Michael Reed wrote:
-- Tuesday, July 25, 2006 4:22 PM, Michael Reed wrote:
Using fibre channel disks, I've noticed that when the system shuts down
that the sd_driver issues a sync cache command to the device. I've
also
noticed that when the lldd is removed via rmmod that this sync cache is
not executed. I would think that the sync cache would be desirable
under this circumstance.
[...]
I'm wondering about the policy of issuing a sync cache. There
are target removal paths which result in it not being issued.
So, the real question is: when a scsi target is removed, is it
policy that sync cache will be issued?
In fibre channel, here are two code paths in which sync cache
is not issued.
- removal of LLDD (rmmod)
- removal of target via sysfs device/delete
[...]
This problem exists with all transports which use scsi_remove_device or
scsi_remove_host for soft device removal, as noted by Mike Christie in
"question about sd_sync_cache and shutdown" on 2006-07-12. He pointed
out that devices are brought into SDEV_CANCEL state before sd_shutdown
is entered. Therefore sd_shutdown does not sync the cache.
This behavior changed a few Linux releases ago. I cannot pinpoint the
responsible changeset. So, to answer your question about policy, there
is no fixed policy in the specification and implementation of the
mid-low SCSI API. Generally, neither "soft shutdown" nor "hot unplug"
can be explicitly signaled from LLD to the SCSI core.
Unless somebody comes up with a general solution for SCSI core, I am
planning to look for a particular solution in the sbp2 transport driver.
(When an sbp2 LU is to be taken down, check if it is still physically
present and somehow trigger SCSI high-level shutdown methods before
scsi_remove_device.)
--
Stefan Richter
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