restore cg comment

1 files changed, 25 insertions, 2 deletions

diff --git a/zenstore/compactcas.cpp b/zenstore/compactcas.cpp
index ade6a7daf..5a0ba974b 100644
--- a/zenstore/compactcas.cpp
+++ b/zenstore/compactcas.cpp
@@ -321,8 +321,6 @@ CasContainerStrategy::Flush()
 void
 CasContainerStrategy::Scrub(ScrubContext& Ctx)
 {
-	ZEN_UNUSED(Ctx);
-
 	RwLock::SharedLockScope _(m_LocationMapLock);
 
 	uint64_t						TotalChunkCount = m_LocationMap.size();
@@ -409,6 +407,31 @@ CasContainerStrategy::Scrub(ScrubContext& Ctx)
 void
 CasContainerStrategy::CollectGarbage(GcContext& GcCtx)
 {
+	// It collects all the blocks that we want to delete chunks from. For each such
+	// block we keep a list of chunks to retain and a list of chunks to delete.
+	//
+	// If there is a block that we are currently writing to, that block is omitted
+	// from the garbage collection.
+	//
+	// Next it will iterate over all blocks that we want to remove chunks from.
+	// If the block is empty after removal of chunks we mark the block as pending
+	// delete - we want to delete it as soon as there are no IoBuffers using the
+	// block file.
+	// Once complete we update the m_LocationMap by removing the chunks.
+	//
+	// If the block is non-empty we write out the chunks we want to keep to a new
+	// block file (creating new block files as needed).
+	//
+	// We update the index as we complete each new block file. This makes it possible
+	// to break the GC if we want to limit time for execution.
+	//
+	// GC can very parallell to regular operation - it will block while taking
+	// a snapshot of the current m_LocationMap state and while moving blocks it will
+	// do a blocking operation and update the m_LocationMap after each new block is
+	// written and figuring out the path to the next new block.
+
+	ZEN_INFO("collecting garbage from '{}'", m_Config.RootDirectory / m_ContainerBaseName);
+
 	uint64_t WriteBlockTimeUs		 = 0;
 	uint64_t WriteBlockLongestTimeUs = 0;
 	uint64_t ReadBlockTimeUs		 = 0;