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Re: [dm-devel] Status of SCSI referrals support in dm-multipath



On 06/01/2014 10:46 PM, Sebastian Herbszt wrote:
Hello,

I am trying to find out the status of SCSI referrals support in dm-multipath.
SCSI referrals support was a topic at LSF [1] but I wasn't able to find any
information. Back in 2010 Hannes suggested the use of multiple entries in a
multipath table [2] but this doesn't seem to work (anymore):

[ 7750.354190] device-mapper: table: Request-based dm doesn't support multiple targets yet

Is there any SCSI referrals support yet?

[1] http://marc.info/?l=linux-scsi&m=129553880112460&w=2
[2] http://www.redhat.com/archives/dm-devel/2010-June/msg00155.html

Not as of now. I've managed to push referrals support into target_core, so you now can set up a backend which supports referrals.

And for the multiple entries I've though to use a linear target on top of the multipath target. The linear target would just split up the device in chunks, according to the referrals layout. And then you would have several multipath devices (all referring to the same disk), one for each supported referrals ALUA configuration. The linear target would then map each chunk to the correct multipath device.

It'll be a beast for failover, but should be possible.
And maybe it's better to use dm-switch here; the number or referrals might overflow dm-linear ...

Cheers,

Hannes

--
Dr. Hannes Reinecke		      zSeries & Storage
hare suse de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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