[libvirt] flush guest page cache and suspend?

Marco Colombo marco at esiway.net
Wed Nov 4 17:16:27 UTC 2009

I'm new to this list. I've searched the archives (and google) but I 
haven't found an answer.

Recently I moved to CentOS 5.4, with libvirt-0.6.3 and kvm-83. They are 
rather new compared to 5.3 ones, so I'm still investigating on the new 

My guests disk images are LV single devices, /dev/vg_virt/<guestname>, 
usually with two partitions, the root fs and swap space.

I'd like to run backups on the node only, taking snapshots of the LVs. 
I'm already doing that, actually. Everything works fine, using bacula 
(with one minor glitch, but that's for another list).

Now I wonder if it makes sense to suspend the guest before creating the 

virsh suspend <guestname>
lvcreate --snapshot ...
virsh resume <guestname>
... perform backup ...
lvremove -f <the snapshot>

but thinking about it a second time it seems it doesn't add anything to 
the equation, the filesystem is still potentially unclean, with data to 
be written still in the guest page cache. The filesystem is ext3, so 
it's not that bad (recovers a consistent state from the journal, with 
standard options even data-consistent, not just metadata-consistent). 
Yet it's like doing a backup of a (potentially badly) crashed 
filesystem. While I trust the recovery step of ext3, my feeling is that 
it can't be 100% reliable. I have (rarely) seen ext3 crash and ask for 
fsck at boot, journal or not (for truth's sake, I can't rule out an hard 
disk problem in those cases).

So, first question: does the suspend command cause a flush in the guest 
OS (details: both guest and node are CentOS 5.4, hypervisor is qemu-kvm)?

I guess not (otherwise I won't be here). So if not, what are the options 
to force the sync?

Ideally, there should be a single atomic 'sync & suspend'. In practice, 
I can think of some workarounds: ssh <guest> -c sync, or the very 
old-fashioned way of enabling the 'sync' user and logging in from the 
serial console, or issuing a sysrq-s, again on the console.

I'm interested in the latter, but I wasn't able to trigger the sysrq 
from either 'virsh console <guestname>' or 'screen `virsh ttyconsole 
<guestname>` (tried minicom, too). The serial console is on pty, I'm not 
even sure you can generate a break on a pty. (and yes, I remembered to 
sysctl -w kernel.sysrq=1 on the guest).

I know XEN has 'xm sysrq', but this is kvm. Is there anything similar? I 
think it can be done if you invoke qemu-kvm from the command line with 
-nographics (it multiplexes the console and the monitor lines on 
stdin/out, and you can send a 'break' with C-a b I think, I've never tried).

So question 2): is there a way to send a sysrq-s to the guest?

My fallback plan is to try and map the serial line over telnet instead 
of pty, and the figure out a way to send the break (which I think is 
part of the telnet protocol) from the command line. I'd rather not mess 
with telnet and local port assignment to the guests, if not necessary.

My really fallback plan is to revert to shutdown/snapshot/create, which 
is overkill for a daily backup.

Question 3): does it _really_ make sense to try and sync the guest OS 
page cache before taking the snapshot? Or is it just me being paranoid?

For reference, here's the qemu cmdline of one of the guests:

/usr/libexec/qemu-kvm -S -M pc -m 512 -smp 1 -name wtitv -uuid 
445d0fe5-c1b6-4baf-a186-da1fe021158c -monitor pty -pidfile 
/var/run/libvirt/qemu//wtitv.pid -boot c -drive 
file=/dev/vg_virt/wtitv,if=ide,index=0,boot=on -net 
nic,macaddr=00:16:3e:57:45:4f,vlan=0,model=e1000 -net 
tap,fd=14,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb 
-usbdevice tablet -vnc

and here's the xml config file for libvirt, same guest:

<domain type='kvm'>
     <type arch='x86_64' machine='pc'>hvm</type>
     <boot dev='hd'/>
   <clock offset='utc'/>
     <disk type='block' device='disk'>
       <source dev='/dev/vg_virt/wtitv'/>
       <target dev='hda' bus='ide'/>
     <interface type='bridge'>
       <mac address='00:16:3e:57:45:4f'/>
       <source bridge='br1'/>
       <model type='e1000'/>
     <console type='pty'>
       <target port='0'/>
     <input type='tablet' bus='usb'/>
     <graphics type='vnc' autoport='yes'/>


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