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From: Zdenek Kabelac <zkabelac@redhat.com>
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Date: Wed, 26 Apr 2017 09:42:46 +0200
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Subject: Re: [linux-lvm] Snapshot behavior on classic LVM vs ThinLVM
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To: Gionatan Danti <g.danti@assyoma.it>, LVM general discussion and development <linux-lvm@redhat.com>

Dne 26.4.2017 v 09:26 Gionatan Danti napsal(a):
> Il 24-04-2017 23:59 Zdenek Kabelac ha scritto:
>> If you set  '--errorwhenfull y'  - it should instantly fail.
> 
> It's my understanding that "--errorwhenfull y" will instantly fail writes 
> which imply new allocation requests, but writes to already-allocated space 
> will be completed.

yes you understand it properly.

> 
> It is possible, without messing directly with device mapper (via dmsetup), to 
> configure a strict "read-only" policy, where *all* writes (both to allocated 
> or not allocated space) will fail?

Nope it's not.

> 
> It is not possible to do via lvm tools, what/how device-mapper target should 
> be used?

At this moment it's not possible.
I do have some plans/idea how to workaround this in user-space but it's 
non-trivial - especially on recovery path.

It would be possible to 'reroute' thin to dm-delay and then write path to 
error and read path leave as is - but it's adding many new states to handle,
to ATM it's in queue...

Using 'ext4' with remount-ro is fairly easy to setup and get exactly this
logic.