[stgt] Write-cache in tgtd during a target-host crash

FUJITA Tomonori fujita.tomonori at lab.ntt.co.jp
Tue Apr 20 10:04:14 CEST 2010

On Wed, 14 Apr 2010 09:35:38 +0100
Chris Webb <chris at arachsys.com> wrote:

> Hi. I have a linux cluster using tgtd as a target and openiscsi as an
> initiator. At the moment, before export, I do something like
>   tgtadm -L iscsi -m logicalunit -o update -t $TID -l 1 -P mode_page=8:0:18:0x10:0:0xff:0xff:0:0:0xff:0xff:0xff:0xff:0x80:0x14:0:0:0:0:0:0
> to disable writeback caching on the target host. I'm wondering about
> dropping this, but I don't properly understand how iscsi handles
> write-caching at the target and therefore whether this is safe or not.
> If the target host crashes or suffers a power failure and is then restarted,
> the initiator will continue where it left off, but data in the target's
> writeback cache (presumably just page cache?) will have been lost.
> Will the initiator have retained the data that hadn't reached disk and
> understand that it needs to resend, or will the volume end up corrupted with
> the initiator's page cache not matching the real content on the disk?

I think that it depends on what you run on the initiator. For example,
when many file systems (such as ext3) hits a nexus loss (the target
crashes), makes the disk offline. Then the page cache on the initiator
will be lost.

Note that data corruption is different from data loss.
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