Il 18-08-2017 14:54 Roger Heflin ha scritto:
I have noticed all of the hardware raid controllers explicitly turn
off the disk's write cache so this would eliminate this issue, but the
cost is much slower write times.
True...
It makes the hardware raid controllers (and disk arrays) become
uselessly slow when their battery backup dies and disables the raid
card and/or arrays write cache.
...true...
Remember, safe, fast and cheap, you only get to pick 2. We generally
pick fast and cheap, the disk arrays/raid controllers pick safe and
fast, but not so cheap as a hardware raid controller with write cache
backup of some sort are quite expensive.
...and true. I am not arguing any of these points.
What really surprised me was to realize that, facing micro-powerlosses,
*even sync() writes* can be vulnerable to undetected data loss, at least
when not using FUAs (using instead the common barrier infrastructure).
So while many (old) mismatch_cnt reports on RAID1/10 arrays where
dismissed as "don't bother, it's a harmless RAID1 thing", I really think
than some were genuine corruptions due to micro powerlosses and similar
causes.
If nothing more, such reports really emphasize the need to have a
"trusted" mismatch_cnt for mirrored arrays, even in the face of some
performance losses (due to no using zero copy anymore).
Regards.
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Danti Gionatan
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