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

Salyzyn, Mark
2005-01-06T14:23:20Z

James Bottomley sez:
> 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.
> However, if you have actual proposals, lets see them.

Proposal would be difficult and illegal for me; I am bound in my
engineering contract to a proprietary world :-( I can just not clean my
room well enough.

However, you would see a similar problem being generated by a need for
quick and hot system failover and cache synchronization, bubbling all
the way up to the Linux disk cache; not just at the physical DASD level.
If the file-system is immune to a deferred failure from cached systems
(disk, RAID or linux cache), it will be hardened to deal with component
to complete system failover.

Brute force, with performance penalty, is to periodically flush cache
(SCSI_SYNC command 0x35 being the weapon of choice), and when that
completes the Journal is cleansed. Added (performance) value, is to
survive a failover or deferred failure without the need to do this
synchronization.

(Note: Adaptec's branch of the aacraid driver supports SCSI_SYNC)

Sincerely -- Mark Salyzyn