexOffset);
 		if (RangeCount > 0)
diff --git a/src/zenstore/gc.cpp b/src/zenstore/gc.cpp
index 370c3c965..f9888722b 100644
--- a/src/zenstore/gc.cpp
+++ b/src/zenstore/gc.cpp
@@ -559,10 +559,12 @@ DiskUsageWindow::FindTimepointThatRemoves(uint64_t Amount, GcClock::Tick EndTick
 
 GcScheduler::GcScheduler(GcManager& GcManager) : m_Log(logging::Get("gc")), m_GcManager(GcManager)
 {
+	m_GcManager.SetDiskWriteBlocker(this);
 }
 
 GcScheduler::~GcScheduler()
 {
+	m_GcManager.SetDiskWriteBlocker(nullptr);
 	Shutdown();
 }
 
@@ -573,6 +575,18 @@ GcScheduler::Initialize(const GcSchedulerConfig& Config)
 
 	m_Config = Config;
 
+	std::error_code Ec;
+	DiskSpace		Space = DiskSpaceInfo(m_Config.RootDirectory, Ec);
+	if (Ec)
+	{
+		m_AreDiskWritesBlocked.store(true);
+		ZEN_WARN("get disk space info FAILED, blocking disk writes, reason: '{}'", Ec.message());
+	}
+	else
+	{
+		CheckDiskSpace(Space);
+	}
+
 	if (m_Config.Interval.count() && m_Config.Interval < m_Config.MonitorInterval)
 	{
 		m_Config.Interval = m_Config.MonitorInterval;
@@ -580,7 +594,7 @@ GcScheduler::Initialize(const GcSchedulerConfig& Config)
 
 	std::filesystem::create_directories(Config.RootDirectory);
 
-	std::error_code Ec = CreateGCReserve(m_Config.RootDirectory / "reserve.gc", m_Config.DiskReserveSize);
+	Ec = CreateGCReserve(m_Config.RootDirectory / "reserve.gc", m_Config.DiskReserveSize);
 	if (Ec)
 	{
 		ZEN_WARN("unable to create GC reserve at '{}' with size {}, reason '{}'",
@@ -664,6 +678,29 @@ GcScheduler::Trigger(const GcScheduler::TriggerParams& Params)
 }
 
 void
+GcScheduler::CheckDiskSpace(const DiskSpace& Space)
+{
+	bool AreDiskWritesBlocked = m_AreDiskWritesBlocked;
+	bool IsLowOnDiskSpace = (m_Config.MinimumFreeDiskSpaceToAllowWrites) != 0 && (Space.Free < m_Config.MinimumFreeDiskSpaceToAllowWrites);
+	if (IsLowOnDiskSpace != AreDiskWritesBlocked)
+	{
+		m_AreDiskWritesBlocked.store(IsLowOnDiskSpace);
+		if (IsLowOnDiskSpace)
+		{
+			ZEN_WARN("Writing to disk is blocked, free disk space: {}, minimum required {}",
+					 NiceBytes(Space.Free),
+					 NiceBytes(m_Config.MinimumFreeDiskSpaceToAllowWrites));
+		}
+		else
+		{
+			ZEN_INFO("Writing to disk is unblocked, free disk space: {}, minimum required {}",
+					 NiceBytes(Space.Free),
+					 NiceBytes(m_Config.MinimumFreeDiskSpaceToAllowWrites));
+		}
+	}
+}
+
+void
 GcScheduler::SchedulerThread()
 {
 	std::chrono::seconds WaitTime{0};
@@ -716,16 +753,21 @@ GcScheduler::SchedulerThread()
 
 		GcClock::TimePoint ExpireTime = MaxCacheDuration == GcClock::Duration::max() ? GcClock::TimePoint::min() : Now - MaxCacheDuration;
 
-		std::error_code		Ec;
 		const GcStorageSize TotalSize = m_GcManager.TotalStorageSize();
 
 		if (Timeout && Status() == GcSchedulerStatus::kIdle)
 		{
-			DiskSpace Space = DiskSpaceInfo(m_Config.RootDirectory, Ec);
+			std::error_code Ec;
+			DiskSpace		Space = DiskSpaceInfo(m_Config.RootDirectory, Ec);
 			if (Ec)
 			{
+				m_AreDiskWritesBlocked.store(true);
 				ZEN_WARN("get disk space info FAILED, reason: '{}'", Ec.message());
 			}
+			else
+			{
+				CheckDiskSpace(Space);
+			}
 
 			const int64_t				PressureGraphLength = 30;
 			const std::chrono::duration LoadGraphTime		= PressureGraphLength * m_Config.MonitorInterval;
diff --git a/src/zenstore/include/zenstore/gc.h b/src/zenstore/include/zenstore/gc.h
index e0354b331..fe9857e6a 100644
--- a/src/zenstore/include/zenstore/gc.h
+++ b/src/zenstore/include/zenstore/gc.h
@@ -27,6 +27,7 @@ class HashKeySet;
 class GcManager;
 class CidStore;
 struct IoHash;
+struct DiskSpace;
 
 /** GC clock
  */
@@ -121,6 +122,14 @@ private:
 	GcManager& m_Gc;
 };
 
+/** Interface for querying if we are running low on disk space, used to deny put/writes to disk
+ */
+class DiskWriteBlocker
+{
+public:
+	virtual bool AreDiskWritesAllowed() const = 0;
+};
+
 /** GC orchestrator
  */
 class GcManager
@@ -139,6 +148,9 @@ public:
 
 	GcStorageSize TotalStorageSize() const;
 
+	const DiskWriteBlocker* GetDiskWriteBlocker() { return m_DiskWriteBlocker; }
+	void					SetDiskWriteBlocker(const DiskWriteBlocker* Monitor) { m_DiskWriteBlocker = Monitor; }
+
 #if ZEN_USE_REF_TRACKING
 	void OnNewCidReferences(std::span<IoHash> Hashes);
 	void OnCommittedCidReferences(std::span<IoHash> Hashes);
@@ -151,7 +163,8 @@ private:
 	mutable RwLock				m_Lock;
 	std::vector<GcContributor*> m_GcContribs;
 	std::vector<GcStorage*>		m_GcStorage;
-	CidStore*					m_CidStore = nullptr;
+	CidStore*					m_CidStore		   = nullptr;
+	const DiskWriteBlocker*		m_DiskWriteBlocker = nullptr;
 };
 
 enum class GcSchedulerStatus : uint32_t
@@ -167,10 +180,11 @@ struct GcSchedulerConfig
 	std::chrono::seconds  MonitorInterval{30};
 	std::chrono::seconds  Interval{};
 	std::chrono::seconds  MaxCacheDuration{86400};
-	bool				  CollectSmallObjects = true;
-	bool				  Enabled			  = true;
-	uint64_t			  DiskReserveSize	  = 1ul << 28;
-	uint64_t			  DiskSizeSoftLimit	  = 0;
+	bool				  CollectSmallObjects				= true;
+	bool				  Enabled							= true;
+	uint64_t			  DiskReserveSize					= 1ul << 28;
+	uint64_t			  DiskSizeSoftLimit					= 0;
+	uint64_t			  MinimumFreeDiskSpaceToAllowWrites = 1ul << 28;
 };
 
 class DiskUsageWindow
@@ -196,7 +210,7 @@ public:
 /**
  * GC scheduler
  */
-class GcScheduler
+class GcScheduler : private DiskWriteBlocker
 {
 public:
 	GcScheduler(GcManager& GcManager);
@@ -220,6 +234,8 @@ private:
 	void			   CollectGarbage(const GcClock::TimePoint& ExpireTime, bool Delete, bool CollectSmallObjects);
 	GcClock::TimePoint NextGcTime(GcClock::TimePoint CurrentTime);
 	spdlog::logger&	   Log() { return m_Log; }
+	virtual bool	   AreDiskWritesAllowed() const override { return !m_AreDiskWritesBlocked.load(); }
+	void			   CheckDiskSpace(const DiskSpace& Space);
 
 	spdlog::logger&				 m_Log;
 	GcManager&					 m_GcManager;
@@ -232,6 +248,7 @@ private:
 	std::mutex					 m_GcMutex;
 	std::condition_variable		 m_GcSignal;
 	std::optional<TriggerParams> m_TriggerParams;
+	std::atomic_bool			 m_AreDiskWritesBlocked = false;
 
 	TCasLogFile<DiskUsageWindow::DiskUsageEntry> m_DiskUsageLog;
 	DiskUsageWindow								 m_DiskUsageWindow;