On Wed, 2010-06-30 at 22:28 +0200, Ragnar Sundblad wrote:
> To be safe, the protocol needs to be able to discover that the devices
> (host or disk) has been disconnected and reconnected or has been reset
> and that either parts assumptions about the state of the other has to
> be invalidated.
>
> I don't know enough about either SAS or SATA to say if they guarantee that
> you will be noticed. But if they don't, they aren't safe for cached writes.
Generally, ZFS will only notice a removed disk when it is trying to
write to it -- or when it probes. ZFS does not necessarily get notified
on hot device removal -- certainly not immediately. (I've written some
code so that *will* notice, even if no write ever goes there... that's
the topic of another message.)
The other thing is that disk writes are generally idempotent. So, if a
drive was removed between the time an IO was finished but before the
time the response was returned to the host, it isn't a problem. When
the disk is returned, ZFS should automatically retry the I/O. (In fact,
ZFS automatically retries failed I/O operations several times before
finally "failing".)
The nasty race that occurs is if your system crashes or is powered off
*after* the log has acknowledged the write, but before the bits get
shoved to main pool storage. This is a data loss situation.
But assuming you don't take a system crash or some other fault, I would
guess that removal of a log device and reinsertion would not cause any
problems. (Except for possibly delaying synchronous writes.)
That said, I've not actually *tested* it.
- Garrett
>
> /ragge
>
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