Hi,
This is a digest of a problem in LVM snapshot which seems to have its roots in dm. I placed it on the LVM list already; here is a copy for dm-devel, just in case anybody has seen this a thousand times before... Our customer is not happy with a 15-snapshot limit.
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 - 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).
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
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.)
Thanks in advance,
Larry Dickson
Cutting Edge Networked Storage
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