On Friday, 19 May 2000 at 12:43:28 -0700, Matthew Dillon wrote:
>
>> Greg Lehey wrote:
>>>
>>> As far as soft updates goes, basically it's incompatible with Vinum,
>>> since there's currently no way of ensuring the sequence of writes
>>> across a number of disks. I'm thinking of ways of doing it, but they
>>> will cause significant loss in performance. There should be no
>>> problems as long as there isn't a crash, of course :-)
>>
>> Do you mean that softupdates is entirely incompatible with Vinum, or
>> just incompatible with Vinum's RAID5?
>
> Wait a sec... softupdates does not depend on write ordering.
> Softupdates issues all non-conflicting writes in parallel and
> doesn't care what order they are written to the disk. When
> those complete, softupdates will then followup with all the
> writes that depend on the original set.
Hmm. Maybe I've misunderstood, then. It was my understanding that
soft updates writes metadata with WRITE_ORDERED set, and thus avoids
having to explicitly wait for completion. It's the WRITE_ORDERED that
Vinum doesn't handle correctly. On a single disk, it's just a
question of not sorting around a WRITE_ORDERED request. Vinum will
keep the WRITE_ORDERED on a single disk, but it won't ensure that a
request for the same volume, but which is destined for a different
disk, will not be written until after all components of a prior
WRITE_ORDERED request for that volume.
Greg
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