[linux-lvm] device-mapper: snapshots: Invalidating snapshot: Unable to allocate exception.

[Tomas Dalebjork]

Many thanks Zdenek ;)

This confirms our theory of what causes the problem for us.

Best regards Tomas

2015-03-16 13:13 GMT+01:00 Zdenek Kabelac <zkabelac at redhat.com>:

> Dne 15.3.2015 v 14:03 Tomas Dalebjork napsal(a):
>
>  Hi,
>>
>> We have configured /etc/lvm/lvm.conf to extend our snapshot automatically.
>> This works fine for slow growing volumes, but if the growth of the volume
>> is
>> faster, than LVM invalidate the snapshot rather than extend the snapshot
>> volume.
>> Because I think that the lvm-monitor does not grow the volume in time.
>>
>> This is the error we see in kernel log (dmesg)
>> device-mapper: snapshots: Invalidating snapshot: Unable to allocate
>> exception.
>>
>> Our snapshot volume is 1GB
>> Our extension is configured like this in /etc/lvm/lvm.conf
>>
>>      snapshot_autoextend_threshold = 50
>>      snapshot_autoextend_percent = 20
>>
>> And we have restarted lvm2-monitor services after this changes using
>> # service lvm2-monitor restart
>>
>> Other slow growing volumes works fine.
>>
>> Can someone hint us of what the issue can be, or how to mitigate this to
>> happen?
>> Or tell me how often the "lvm2-monitor" is checking the utilization
>> threashold
>> of the volumes?
>>
>
>
> It's a known issue - the advice is to switch to thin provisioning were
> there is no such problem.
>
> 'dmeventd' is not fast enought for old snapshots.
>
> If you want to use old snaps - you need to have reserve for  10 x
> speed_of_your_drive.
>
> So i.e. if you want to be sure you will not get invalid snapshot and
> you could push  0.5G/s through your SSD - you simply need 5GB reserve.
>
> Back to your example - for 1GB origin you need to provide 100% size for
> snapshot otherwise you are in danger of getting invalid snapshot when you
> rapidly write to your origin.
>
> There is no other advice then to switch to thinly provisioned devices for
> such case.
>
>
> Regards
>
> Zdenek
>
>
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