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Re: [dm-devel] LVM Snapshot Feature



On Mon, Aug 4, 2008 at 4:15 PM, Mikulas Patocka <mpatocka redhat com> wrote:
> On Fri, 1 Aug 2008, Adam Hamsik wrote:
>
>> On Aug,Friday 1 2008, at 9:35 AM, Bombardier, Pascal wrote:
>>
>> > Hi,
>> >
>> > In your opinion, does it make sense to be able to create non-persistent
>> > snapshots ?
>> > In the case of a filesystem backup, a non-persistent snapshot would be a
>> > snapshot that we agree to lose in case of failure or reboot. If anything
>> > happens, a server administrator would have to recreate a new snapshot
>> > and then restart the backup.
>> > In this case, I think metadata could be stored in memory instead of on
>> > disk, which should improve LVM snapshot performance.
>> >
>> > Is this theory wrong, and is this something which can be implemented
>> > easily in the current LVM code ?
>> >
>>
>> AFAIK non-persistent snapshots are already implemented see
>> DM_PERSISTENT_DEV_FLAG ? I haven't look at linux kernel code how is
>> it done but when I solve my lvcreate -s bug on NetBSD non-persistent
>> snapshots are next :).
>>
>> Regards
>>
>> Adam.
>
> Yes. They are implemented in device mapper in kernel --- but no userspace
> lvm tool can use them. You can only create them using dmsetup.
>
> Mikulas


Do you have list of commands which I have to run to create persistent dm
snapshot with dmsetup ? It would be handy to test my driver with dmsetup
first and only after that use lvm2tools to create persistent/non-persistent
snapshots.

Regards

Adam.


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