[dm-devel] [PATCH] [RFC] switch DM tables to readonly automatically

[Hannes Reinecke]

Hi Christophe,

Christophe Varoqui wrote:
>> Hi all,
>>
>> this patch switches the device-mapper table to read-only status automatically if one underlying device returns -EROFS.
>>
>> Rationale:
>> Whenever a SCSI device is switched to read-only a table reload from multipath-tools fails, without any indication about the reason. And it's actually quite tricky to detect the read-only status from userland. And quite pointless, too, as the kernel already knows about it.
>>
>> And we now can create tables for CD-ROMs, too, without having to use the '-r' flag to dmsetup ...
>>
>> Christophe, this might also fix your problem.
>>
> It seems it will solve the map creation error on read-only LU, but what about these LU becoming writable ? ...
> as is the case with the Symmetrix R2 upon spliting the synchronisation link.
> Will the devmap become writable too automagically or is the multipathd daemon expected to take action the promote the map RW ?
> 
Tricky business. We'll first have to be notified that the LU is becoming writeable. The current SCSI stack is not
very good at providing that sort of information.

I would think the best way here would be to modify the 'tur' checker to check for the READ-ONLY state, too.
That would mean we'll have to implement a new path state 'READONLY', but that should be okay methinks.
But this would have to be done at the SCSI level as the current 'sd' driver is not capable of switching
between read-only and read-write on the fly, either.

Maybe I'll give it a go if I find some time.

Cheers,

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