On Fri, 17 Jun 2016, Allen Samuels wrote: > I donʼt understand the ref_map and unused stuff. What is their purpose? ref_map: We allocate blobs and write uncompressed data in. Later, we logically overwrite part of that reference, such that our big 1MB allocation only has part of it referenced. When that happens we want to release the unreferenced part back to the allocator for use by something else. ref_map lets us do that, which some additional complexity that counts references (from multiple clones). unused: We might allocate a full min_alloc_size but only write one block into it. If we do another small write in an adjacent block, we want to write into the existing blob. In order to do that without with a WAL event, we need to know that the block isn't currently referenced by anything. ref_map isn't quite sufficient for this because we don't have a complicated commit/persist lifecycle sequence on update, and we need to make sure the *committed* state has no references before we can safely overwrite a block. After pondering this a while I decided that would be very complex to implement that, with marginal benefit, and in reality this mostly matters for newly-allocated but never-written blobs... hence unused. I decided we don't care the case when you partially occlude part of a blob (but less that min_alloc_size so it wasn't released back to the allocator) and *then* also do a small overwrite such that the space could be reused. sage
