Now that the hybrid allocator appears to be enabled by default in
Octopus, is it safe to change bluestore_min_alloc_size_hdd to 4k from
64k on Octopus 15.2.10 clusters, and then redeploy every OSD to switch
to the smaller allocation size, without massive performance impact for
RBD? We're seeing a lot of storage usage amplification on EC 8+3
clusters which are HDD backed that lines up with a lot of the mailing
list posts we've seen here. Upgrading to Pacific before making this
change is also a possibility once a more stable release arrives, if
that's necessary.
Second part of this question - we are using RBDs currently on the
clusters impacted. These have XFS filesystems on top, which detect the
sector size of the RBD as 512byte, and XFS has a block size of 4k.
With the default of 64k for bluestore_min_alloc_size_hdd, let's say a
1G file is written out to the XFS filesystem backed by the RBD. On the
ceph side, is this being seen as a lot of 4k objects thus a
significant space waste is occurring, or is RBD able to coalesce these
into 64k objects, even though XFS is using a 4k block size?
XFS details below, you can see the allocation groups are quite large:
meta-data=/dev/rbd0 isize=512 agcount=501, agsize=268435440 blks
= sectsz=512 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=1, rmapbt=0
= reflink=1
data = bsize=4096 blocks=134217728000, imaxpct=1
= sunit=16 swidth=16 blks
naming =version 2 bsize=4096 ascii-ci=0, ftype=1
log =internal log bsize=4096 blocks=521728, version=2
= sectsz=512 sunit=16 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
I'm curious if people have been tuning XFS on RBD for better
performance, as well.
Thank you!
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