Will need benchmarking, but might just work.
Still need barriers for reasonable performance + integrity on systems
without NCQ, like the kit on my desk. Without barriers I have to turn
write-cache off (I do see those power-off errors). But that's too slow.
I grant you, it is all about pushing ordering down as far as reasonable.
That's right. You can keep it quite simple: just distinguish barriers
from flushes. Or make it more detailed, distinguishing different
block sets and partial barriers between them.
I think simply distinguishing barrier from flush is relatively
understandable, isn't it?
Yeah, if you're going to do it _properly_ that's the way :-)
But the I/O failure strategy is really a different problem, and shouldn't
necessarily distract from barrier I/O elevator scheduling.
The I/O failure strategy applies just the same even with no barriers
and no disk cache at all. Even there, you want a failed write to
block subsequent dependent writes.
True, but fortunately it's quite a simple thing. fsync et al need to
ask for flush, nothing else does. You simply don't need consistency
from journalling writes without fsync. Journal writes without fsync
happen at times determined by arcane kernel daemons. Applications and
even users don't have any say, and therefore no particular
expectation.
Mail servers seem like the most vulnerable systems to me too.
They are often designed to relay a successful fsync() result across
the network. RAID doesn't protect against power-fails with flushless
fsync, and SMTP does not redundantly hold the same email at different
locations for recovery. (The most solid email services will use a
distributed database which is not vulnerable to these problems. But
the most common mail systems are).
That depends on the disk implementation. Another use of the 32MB is
to hold write-through NCQ commands in flight while they are sorted.
If the trend is towards that, perhaps it's quite resistant to power
failure.
Still, I'm on the side of safety - having witnessed these power-fail
corruptions myself. The type I've seen will get more common, as the
trend in home "appliances" is towards pulling the plug to turn them
off, and to have these larger storage devices in them.
-- Jamie
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