On 06/11/2014 04:24 PM, James Bottomley wrote:
On Thu, 2014-06-05 at 09:26 +0200, Hannes Reinecke wrote:
REPORT_LUN_SCAN does not report any outstanding unit attention
condition as per SAM. However, the target might not be fully
initialized at that time, so we might end up getting a
default entry (or even a partially filled one).
But as we're not able to process the REPORT LUN DATA HAS CHANGED
unit attention correctly we'll be missing out some LUNs during
startup.
So it's better to send a TEST UNIT READY for modern implementations
and wait until the unit attention condition goes away.
Are you sure this is a good idea: we just spent ages tuning SCSI init so
we don't slow systems down. This patch, in the event the array is
having a power on problem, takes us right back to waiting for init
again ... basically the busy wait in scsi_test_lun.
Since the array should send us a UA anyway when it's got itself sorted
out, what's wrong with just processing the report luns data has changed
condition?
Because we can't.
_If_ we were attempting this we'd run into several issues:
a) Boot will fail, as REPORT LUNs will return 0 LUNs (or just LUN 0).
So the scanning code will assume everything's fine. Booting will
continue, only to figure out that no LUNs are present.
As there is _no_ indication that REPORT LUNs should indeed have
returned an error (only it can't due to SAM) we wouldn't even
now that there _is_ an issue.
(In fact, that's what triggered the patchset in the first place.)
b) Even _if_ we're able so somehow recover from that we will have
to rescan the host and any attached devices.
The only way to do this currently is to _remove_ all devices
from that host and then do a full rescan.
Trying this with any devices which are already part of some
complex setup will become ... interesting.
So the easy way out here is indeed just to send a TEST UNIT READY.
And as we're checking for a reasonably SCSI compliance we should
be catching most of the oddballs.
Cheers,
Hannes
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