snapshot has post-snapshot files
Yesterday (04 Dec) at 23:17 I started a backup job which creates snapshots as its first step. The backup is still running.
The snapshot appears to have files that changed after it was created, e.g.,
corn:/var/spool/cyrus/mail/r/user/ross# ls -l debian/user/cyrus.squat -rw------- 1 cyrus mail 1137271795 2007-08-05 03:30 debian/user/cyrus.squat # ls -l /snap/var/spool/cyrus/mail/r/user/ross/debian/user/cyrus.squat -rw------- 1 cyrus mail 1137271795 2007-08-05 03:30 /snap/var/spool/cyrus/mail/r/user/ross/debian/user/cyrus.squat while mount shows /dev/mapper/daisy-snapspool on /snap/var/spool/cyrus type ext3 (rw) /dev/mapper/daisy-snaplib on /snap/var/lib/cyrus type ext3 (rw)
There are 2 mysteries: why is the newer file on the snapshot, and why is the snapshot working at all? That file alone is about the size I allocated for the snapshot, and there are other large files like it. However,
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# date; lvdisplay daisy/snapspool
Wed Dec 5 14:01:35 PST 2007
--- Logical volume ---
LV Name /dev/daisy/snapspool VG Name daisy LV UUID k0AK2H-dCCX-xu30-aVSk-At3g-21n6-eDZgdn LV Write Access read/write LV snapshot status active destination for /dev/daisy/cyrspool LV Status available # open 1 LV Size 19.62 GB Current LE 628 COW-table size 1.00 GB COW-table LE 32 Allocated to snapshot 42.25% Snapshot chunk size 8.00 KB Segments 1 Allocation inherit Read ahead sectors 0 Block device 254:23
# vgdisplay daisy --- Volume group --- VG Name daisy System ID Format lvm2 Metadata Areas 2 Metadata Sequence No 17 VG Access read/write VG Status resizable MAX LV 0 Cur LV 11 Open LV 8 Max PV 0 Cur PV 2 Act PV 2 VG Size 164.84 GB PE Size 32.00 MB Total PE 5275 Alloc PE / Size 3891 / 121.59 GB Free PE / Size 1384 / 43.25 GB VG UUID z2Dbsr-Ywb0-J56t-BMDT-cfxH-7cum-Fodpe9 -------------------------------------------------------------------
I running Debian GNU/Linux with a Debian stock 2.6.18-5-686 kernel (2.6.22-3 is now current, so I suppose there could be some version mismatch issues). The volumes were created under EVMS and recently converted to vanilla LVM. There are some other LVM1 VG's on the system.
I forgot when I created the snapshot that the *squat files would be regenerated overnight; they are index files. Relatively little of the material in them changes from day to day. My assumption is that the snapshot needs a copy of the whole file if anything changes; is that correct? If it's smarter than that, it might explain why I have not blown through the space I allocated, though not why the snapshot isn't frozen.
The script, ran as root, at the start of the backup, is
BTW, is sync necessary before snapshot, or is it automatic?
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#! /bin/sh
echo "$0 invoked with argument $1"
/usr/sbin/invoke-rc.d cyrus2.2 stop
# ctl_mboxlist must run as cyrus # but su says it needs a terminal # sudo does not gives a password prompt if run as root sudo -u cyrus /usr/sbin/ctl_mboxlist -d > /var/lib/cyrus/mailboxlist.txt
chown cyrus /var/lib/cyrus/mailboxlist.txt
sync # may be redundant with snapshot
# create snapshots lvcreate -L1G -s -n snapspool /dev/daisy/cyrspool lvcreate -L100M -s -n snaplib /dev/daisy/cyrlib
# resume operation /usr/sbin/invoke-rc.d cyrus2.2 start
# mount snapshots mkdir -p /snap/var/spool/cyrus mkdir -p /snap/var/lib/cyrus mount /dev/daisy/snapspool /snap/var/spool/cyrus mount /dev/daisy/snaplib /snap/var/lib/cyrus
# I should be more careful about passing failure codes back
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It might also be relevant that the origin of the snapshot is mounted noatime for an ext3 filesystem:/dev/mapper/daisy-cyrlib on /var/lib/cyrus type ext3 (rw) /dev/mapper/daisy-cyrspool on /var/spool/cyrus type ext3 (rw,noatime)
I'm guessing I should have mounted the snapshots ro, but I haven't been writing to them.
Ross