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Re: [Xen-devel] A snapshot is not (really) a cow

I'm not aware of anything that is trying to use NBD - I did experiment a bit a while back, but didn't get anywhere. I thought xen might be using it behind the scenes in some way.

I have attached some tar-zipped output which may help.

lvm2-nbd-problem-20040916/messages-log # messages during vchange -ay
lvm2-nbd-problem-20040916/services-list    # services that were enabled
lvm2-nbd-problem-20040916/syslog-start     # syslog start at reboot
lvm2-nbd-problem-20040916/mem-slab-1    # before vchange -ay
lvm2-nbd-problem-20040916/mem-slab-2 # after vchange -ay lvm2-nbd-problem-20040916/vgchange-ay-result # response to command
lvm2-nbd-problem-20040916/cmdline   # xen0 kernel command line

xen version date: xen-2.0-20040924.

I am puzzled that in /var/log/messages it almost immediately starts trying to access nbd devices - none are listed as existing in /dev. Also I'm not sure what the '[' relates to.

There is a note about the symbols matching 2.6.8, but I am running Also it says kernel modules ere not enabled, but lsmod shows:

... # lsmod
Module                  Size  Used by
nfsd                   91720  8
exportfs                5248  1 nfsd

Should I be doing anything about kernel symbols? I build and install straight out of the box with no special config:

make world     (as orndinary user)
make install     (as root)

Too late. I hope this helps.


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 ...

Are you using NBD? Our default kernel has it compiled in, but it
looks like some of your start up scripts are trying to initialise
it, or you have some daemon that's polling the devices.

It looks like the error messages are hammering the machine pretty
hard from time to time. It's definitely worth figuring out what's
causing it. Or, compile nbd out of the kernel...

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]

Seeing the output of /proc/slabinfo and /proc/meminfo might be
interesting. And idea why the machine might be under memory pressure?


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Attachment: lvm2-nbd-problem-20040916-tar.gz
Description: GNU Zip compressed data



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