Write cache, is write cache, is write cache?

[Šimun Mikecin]

2011/1/24 Jeremy Chadwick <freebsd at jdc.parodius.com>

> The term "flush" means many different things.  fsync(2), for example,
> behaves differently on UFS than it does on ZFS.  People think that
> "flush" means "guarantee the data written was written to disk", but
> ensuring an actual ATA/SCSI command completes **and** has had its data
> written to the platters is an entirely different beast (IMO) than "flush
> kernel buffers to disk and hope for the best".
>

AFAIK, BIO_FLUSH should complete when data was written to disk (not before),
or in the case of battery backed cache: when data was written to battery
backed cache.
AFAIK, ZFS doesn't only "flush kernel buffers" (like UFS does), but also
executes BIO_FLUSH every X seconds (X=5 if my memory serves me well). So,
you shouldn't loose data that was written before BIO_FLUSH executed.

   With single disks, all I've seen are read/write errors which can't be

> repaired.  "zpool status" will actually show what files got affected as
> a result of the issue, though sometimes "zpool scrub" needs to be run
> before this can be detected.


Adding redudancy does make your data safer, but if everything is working as
explained, sudden power-loss shouldn't be the cause of those errors even
with single disks. Correct me if you think that I'm mistaken.