Hello,
I am using XFS on LVM2 and I have a problem creating snapshots.
I will only be mounting the snapshot read-only, and when the XFS
filesystem is frozen, the log is flushed (is this true?), so I believe I
should not need to snapshot the log, only the data device.
Problem: if I try to mount the filesystem without specifying a logdev=
option, even with ro,norecovery, mount complains with error 990.
So I created a dummy log filled with 0's to use when mounting read-only
snapshots. Strange thing: if the dummy log device is read only, mount
complains, even though I am specifying the ro option. So I had to make
the dummy log device writable--but I note that XFS never actually writes
anything to it.
This approach sort of works, but if files are being modified when the
snapshot is made, they can be corrupt in the snapshot. So I guess the
filesystem is not being frozen properly, or else freezing does not
completely flush the log, and so I would need to snapshot the log too
(but that seems silly).
Anyway, I cannot snapshot the log, because there is no way to atomically
freeze the filesystem, create two snapshots (log and data device) and
then unfreeze. I tried using xfs_freeze to do this, but then lvcreate
hangs, as described elsewhere on linux-lvm.
Can anyone shed some light on these problems? Any suggestions will be
much appreciated.
For reference, I am using kernel 2.6.12.5 on AMD64, device-mapper
1.01.04. and LVM2 2.01.09, which I believe are the latest stable
versions. I am about to try kernel 2.6.13.
The commands I use to create and mount the snapshot are, for example:
mkdir -p /export/home_backup
lvcreate -L4G -s -pr -nhome_backup /dev/vg0/home
mount -t xfs -o nouuid,ro,norecovery,logdev=/dev/log0/dummy_log /dev/vg0/home_backup /export/home_backup
The original filesystem that I am trying to make a snapshot of is
mounted like this:
mount -t xfs -o logbufs=8,logdev=/dev/log0/homelog /dev/vg0/home /export/home
Further information:
# lvdisplay /dev/vg0/home
--- Logical volume ---
LV Name /dev/vg0/home
VG Name vg0
LV UUID hegvc2-MIwz-iSmN-m2af-LObQ-pfVp-A3UlWy
LV Write Access read/write
LV Status available
# open 1
LV Size 32.00 GB
Current LE 8192
Segments 2
Allocation inherit
Read ahead sectors 0
Block device 253:0
# xfs_info /export/home
meta-data=/export/home isize=256 agcount=18, agsize=491520
blks
= sectsz=512
data = bsize=4096 blocks=8388608, imaxpct=25
= sunit=0 swidth=0 blks, unwritten=1
naming =version 2 bsize=4096
log =external bsize=4096 blocks=32768, version=1
= sectsz=512 sunit=0 blks
realtime =none extsz=65536 blocks=0, rtextents=0
# vgdisplay
--- Volume group ---
VG Name vg0
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 124
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 5
Open LV 4
Max PV 0
Cur PV 1
Act PV 1
VG Size 223.52 GB
PE Size 4.00 MB
Total PE 57221
Alloc PE / Size 29440 / 115.00 GB
Free PE / Size 27781 / 108.52 GB
VG UUID N0Vkgh-i7Zn-ohXQ-M64S-GQxa-bncH-JWPxGG
--- Volume group ---
VG Name log0
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 15
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 4
Open LV 3
Max PV 0
Cur PV 1
Act PV 1
VG Size 16.85 GB
PE Size 4.00 MB
Total PE 4314
Alloc PE / Size 128 / 512.00 MB
Free PE / Size 4186 / 16.35 GB
VG UUID vDyLlP-dwfW-9Nj5-dmHh-Ya7O-zUAo-81rUf9
# pvdisplay
--- Physical volume ---
PV Name /dev/md20
VG Name vg0
PV Size 223.52 GB / not usable 0
Allocatable yes
PE Size (KByte) 4096
Total PE 57221
Free PE 27781
Allocated PE 29440
PV UUID f47Jdk-arEl-RpRL-MI7f-E6H4-sVSL-xVK4CC
--- Physical volume ---
PV Name /dev/md11
VG Name log0
PV Size 16.85 GB / not usable 0
Allocatable yes
PE Size (KByte) 4096
Total PE 4314
Free PE 4186
Allocated PE 128
PV UUID pFQgKc-RrM1-M5Ui-ChnC-t3Md-Qo0E-nqu9oW
--
Mario Becroft <mb@gem.win.co.nz>
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