[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [linux-lvm] 15 snapshot limit?



More details: I found a /var/log/messages note that corresponded exactly to the following bug:
==========================

http://www.linux-archive.org/debian-kernel/7067-bug-453254-linux-2-6-18-5-xen-686-unable-create-more-than-32-lvm-snapshots-total.html

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
Package: linux-2.6.18-5-xen-686
Severity: normal


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).

Larry

On 9/2/08, Larry Dickson <ldickson 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.
 
Larry


On 8/29/08, Larry Dickson <ldickson 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.
 
Larry
 
On 8/29/08, Milan Broz <mbroz 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 redhat com


_______________________________________________
linux-lvm mailing list
linux-lvm redhat com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/




[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]