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: