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

[Chris Webb]

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?

Best wishes,

Chris.
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