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Date: Fri, 29 Aug 2008 22:31:50 +0200
From: Milan Broz <mbroz@redhat.com>
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Subject: Re: [linux-lvm] 15 snapshot limit?
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To: LVM general discussion and development <linux-lvm@redhat.com>

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@redhat.com