Re: Help: pool not responding

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Le 29/02/2016 20:43, Mario Giammarco a écrit :
> [...]
> I said SSHD that is a standard hdd with ssd cache. It is 7200rpms but in
> benchmarks it is better than a 10000rpm disk.

Lies, damn lies and benchmarks...
SSHD usually have very small flash caches (16GB or less for 500GB of
data or more) and AFAIK there's no distribution supporting cache hints
or to be of any use here Ceph OSD cache hint support : the drive makes
the decisions about when to use the cache and you can trust only one
thing: the fact that they are optimized for benchmarks and certainly not
Ceph OSD usage patterns (with or without internal journal).

There are probably 2 kinds of optimizations that SSHD can perform :
- buffering random writes with a writeback cache algorithm targeting
random writes. With only 8 to 16GB of flash this would probably mean
that under heavy random write usage (typical for OSD) the flash will die
very fast which would kill the entire drive or lose data and so it's
unlikely that this is what they use.
- write the most used data (what is first loaded on system boot and what
is most used) to the flash cache to speed up the OS boot sequence and
access to the most used applications or data. As OSDs don't have any
recognizable pattern this is useless in most cases.

So SSHD for OSD are almost certainly useless. You are better off saving
money by buying more ordinary HDD SATA drives or as many HDD and a few
good SSDs for journal if you can afford them.

In fact if the SSHD tries to cache writes and doesn't die early in the
process you may get even worse performance than a pure HDD setup because
most consumer-level SSD (and probably SSHD) are absolute crap for the
type of access Ceph OSD do with journals (see
http://www.sebastien-han.fr/blog/2014/10/10/ceph-how-to-test-if-your-ssd-is-suitable-as-a-journal-device/
for the horror stories).

Best regards,

Lionel
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