RE: [linux-iscsi-devel] [question] deferred sense

Bryan Henderson
2005-01-06T23:11:21Z

>> Change the assumptions in Journalling filesystems and make it work in a
>> deferred sense environment.
>
>It's possible, but I think it will involve large changes to the kernel.
>
>> Don't delete the Journal until a safe point of time when the writeback
>> is confirmed either by underlying knowledge, time (flush interval
>> passed), amount of data (dirty cache is forced flushed), or a read-back
>> (resulting from the deferred error investigation). RAID cards and
>> external enclosures do this all the time, but to be fair with usually
>> more knowledge of all the cache handling.
>
>Well ... this proposal would change every filesystem in the kernel for
>something that occurs rather rarely (I've never actually seen it
>happen), so it is rather a lot of work for not very much benefit.

It's especially little benefit because there's no reason for the SCSI
target to be in write back mode. Caching and I/O scheduling within Linux
and SCSI command queueing do pretty much everything write back caching in
the target is intended for. So I think the right solution for this
problem (which I agree is a major problem) is for journalled filesystems
to keep the disk in non-writeback mode.

Don't you usually have to go out of your way to get a disk drive to do
writeback?

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems