Hi Mike,

On Mon, 2008-06-30 at 11:27 -0500, Mike Christie wrote:
> Konrad Rzeszutek wrote:
> >> Ah if your disk are using write back cache then you are going to hit 
> >> some problems. So if you see this in /var/log/messages when you loging:
> >>
> >> kernel: sd 9:0:0:1: [sdb] Write cache: enabled,
> >>
> >> then later when you run iscsiadm to log out you see:
> >>
> >> kernel: sd 9:0:0:1: [sdb] Synchronizing SCSI cache
> >>
> >> Then you are going to hit problems due to the scsi sysfs interface 
> >> changing on us. iscsiadm is going to hang. IO is going to hang. You 
> >> basically have to reboot the box by hand.
> > 
> > Mike,
> > 
> > Are you sure about this? When the sysfs entries are deleted (during the 
> > iscsiadm logout phase), the SCSI ml finishes all of the I/Os and the last
> > operation is sending the SCSI Cache command. Wouldn't that quiesce I/O ? 
> > Granted
> See below. You are right if everything goes ok.
> > this means doing these steps which are outside the normal iscsiadm:
> >  1). flush dirty pages (call 'sync')
> >  2). delete the sysfs entries (echo 1 > /sys/block/sdX/device/delete)
> >  3). wait until /sys/class/scsi_host/hostZ/host_busy reaches zero
> >  4). iscsiadm -m logout
> >
> The problem that I described occurs when we run the iscsiadm logout 
> command and we used the sysfs delete file. When iscsiadm wrote to that 
> attr in 2.6.18 it would return when the cache sync was sent and the 
> device was fully deleted in the kernel. In 2.6.21 and above it returned 
> right away. So iscsiadm's logout code would write to that attr and think 
> the devices were cleaned up, then it would call the iscsi shutdown code 
> which would send a logout and cleanup the kernel session, connection and 
> host structs, thinking that the devices were properly flushed but IO 
> could still be waiting to get written so all types of fun things can 
> happen like....


> We could get to the scsi host remove call and all the scsi device sysfs 
> delete calls would still be starting up, so the host remove call and 
> those delete calls would race (so this is we would have bypassed the 
> host_busy check in the connection deletion function in the kernel). When 
> this happens if the sysfs delete device got the scan mutex first, but 
> the iscsi shutdown code had blocked the devices, while we were trying to 
> remove the host then the iscsiadm logout command will hang, because the 
> delete device would wait forever to try and send the command (it is not 
> yet in the host so the command timer is not running and the device is 
> blocked), and the remove host call is waiting on the scan mutex which 
> the device has.
> If you have multiple devices then the remove host command can also end 
> up failing IO, because we will have sent the logout and later set the 
> session internal state to terminiate and incoming IO on the other 
> devices that was queued will be failed when the remove devices functions 
> flush the IO.
> If you do not have a write back cache we have other problems, where IO 
> can be failed when it should not have for the reason above where the 
> logout is sent, the terminate bit is set, and the remove host runs 
> before the devices were properly removed and that causes IO to be failed.
> And actually in some kernels you can hang (the app would hang not 
> iscsiadm in this case) when a cache sync was not needed, because if a 
> cache sync was not needed when we would remove the host and it would 
> delete the device but IO would be stuck in the queues and no one did a 
> unplug of the queue when the scsi device was removed. We added a 
> iscsi_unblock_session in the iscsi_session_teardown to flush everything 
> so at least apps would not hang there (but that resulted in IO getting 
> failed like above).

Ok, so with Open/iSCSI 2.0-869.2 running on w/ LIO-Core IBLOCK
+ DRBD with write cache disabled, we are no longer seeing hanging
umounts during the rsc_stop() section of ISCSI_MOUNT VHACS cluster
resource.  Thanks for the helpful info mnc!! :-)

Things have started to look up with the latest kernel + open/iscsi
userspace code.  The only item left that I have been running into on
Etch are strange sshd segfaults (requring a service restart) during
ISCSI_MOUNT:rsc_stop.  They seem to be somehow be Open/iSCSI related
(not exactly sure how yet) and happens on multiple machines, from both
myself and Jerome's clusters.  We have been trying to track this issue
down with little luck, but with the new code it definately seems to be
happening alot less frequently on my setup (only once so far with the
latest Open/iSCSI).  Pretty vague I know.. :)

Oh yeah, jerome also mentioned something to me this morning about
left-over iscsid processes after ISCSI_MOUNT:rsc_stop() as well.  What
was the problem case on this one again..?


> > 

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