[linux-lvm] 15 snapshot limit?
ldickson at cuttedge.com
Tue Sep 2 22:19:53 UTC 2008
More details: I found a /var/log/messages note that corresponded exactly to
the following bug:
11-28-2007, 01:34 AM
Peter de Zwart
Default Bug#453254: linux-2.6.18-5-xen-686: Unable to create more than 32
LVM snapshots in total
When attempting to create more than 32 LVM snapshots, the following error
message is sent from the Kernel:
device-mapper: table: 254:142: snapshot: Could not create kcopyd client
device-mapper: ioctl: error adding target to table
(End of reference; no follow-up found.) Except that in my case 15 is the max
snapshot count, not 32. Increasing memory from 4GB to 6GB does not help. I
notice mentions of this "Could not create kcopyd client" bug on the web all
the way back to 2004. Does anybody know any workaround? Our customer is not
happy about a 15-snapshot limit. Versions are (variants of) lvm2-2.02.38,
device-mapper-1.02.26, kernel-smp-2.6.20 (64 bit).
On 9/2/08, Larry Dickson <ldickson at cuttedge.com> wrote:
> Sorry, I did not make that last question clear:
> What (if any) is the formula by which I can predict the number of snapshots
> that are possible, given a certain size of the parent volume and/or
> snapshot and a certain amount of available memory? Milan implied there was
> such a limit, and I seem to remember it, but I can't seem to find it spelled
> out anywhere.
> On 8/29/08, Larry Dickson <ldickson at cuttedge.com> wrote:
>> 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.
>> 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
>>> > 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).
>>> 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
>>> (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).
>>> mbroz at redhat.com
>>> linux-lvm mailing list
>>> linux-lvm at redhat.com
>>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
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