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Date: Thu, 25 Oct 2012 17:20:42 +0200
From: Milan Broz <gmazyland@gmail.com>
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Subject: Re: [dm-crypt] Toggle read-only status of mapping
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To: Dennis Birkholz <dm-crypt@dennis.birkholz.biz>
Cc: dm-crypt@saout.de

On 10/25/2012 04:50 PM, Dennis Birkholz wrote:
> Am 25.10.2012 12:15, schrieb Milan Broz:

> The reason is relatively easy: if my raid is read-only, it can crash and
> I can reassemble it without a problem, but I cannot make it read-only if
> the crypt-mapping is read-write.

If you raid is mapped read-write, and there is a device failure, it either
continue to be read-write (in degraded mode, that's exactly what "redundant"
in RAID means) or the array is lost completely (then you must deactivate
mapped device anyway).

And raid allows resync while read-write so I really do not see real
use for this switch. 
(And even read-only fs mount sometime means filesystem journal reply,
specifically after fs crash.)

Anyway, it is easily to be done with dmsetup:

  NAME=<your crypt device name>

  # load identical but read-only table into inactive slot
  dmsetup reload $NAME --readonly --table "$(dmsetup table --showkeys $NAME)"
  # switch tables
  dmsetup resume $NAME

Milan