It's very encouraging to know that I misunderstood the situation. But I
do have a problem:
It may be that I am genuinely running out of memory, but I am also
getting a lot of messages about nbdNNN, and the overall effect is pretty
disastrous.
I successfully created 2 new snapshots while another was in use as the
root file system of a xenU domain. Previously this had failed
immediately. Then the next lvcreate -s failed with a message about
running out of memory, as before.
I wonder whether the timing problem I have which affects rpm is also
causing some problem?
The relevant bit of /var/log/messages is:
... lots of nbdNNN messages ...
Sep 26 19:47:33 a4 kernel: nbd126: Request when not-ready
Sep 26 19:47:33 a4 kernel: end_request: I/O error, dev nbd126, sector 0
Sep 26 19:47:33 a4 kernel: nbd127: Request when not-ready
Sep 26 19:47:33 a4 kernel: end_request: I/O error, dev nbd127, sector 0
Sep 26 19:48:26 a4 net.agent[7343]: add event not handled
Sep 26 19:48:26 a4 kernel: device vif6.0 entered promiscuous mode
Sep 26 19:48:27 a4 kernel: xen-br0: port 3(vif6.0) entering learning state
Sep 26 19:48:27 a4 kernel: xen-br0: topology change detected, propagating
Sep 26 19:48:27 a4 kernel: xen-br0: port 3(vif6.0) entering forwarding state
Sep 26 19:50:00 a4 CROND[7360]: (mail) CMD (/usr/bin/python -S
/usr/lib/mailman/cron/gate_news)
Sep 26 19:53:40 a4 kernel: nbd0: Request when not-ready
Sep 26 19:53:40 a4 kernel: end_request: I/O error, dev nbd0, sector 0
Sep 26 19:53:40 a4 kernel: nbd1: Request when not-ready
Sep 26 19:53:40 a4 kernel: end_request: I/O error, dev nbd1, sector 0
Sep 26 19:53:40 a4 kernel: nbd2: Request when not-ready
... lots more nbdNNN messages ...
Sep 26 19:53:44 a4 kernel: end_request: I/O error, dev nbd124, sector 0
Sep 26 19:53:44 a4 kernel: nbd125: Request when not-ready
Sep 26 19:53:44 a4 kernel: end_request: I/O error, dev nbd125, sector 0
Sep 26 19:53:44 a4 kernel: nbd126: Request when not-ready
Sep 26 19:53:44 a4 kernel: end_request: I/O error, dev nbd126, sector 0
Sep 26 19:53:44 a4 kernel: nbd127: Request when not-ready
Sep 26 19:53:44 a4 kernel: end_request: I/O error, dev nbd127, sector 0
Sep 26 19:53:44 a4 kernel: lvcreate: page allocation failure. order:0,
mode:0xd0
Sep 26 19:53:44 a4 kernel: [__alloc_pages+824/842]
__alloc_pages+0x338/0x34a
Sep 26 19:53:44 a4 kernel: [<c013a16f>] __alloc_pages+0x338/0x34a
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [kmem_cache_alloc+89/100]
kmem_cache_alloc+0x59/0x64
Sep 26 19:53:44 a4 kernel: [<c013eb87>] kmem_cache_alloc+0x59/0x64
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [alloc_pl+48/76] alloc_pl+0x30/0x4c
Sep 26 19:53:44 a4 kernel: [<c0328a54>] alloc_pl+0x30/0x4c
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [client_alloc_pages+40/129]
client_alloc_pages+0x28/0x81
Sep 26 19:53:44 a4 kernel: [<c0328ba7>] client_alloc_pages+0x28/0x81
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [vmalloc+32/36] vmalloc+0x20/0x24
Sep 26 19:53:44 a4 kernel: [<c014f4d1>] vmalloc+0x20/0x24
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [kcopyd_client_create+104/199]
kcopyd_client_create+0x68/0xc7
Sep 26 19:53:44 a4 kernel: [<c03296cd>] kcopyd_client_create+0x68/0xc7
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [dm_create_persistent+199/320]
dm_create_persistent+0xc7/0x140
Sep 26 19:53:44 a4 kernel: [<c032b769>] dm_create_persistent+0xc7/0x140
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [snapshot_ctr+715/892] snapshot_ctr+0x2cb/0x37c
Sep 26 19:53:44 a4 kernel: [<c0329f7f>] snapshot_ctr+0x2cb/0x37c
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [dm_table_add_target+323/464]
dm_table_add_target+0x143/0x1d0
Sep 26 19:53:44 a4 kernel: [<c0324aa7>] dm_table_add_target+0x143/0x1d0
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [populate_table+125/216]
populate_table+0x7d/0xd8
Sep 26 19:53:44 a4 kernel: [<c0327105>] populate_table+0x7d/0xd8
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [table_load+103/309] table_load+0x67/0x135
Sep 26 19:53:44 a4 kernel: [<c03271c7>] table_load+0x67/0x135
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [ctl_ioctl+236/330] ctl_ioctl+0xec/0x14a
Sep 26 19:53:44 a4 kernel: [<c03279d3>] ctl_ioctl+0xec/0x14a
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [table_load+0/309] table_load+0x0/0x135
Sep 26 19:53:44 a4 kernel: [<c0327160>] table_load+0x0/0x135
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [ctl_ioctl+0/330] ctl_ioctl+0x0/0x14a
Sep 26 19:53:44 a4 kernel: [<c03278e7>] ctl_ioctl+0x0/0x14a
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [sys_ioctl+484/670] sys_ioctl+0x1e4/0x29e
Sep 26 19:53:44 a4 kernel: [<c0167606>] sys_ioctl+0x1e4/0x29e
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: [syscall_call+7/11] syscall_call+0x7/0xb
Sep 26 19:53:44 a4 kernel: [<c010d967>] syscall_call+0x7/0xb
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: device-mapper: : Could not create kcopyd client
Sep 26 19:53:44 a4 kernel:
Sep 26 19:53:44 a4 kernel: device-mapper: error adding target to table
... at this point lvm2 is unusable until after a reboot (unless some
service can be restarted?) and xenU domains (or at least those with lvm2
snapshot based file systems) are dead.
I certainly have no intention of writing to the original image.
Thanks
Peri
Christian Limpach wrote:
On Sun, Sep 26, 2004 at 12:38:06PM +0100, Peri Hankey wrote:
I always found the lvm2 'snapshot' terminology confusing - the thing
created as a 'snapshot' is what accepts changes while a backup is made
of the original volume.
I don't think that's the terminology the LVM2 people use. The regular
use is to create a snapshot and backup this snapshot while you keep
using the original.
# drat - I needed another domain
lvcreate -L512M -s -n u4 /dev/vmgroup/root_file_system
... nasty messages .... all xenU domains dead ....
... lmv2 system in inconsistent state ...
... /dev/vmgroup/u4 doesn't exist ...
... /dev/mapper/root_file_system-u4 does exist ...
This should work, if it doesn't then it would seem to be a bug in
LVM2. Since you mention out of memory error messages, are you sure
that you're not running out of memory in dom0?
The problem is that the 'snapshot' cows hold onto each other's tails -
they seem to be held in a list linked (I think) from the original
logical volume (here /dev/vmgroup/root_file_system). For their intended
use as enabling backup, this seems to be meant to allow writes to the
original volume to be propagated to all 'snapshots' created against that
volume - there are comments about getting rid of the 'snapshots' after
the backup has been done because this propagation of writes hits
performance.
For my requirements, and I imagine for most others reading this list,
all of this is superfluous. I don't need
original -> snap1 -> snap2 -> snap3 ...
This is not the layout LVM2 uses. If you look at the output of
``dmsetup table'', you'll see that each snapshot is independent
and only refers to the device it is a snapshot of and to its cow
device which will hold modifications.
so that I can't create a new snap4 while any of the others are in use.
I just need
original <- cow1
original <- cow2
original <- cow3
original <- cow4
...
where A '<-' B means B is a cow image of A, and where each of the cows
is independent of the others so that a new cow can be created at any
time, regardless how many others are active.
This is the layout LVM2 uses. And it is indeed simple (and should be
quite robust) as long as you don't want to write to the original.
If you write to the original, you will have to copy the changed
blocks to every snapshot's cow device. I think I've seen this
fail when having multiple snapshots and writing to the original.
But since you didn't write to the original (and one generally doesn't
need/want to write to the original in our case), that problem
is unlikely to be relevant to the failure you've seen.
christian
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