Xen devel - Jan 2006 - can not lvcreate after lvm snapshot and "xm mem-set" and lvremove

I am not sure if it''s a xen bug or lvm bug, or just my bad hardware,
but here''s how to reproduce:

/dev/vg0/centos1 is lvm block used by a domainU, it''s 8G in size.

#lvcreate -L8192M -s -n snap1 vg0
#mount /dev/vg0/snap1 /mnt
do some copying files here
#umount /mnt
#lvremove /dev/vg0/snap1
not removed! it says can''t allocated memory

#xm mem-set 0 512
#lvremove /dev/vg0/snap1
now it removed successfully

#lvcreate -L4096M -n fedora1 vg0
System lockup at this point.   I have to login from another terminal. 
I couldn''t kill the lvcreate process, couln''t do anything lvm.
I have
to reboot the machine.

My vg0 is on /dev/md1, which is a software raid 1.
My dom0 os is fedora core 4.  Xen version is 3.0-testing

So what causes this?  What''s my work around I can use to do lvm
snapshot.


Wensheng

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in the last post:

The snapshot is created by
lvcreate -L8192M -s -n snap1 /dev/vg0/centos1

not
lvcreate -L8192M -s -n snap1 vg0


Wensheng

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Hi,

I would strongly suggest not to use the lvm snapshot feature.

There were some post in the ML where people reported that the lvm snapshot 
feature is not ready for production use. In my opinion it is even worse. At 
least in combination with xen it really unstable and unusable (at least in my 
tests).

I tried the following, because I though it would be a quite good backup 
solution:

- suspend a running xenU Domain in a file via "xm save <domainname> 
<suspend-filename>"
- make a lvm snapshot of the corresponding lv
- restore the stopped xen domain
- create a backup of <suspend-file> and the lvm snapshot
- delete the lvm snapshot and <suspend-file>

If this would work correctly, you would have a perfect snapshot of the 
filesystem and your running domain and can backup it without downtime of your 
system. the only downtime would be the time between the xm save and the xm 
restore (max. 3 seconds maybe)

But even if in theory this should work, I had some ugly problems with this in 
xen2.0.7 and also xen3

most of the time the xenU Domain became unreponsive (I guess at the point at 
which the domain wants to access HDD data) and sometimes the xenU domain even 
crashed, sometimes even after the lvm snapshot was already backuped and 
deleted.

--Ralph

Am Donnerstag, 5. Januar 2006 11:12 schrieb Wensheng
Wang:
> in the last post:
>
> The snapshot is created by
> lvcreate -L8192M -s -n snap1 /dev/vg0/centos1
>
> not
> lvcreate -L8192M -s -n snap1 vg0
>
>
> Wensheng
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
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