Either more weird then, what drives is in the other cluster?
Desktop Toshiba and Seagate Constellation 7200rpm
As I understand by now the main impact is for SSD+HDD clusters. Enabled
HDD write cache causes kernel to send flush requests for it (when write
cache is disabled it doesn't bother about that) and probably it affects
something else and causes some extra waits for SSD journal (although it's
strange and looks like a bug to me). I tried to check latencies in `ceph
daemon osd.xx perf dump` and both kv_commit_lat and commit_lat decreased
~10 times when I disabled HDD write cache (although both are SSD-related
as I understand).
Maybe your HDD are connected via some RAID controller and when you disable
cache it doesn't really get disabled, but the kernels just stops to issue
flush requests and makes some writes unsafe?
--
With best regards,
Vitaliy Filippov
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