Single snapshots do weird things?

linux-lvm Glen Harris 3 participants 7 messages Sep 15, 2003 – Sep 16, 2003

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Glen Harris

author

2003-09-15 17:10 UTC #0 lore raw

Since my last message to the list, when I found I was using an old version of the driver(4.0.2), I've done a completly new install of the kernel.org 2.4.21 sources, and the device-mapper.1.00.05 and LVM2.2.00.06 tarballs. This is still on Debian unstable, but no Debian lvm packages or kernel patches are installed this time.

I've patched the kernel with the device-mapper and VFS patch, compiled and installed both tarballs, and can insmod the dm-mod with no errors.

Now, when I copy 2Mb onto the filesystem and create an 8Mb snapshot, it's immediately flagged INACTIVE, "Allocated to snapshot" is 100% and is not mountable.

I'm using a 256k PE size and a 512Mb LV on a 10Gb disk.

Here's the sequence of commands, with output omitted where the command works normally.

lvm:~# pvcreate /dev/hdb
lvm:~# vgcreate -s 256k vg0 /dev/hdb
lvm:~# vgdisplay vg0
  --- Volume group ---
  VG Name               vg0
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  1
  VG Access             read/write
  VG Status             resizable
  MAX LV                256
  Cur LV                0
  Open LV               0
  Max PV                256
  Cur PV                1
  Act PV                1
  VG Size               9.54 GB
  PE Size               256.00 KB
  Total PE              39092
  Alloc PE / Size       0 / 0
  Free  PE / Size       39092 / 9.54 GB
  VG UUID               w7c4AZ-nmm5-BI4q-#OAm-yvSc-E0eh-#dRhSk

lvm:~# lvcreate -L 512m -n lv0 vg0
lvm:~# lvdisplay /dev/vg0/lv0
  --- Logical volume ---
  LV Name                /dev/vg0/lv0
  VG Name                vg0
  LV UUID                fAfvNg-fBhr-DCWS-Gwm6-RWfX-hHpz-EPi7zn
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                512.00 MB
  Current LE             2048
  Segments               1
  Allocation             next free (default)
  Read ahead sectors     0
  Block device           254:0

lvm:~# mke2fs /dev/vg0/lv0
lvm:~# mount /dev/vg0/lv0 fs
lvm:~# df -k fs
/dev/vg0/lv0            507748        13    481521   1% /root/fs
lvm:~# cp -R /usr/src/device-mapper.1.00.05* fs
lvm:~# sync
lvm:~# df -k fs
/dev/vg0/lv0            507748      2028    479506   1% /root/fs
lvm:~# lvcreate -s -L 8m -p r -n snap0 -d -v /dev/vg0/lv0
    Setting chunksize to 16 sectors.
    Finding volume group "vg0"
    Creating logical volume snap0
    Archiving volume group "vg0" metadata.
    Creating volume group backup "/etc/lvm/backup/vg0"
    Found volume group "vg0"
    Loading vg0-snap0
    Found volume group "vg0"
    Zeroing start of logical volume "snap0"
    Found volume group "vg0"
    Removing vg0-snap0
    Found volume group "vg0"
    Found volume group "vg0"
    Found volume group "vg0"
    Loading vg0-lv0-real
    Loading vg0-snap0-cow
    Loading vg0-snap0
    Loading vg0-lv0
    Creating volume group backup "/etc/lvm/backup/vg0"
  Logical volume "snap0" created
lvm:~# lvdisplay /dev/vg0/snap0
  --- Logical volume ---
  LV Name                /dev/vg0/snap0
  VG Name                vg0
  LV UUID                WL5TXy-gE0H-jQu!-BJod-XPmk-WnFL-VVgLus
  LV Write Access        read only
  LV snapshot status     INACTIVE destination for /dev/vg0/lv0
  LV Status              available
  # open                 0
  LV Size                512.00 MB
  Current LE             2048
  Segments               1
  Snapshot chunk size    8.00 KB
  Allocated to snapshot  100.00%
  Allocation             next free (default)
  Read ahead sectors     0
  Block device           254:4

lvm:~# mount -t ext2 /dev/vg0/snap0 snap/
mount: block device /dev/vg0/snap0 is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/vg0/snap0,
       or too many mounted file systems

My syslog now shows

device-mapper: 4.0.4-ioctl (2003-08-30) initialised: [email protected]
device-mapper: snapshot is marked invalid
EXT2-fs: unable to read superblock

Here's some version numbers:

lvm:~# lvm version
  LVM version:     2.00.06 (2003-08-20)
  Library version: 1.00.05-ioctl-cvs (2003-09-01)
  Driver version:  4.0.4

lvm:~# uname -a
Linux lvm 2.4.21-dm #1 Fri Sep 12 12:52:48 EST 2003 i686 GNU/Linux

lvm:~# gcc -v
Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.4/specs
gcc version 2.95.4 20011002 (Debian prerelease)

lvm:/usr/src# lsmod
Module                  Size  Used by    Not tainted
dm-mod                 40364   7
3c59x                  25096   1
af_packet              11976   1
rtc                     6236   0  (autoclean)
unix                   14020   4  (autoclean)

Thanks, glen.

Alasdair G Kergon

2003-09-15 17:37 UTC #1 lore raw

Quoted 3 lines · Glen Harris

Now, when I copy 2Mb onto the filesystem and create an 8Mb snapshot, it's immediately flagged INACTIVE, "Allocated to snapshot" is 100% and is not mountable.

Thanks for those diagnostics: I'll look into this further tomorrow.

Stick to using read/write snapshots for now. With read-only ones, it seems the COW device is incorrectly getting set read-only.

 # dmsetup info vg0-snap0-cow           
 Name: vg0-snap0-cow
 State:             ACTIVE (READ-ONLY)

Alasdair

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Stephan Budach

2003-09-15 17:38 UTC #2 lore raw

Ich bin vom 15.09.2003 bis einschliesslich 17.11.2003 im Urlaub und nicht erreichbar. Bitte wenden Sie sich in dieser Zeit an meinen Kollegen Jens Jordan, [email protected].

Mit freundlichem Gruss,
Stephan Budach

I am out of office from Sep.,15th until Nov.,17th 2003. In the meantime, please contact Jens Jordan, [email protected]

Yours sincerely,
Stephan Budach

Glen Harris

author

2003-09-15 18:13 UTC #3 lore raw

In reply to Alasdair G Kergon

Hey, that fixed it! I've created 5 snapshots with copying/deleting files in between, and they've all worked!

Well, except for one. I copied a file and took a snapshot and the file wasn't in the snapshot. Running sync and making another snapshot *did* have the file, though.

Is it mandatory to sync the filesystem before taking a snapshot?

Quoted 2 lines · Alasdair G Kergon

Stick to using read/write snapshots for now. With read-only ones, it seems the COW device is incorrectly getting set read-only.

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Alasdair G Kergon

2003-09-16 07:13 UTC #4 lore raw

Quoted 1 line · Glen Harris

Is it mandatory to sync the filesystem before taking a snapshot?

If you want the snapshot to be consistent: that's what the VFS lock patch attempts to do automatically whenever you take a snapshot: presumably you haven't applied that patch?

Alasdair

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Glen Harris

author

2003-09-16 17:26 UTC #5 lore raw

Quoted 10 lines · Alasdair G Kergon

Quoted 1 line · Glen Harris
Is it mandatory to sync the filesystem before taking a snapshot?
If you want the snapshot to be consistent: that's what the VFS lock patch attempts to do automatically whenever you take a snapshot: presumably you haven't applied that patch?
Alasdair

Yes, I applied linux-2.4.21-VFS-lock.patch as well as the devmapper patch. The behaviour is reproducable, I just did it again:

lvm:~# mount /dev/vg0/lv0 fs
[Several files/dirs exist, but not device-mapper.1.00.05.tgz]

lvm:~# lvcreate -s -L 8m --n snap0 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap0 snap/0
[Not there yet!]

lvm:~# cp /usr/src/device-mapper.1.00.05.tgz fs
lvm:~# lvcreate -s -L 8m --n snap1 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap1 snap/1
[Does *not* exist in the mounted snapshot filesystem]

lvm:~# sync
lvm:~# lvcreate -s -L 8m --n snap2 /dev/vg0/lv0
lvm:~# mount /dev/vg0/snap2 snap/2
[Ahhh! Now it exists in *this* mounted snapshot filesystem!]

Oh, and after the file appears in the final snapshot, it doesn't magically appear in the previous one.

glen.

Signature · 13 lines

| Nine megs for the secretaries fair,      | One disk to rule them all,     |
| Seven megs for the hackers scarce,       | One disk to bind them,         |
| Five megs for the grads in smoky lairs,  | One disk to hold the files     |
| Three megs for system source;            | And in the darkness grind 'em. |
-----------------------------------------------------------------------------
| Glen Harris | [email protected] | 07 47500893 | http://www.gbrmpa.gov.au |
-----------------------------------------------------------------------------

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If you have received this transmission in error please notify us immediately
by return email and delete all copies. Any unauthorised use, disclosure or
distribution of this email is prohibited.
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Alasdair G Kergon

2003-09-16 11:40 UTC #6 lore raw

In reply to Glen Harris

Quoted 2 lines · Glen Harris

Hey, that fixed it! I've created 5 snapshots with copying/deleting files in between, and they've all worked!

I've put a new tarball up on the ftp site containing a fix for the read-only snapshot problem together with some other minor fixes.

ftp://ftp.sistina.com/pub/LVM2/tools/LVM2.2.00.07.tgz

Alasdair

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