Re: fibre channel sync cache question

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

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
-=====-=-==- -=== ==-=-
http://arcgraph.de/sr/