[dm-devel] Could not create kcopyd client

[Larry Dickson]

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:
==========================

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.)

Thanks in advance,

Larry Dickson

Cutting Edge Networked Storage
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