pacific doesn't defer small writes for pre-pacific hdd osds

[Dan van der Ster <dvanders@xxxxxxxxx>]

Hi Igor and others,

(apologies for html, but i want to share a plot ;) )

We're upgrading clusters to v16.2.9 from v15.2.16, and our simple "rados bench -p test 10 write -b 4096 -t 1" latency probe showed something is very wrong with deferred writes in pacific.

Here is an example cluster, upgraded today:

The OSDs are 12TB HDDs, formatted in nautilus with the default bluestore_min_alloc_size_hdd = 64kB, and each have a large flash block.db.

I found that the performance issue is because 4kB writes are no longer deferred from those pre-pacific hdds to flash in pacific with the default config !!!

Here are example bench writes from both releases: https://pastebin.com/raw/m0yL1H9Z

I worked out that the issue is fixed if I set bluestore_prefer_deferred_size_hdd = 128k (up from the 64k pacific default. Note the default was 32k in octopus).

I think this is related to the fixes in https://tracker.ceph.com/issues/52089 which landed in 16.2.6 -- _do_alloc_write is comparing the prealloc size 0x10000 with bluestore_prefer_deferred_size_hdd (0x10000) and the "strictly less than" condition prevents deferred writes from ever happening.

So I think this would impact anyone upgrading clusters with hdd/ssd mixed osds ... surely we must not be the only clusters impacted by this?!

Should we increase the default bluestore_prefer_deferred_size_hdd up to 128kB or is there in fact a bug here?

Best Regards,

Dan

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