Hi,
Do the I/O problems go away when only one of the SSDs is attached?
No, the problem stays with only one SSD. The problem is only less when
resilvering, but not totally disappeared (maybe because of the resilver
overhead).
Frankly, I'm betting that your SSDs are wearing out. Resilvering
will essentially be one big streaming write, which is optimal for SSDs
(even an SLC-based SSD, as you likely have, performs far better when
writing large amounts of data at once). NFS (and to a lesser extent
iSCSI) is generally a whole lot of random small writes, which are hard
on an SSD (especially MLC-based ones, but even SLC ones). The
resilvering process is likely turning many of the random writes coming
in to the system into a large streaming write to the /resilvering/ drive.
Hmm, interesting theory. Next I well execute only a resilver to see if
the same happens. I assume when adding a new disk, even though it's only
a slog disk, the whole tank will resilver? If I look to the zpool iostat
currently I see a lot of reads on the separate SATA disks (not on the
tank/or raidz2 pools), assuming resilvering takes place there and the
SSD's are already synced.
My guess is that the SSD you are having problems with has reached the
end of it's useful lifespan, and the I/O problems you are seeing
during normal operation are the result of that SSD's problems with
committing data. There's no cure for this, other than replacing the
SSD with a new one.
SSDs are not hard drives. Even high-quality modern ones have
/significantly/ lower USE lifespans than an HD - that is, a
heavily-used SSD will die well before a HD, but a very-lightly used
SSD will likely outlast a HD. And, in the case of SSDs, writes are
far harder on the SSD than reads are.
Is about half a year for these disk not really short? Sure, we have some
I/O, but not that many write operations, about ~80-140 iops, anyway, I
will try to get new disks from SUN (we have SLC disks from Sun). Is
there any knowledge about the life time of SSD's? Maybe in terms of
amount of I/O Operations?
Regards,
Mart van Santen
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