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.

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