Basically they max out at around 1000 IOPS and report 100%
utilization and feel slow.
Haven't seen the 5200 yet.
Micron 5100s performs wonderfully!
You have to just turn its write cache off:
hdparm -W 0 /dev/sdX
1000 IOPS means you haven't done it. Although even with write cache
enabled I observe like ~5000 iops, not 1000, but that delta is probably
just eaten by Ceph :))
With write cache turned off 5100 is capable of up to 40000 write iops.
5200 is slightly worse, but only slightly: it still gives ~25000 iops.
Funny thing is that the same applies to a lot of server SSDs with
supercapacitors. As I understand when their write cache is turned on
every `fsync` is translated to SATA FLUSH CACHE, and the latter is
interpreted by the drive as "please flush all caches, including
capacitor-protected write cache".
And when you turn it off the drive just writes at its full speed and
doesn't flush the cache because it has capacitors to account for a
possible power loss.
You don't need to disable cache explicitly only with some HBAs that do
it internally.
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