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=sharing
The 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?
sage
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