On Tue, 2008-04-22 at 12:12 -0400, Brian J. Murrell wrote: > On Tue, 2008-04-22 at 11:47 +0100, Stephane Chazelas wrote: > > > > but > > when you write to it, the system makes sure that unless it has > > been done already, the block you're modifying are being copied > > first from the "real" volume to COW volume before being modified > > in the "real" volume. > > And to be clear, the COW volumes of _all_ snapshots[1]. This is where > the snapshot scaling problem arises. Could someone say a bit more, because I definitely don't follow this. If the [1] after snaphots means something, I don't know what. I've seen references to this scaling problem before, and have never quite gotten it. The quoted paragraph above sounds as if it means whenever there is a write to any volume, the COW tables for all snapshots get the contents that are about to be overwritten. I have multiple snapshots active, and it doesn't seem to work this way. Some volumes have big snapshot volumes, into which substantial material is written. Others have small snapshot volumes, into which very little is written. In particular, the little ones don't seem to be getting the writes to the big ones. I judge how much of the COW is in use from the output of lvdisplay (attached at bottom) If all COW volumes get the overwritten material, then they'd all have the same size in use. And they don't seem to. As I said, I'm missing something. Could someone enlighten me? Ross Boylan lvdisplay for big volume shows LV Name /dev/daisy/_var_spool_cyrus VG Name daisy LV UUID vUs6Bj-2DRx-4OVt-7FBr-K2dW-3gTV-AHpNRZ LV Write Access read/write LV snapshot status active destination for /dev/daisy/cyrspool LV Status available # open 0 LV Size 23.62 GB Current LE 756 COW-table size 12.00 GB COW-table LE 384 Allocated to snapshot 29.78% Snapshot chunk size 4.00 KB Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 254:41 and for the little one it shows LV Name /dev/daisy/_var_lib_cyrus VG Name daisy LV UUID qkcv3k-XBYW-2gcx-pjbM-oF0g-Mlfk-P28QcH LV Write Access read/write LV snapshot status active destination for /dev/daisy/cyrlib LV Status available # open 0 LV Size 96.00 MB Current LE 3 COW-table size 128.00 MB COW-table LE 4 Allocated to snapshot 33.32% Snapshot chunk size 4.00 KB Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 254:38 I think this means the first one used 3.6G=12G*.30 while the second used 42.7MB = 128MB*.33. The amount written to the first one exceeds the total size available (128MB) for the 2nd snapshot. I'm running Linux kernel 2.6.18 as packaged for Debian. _______________________________________________ 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/
