[linux-lvm] 15 snapshot limit?

[Larry Dickson]

You said:

> (But technically I tested >50 snapshot from one LV - should be limited
> only by available memory in recent kernels)

What is that limit? I suspect that is what I have run up against.

Larry

On 8/29/08, Milan Broz <mbroz at redhat.com> wrote:
>
> Larry Dickson wrote:
> > With a 768 GB logical volume in a 2235 GB volume group, and 4 GB of
> > memory, I try to set up lots of snapshots (of 45 or 50 GB each) and
> > can't get past 15. Sometimes I'll succeed in generating more than 15
> > COWs in /dev/mapper, but lvdisplay never shows more than 15 snapshots in
> > the "LV snapshot status source of" listing under the parent volume. I
> > searched and found 15 snapshots mentioned in a couple places like
> > wiki.samba.org/index.php/Shadow_Copies_with_Snapshots
> > <http://wiki.samba.org/index.php/Shadow_Copies_with_Snapshots> - but
> > nothing really authoritative.
> >
> > Is there really such a limit? Or is it a memory requirement - I seem to
> > remember so many MB memory per GB of storage, but can't find that
> > either. Versions are (variants of) lvm2-2.02.38, device-mapper-1.02.26,
> > kernel-smp-2.6.20 (64 bit).
>
> Hi,
> no, there is no hard limit for snapshots.
>
> But IIRC patches which solves some problems related with this (using
> private dm-io
> mempool) were commited in 2.6.22 kernel.
> But anyway, many snapshot from one origin causes serious performance
> degradation.
> (Current snapsot implementation is not ideal in this situation - every
> write to origin
> is replicated N-times to COW snapshot area - iow: every snapshot is
> independent and
> need copy changed area separately.)
>
> (But technically I tested >50 snapshot from one LV - should be limited only
> by available
> memory in recent kernels).
>
> Milan
> --
> mbroz at redhat.com
>
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/linux-lvm/attachments/20080829/e1651dd0/attachment.htm>