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On 11/22/2019 12:50 AM, Sage Weil
wrote:
On Thu, 21 Nov 2019, Mark Nelson wrote:Hi Folks, We're discussing changing the minimum allocation size in bluestore to 4k. For flash devices this appears to be a no-brainer. We've made the write path fast enough in bluestore that we're typically seeing either the same or faster performance with a 4K min_alloc size and the space savings for small objects easily outweigh the increase in metadata for large fragmented objects. For HDDs there are tradeoffs. A smaller allocation size means more fragmentation when there are small overwrites (like in RBD) which can mean a lot more seeks. Igor was showing some fairly steep RBD performance drops for medium-large reads/writes once the OSDs started to become fragmented. For RGW this isn't nearly as big of a deal though since typically the objects shouldn't become fragmented. A small (4K) allocation size does mean however that we can write out 4K random writes sequentially and gain a big IOPS win which theoretically should benefit both RBD and RGW. Regarding space-amplification, Josh pointed out that our current 64K allocation size has huge ramifications for overall space-amp when writing out medium sized objects to EC pools. In an attempt to actually quantify this, I made a spreadsheet with some graphs showing a couple of examples of how the min_alloc size and replication/EC interact with each other at different object sizes. The gist of it is that with our current default HDD min_alloc size (64K), erasure coding can actually have worse space amplification than 3X replication, even with moderately large (128K) object sizes. How much this factors into the decision vs fragmentation is a tough call, but I wanted to at least showcase the behavior as we work through deciding what our default HDD behavior should be. https://docs.google.com/spreadsheets/d/1rpGfScgG-GLoIGMJWDixEkqs-On9w8nAUToPQjN8bDI/edit?usp=sharingThe key difference (at the bluestore level) between RGW and RBD/CephFS writes is that RGW passes down the CEPH_OSD_ALLOC_HINT_FLAG_IMMUTABLE | CEPH_OSD_ALLOC_HINT_FLAG_APPEND_ONLY hints. The immutable one in particular is what we really care about, since it's the mutable objects that get overwrites that lead to (most) fragmentation. We should use this to decide whether to create minimal (min_alloc_size) blobs or whether we should keep the blobs larger to limit fragmentation. I'm not sure what we would call the config option that isn't super confusing... maybe bluestore_mutable_min_blob_size? bluestore_baseline_min_blob_size? I'm thinking about introducing the following behavior for spinner-based stores (preliminary notes for now): 1) Unconditionally pin allocator's granularity to 4K (block size), i.e. untie it from min_alloc_size setting This will probably result in additional overhead in allocator when one needs to allocate contiguous 64K blobs (see 3) below). But it looks like new avl allocator is great in dealing with
that. 3) Use existing min_alloc_size defaults (64K) for all(?) other
objects 4) Tag object with alloc size it's using. ------- The above is enough enough to deal with RGW space
amplification for small objects which is the worst case for now
IMO.---------------- Additional points to consider: 5) add per-object (more flexible and complex) hint or per-store
setting to optimize onode keeping for size vs. for speed. May be
even spit the latter into optimize for reads vs. optimize for
overwrites? EC and/or other concerned entries to indicate
"OPTIMIZE_FOR_SPACE". Unconditionally or using some logics? 6) apply appropriate alloc size depending on the strategy determined at 5) 7) Even more versatile approach would be vary alloc size at onode's blob level. Hence tag blob rather than object with applied alloc size (needs single bit in fact).May be useful to deal with space amplification for tail blobs or
sparse objects.
Thoughts?
Thanks, Igor sage |
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