Mark Hahn wrote:
The write cache in modern drives is multiple megabytes - 8 or 16MB is
not uncommon. The chances that you have data that is lost on a power
failure in the write cache is actually quite high...
but we're not talking about power failures in the middle of peak activity.
afaikt, drives also never dedicate their whole cache to writeback - they
keep plenty available for reads, as well. it would also be rather surprising
if the firmware was completely oblivious about limiting the age of
writebacks; after all always delaying writes until you run out of cache
capacity is _not_ a winning strategy (even ignoring safety issues.)
If you have drives/hardware to test on, you can easily verify (which we
do on a regular basis) that running with barriers over power fail
testing gets you a solid recovery. Running with write cache on and no
barriers gets you file system corruption. As I said before, the data you
just wrote (or the file system wrote for you) most recently is the same
data that you stand to lose on a powerloss.
during a normal shutdown, can you think of some reason the drive would have
LOTS of outstanding writes? that's the real point. depending on kernel
version, linux should be doing a cache-flush command and standby, then
eventually calling bios poweroff. it's very possible that this is going
wrong (rumors of disks that claim to implement, but ignore cache-flush,
or perhaps ones that stupidly don't flush on standby, or even bios poweroff
that happens so fast that the disks isn't done flushing...) but turning
off all writeback is overkill (especially when there's some other obvious
sign of distress...)
We don't test every make of drive, but the modern drives we do test do
honor the cache flush commands. It is important to note that drive
firmware is like any other bit of code - it can have bugs so this
support does need to be reverified on each drive (and version of
firmware) before you can trust high value data ;-)
If there is a hole in the sequence, dropping to standby could be the
source of issues...
ric
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