Stefan Bader wrote:

2007/5/30, Phillip Susi [off-list ref]:

quoted

Stefan Bader wrote:

quoted

Since drive a supports barrier request we don't get -EOPNOTSUPP but
the request with block y might get written before block x since the
disk are independent. I guess the chances of this are quite low since
at some point a barrier request will also hit drive b but for the time
being it might be better to indicate -EOPNOTSUPP right from
device-mapper.

The device mapper needs to ensure that ALL underlying devices get a
barrier request when one comes down from above, even if it has to
construct zero length barriers to send to most of them.

And somehow also make sure all of the barriers have been processed
before returning the barrier that came in. Plus it would have to queue
all mapping requests until the barrier is done (if strictly acting
according to barrier.txt).

But I am wondering a bit whether the requirements to barriers are
really that tight as described in Tejun's document (barrier request is
only started if everything before is safe, the barrier itself isn't
returned until it is safe, too, and all requests after the barrier
aren't started before the barrier is done). Is it really necessary to
defer any further requests until the barrier has been written to save
storage? Or would it be sufficient to guarantee that, if a barrier
request returns, everything up to (including the barrier) is on safe
storage?

Well, what's described in barrier.txt is the current implemented
semantics and what filesystems expect, so we can't change it underneath
them but we definitely can introduce new more relaxed variants, but one
thing we should bear in mind is that harddisks don't have humongous
caches or very smart controller / instruction set.  No matter how
relaxed interface the block layer provides, in the end, it just has to
issue whole-sale FLUSH CACHE on the device to guarantee data ordering on
the media.

IMHO, we can do better by paying more attention to how we do things in
the request queue which can be deeper and more intelligent than the
device queue.

Thanks.

-- 
tejun