If I understand correctly, ZFS now adays will only flush data to
non volatile storage (such as a RAID controller NVRAM), and not
all the way out to disks. (To solve performance problems with some
storage systems, and I believe that it also is the right thing
to do under normal circumstances.)
Doesn't this mean that if you enable write back, and you have
a single, non-mirrored raid-controller, and your raid controller
dies on you so that you loose the contents of the nvram, you have
a potentially corrupt file system?
ZFS requires,that all writes be flushed to non-volatile storage.
This is needed for both transaction group (txg) commits to ensure pool integrity
and for the ZIL to satisfy the synchronous requirement of fsync/O_DSYNC etc.
If the caches weren't flushed then it would indeed be quicker but the pool
would be susceptible to corruption. Sadly some hardware doesn't honour
cache flushes and this can cause corruption.
Neil.
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