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Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later
From: |
Andrew Martin |
Subject: |
Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later |
Date: |
Tue, 19 Aug 2014 18:20:38 -0500 (CDT) |
----- Original Message -----
> From: "Stefan Hajnoczi" <address@hidden>
> To: "Andrew Martin" <address@hidden>
> Cc: address@hidden
> Sent: Tuesday, August 19, 2014 9:59:25 AM
> Subject: Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later
>
> If you strace -f the QEMU process on the host, you will see fdatasync(2)
> system calls when the guest flushes the disk.
>
> You can find the file descriptor number by checking ls -l
> /proc/$PID_OF_QEMU/fd and looking for the disk image file.
When the disk is set to cache=writethrough on one of the same VMs, I see
frequent
fdatasync(2) calls (every few seconds). However, when I change the disk over to
cache=writeback, since boot I have not yet seen a single fdatasync(2) call, even
after writing data 2x the amount of RAM:
# time strace -ft -p4113 2>&1 | grep fdatasync
^C
real 15m39.245s
user 0m7.940s
sys 0m18.280s
Note that the disk is defined as follows:
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2' cache='writeback'/>
<source file='/var/lib/libvirt/images/vm.img'/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04'
function='0x0'/>
</disk>
> > I recently experienced UPS failure on several hosts which caused a hard
> > shutdown. After restarting, 3 of the guests had corruption on their disks
> > and
> > required a fairly long fsck to fix. Afterwards, data that had been written
> > to
> > the disks several hours before the crash was corrupted, which makes me
> > think
> > that it was never fsync()-ed to the non-volatile storage.
>
> What exactly was the "corruption" you encountered? Which application,
> error message, etc.
Two of the servers are web servers with apache2. In one case, a python daemon
copies JPGs onto the server - the last 100 copied onto the server were
corrupted.
In another case, some files had been uploaded several days prior to the
www-root,
but after the hard reset said files were no longer present in the filesystem.
> > Is it safe in this setup to use cache=writeback? Or, should I use
> > cache=writethrough instead?
>
> Ubuntu 12.04 is recent and sends write cache flushes.
>
> Are you sure the file system and/or application workload are flushing
> the disk cache? Please check the mount options and application-specific
> configuration.
The mount options for the ext4 filesystem in the VM in both cases are:
rw,relatime,errors=remount-ro,data=ordered
Similarly, the host's ext4 filesystem holding the images is mounted with:
rw,relatime,data=ordered
I did not see any errors in the kernel log in the guest, probably because the
root filesystem was read-only until the fsck had completed.
Thanks,
Andrew
- [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Andrew Martin, 2014/08/15
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Stefan Hajnoczi, 2014/08/19
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later,
Andrew Martin <=
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Stefan Hajnoczi, 2014/08/21
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Andrew Martin, 2014/08/25
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Paolo Bonzini, 2014/08/26
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Andrew Martin, 2014/08/27
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Paolo Bonzini, 2014/08/27
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Andrew Martin, 2014/08/27
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Paolo Bonzini, 2014/08/27
- Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later, Stefan Hajnoczi, 2014/08/28