Re: A snapshot is not (really) a cow

[Peri Hankey <mpah@thegreen.co.uk>]

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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/
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 
2.6.8.1. 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.

Thanks
Peri

>>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?
>
>Ian
>
>
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