Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Mar 17, 2010 at 03:14:10PM +0000, Chris Webb wrote:
> Anthony Liguori <anthony@codemonkey.ws> writes:
> 
> > This really gets down to your definition of "safe" behaviour.  As it
> > stands, if you suffer a power outage, it may lead to guest
> > corruption.
> > 
> > While we are correct in advertising a write-cache, write-caches are
> > volatile and should a drive lose power, it could lead to data
> > corruption.  Enterprise disks tend to have battery backed write
> > caches to prevent this.
> > 
> > In the set up you're emulating, the host is acting as a giant write
> > cache.  Should your host fail, you can get data corruption.
> 
> Hi Anthony. I suspected my post might spark an interesting discussion!
> 
> Before considering anything like this, we did quite a bit of testing with
> OSes in qemu-kvm guests running filesystem-intensive work, using an ipmitool
> power off to kill the host. I didn't manage to corrupt any ext3, ext4 or
> NTFS filesystems despite these efforts.
> 
> Is your claim here that:-
> 
>   (a) qemu doesn't emulate a disk write cache correctly; or
> 
>   (b) operating systems are inherently unsafe running on top of a disk with
>       a write-cache; or
> 
>   (c) installations that are already broken and lose data with a physical
>       drive with a write-cache can lose much more in this case because the
>       write cache is much bigger?
> 
> Following Christoph Hellwig's patch series from last September, I'm pretty
> convinced that (a) isn't true apart from the inability to disable the
> write-cache at run-time, which is something that neither recent linux nor
> windows seem to want to do out-of-the box.
> 
> Given that modern SATA drives come with fairly substantial write-caches
> nowadays which operating systems leave on without widespread disaster, I
> don't really believe in (b) either, at least for the ide and scsi case.
> Filesystems know they have to flush the disk cache to avoid corruption.
> (Virtio makes the write cache invisible to the OS except in linux 2.6.32+ so
> I know virtio-blk has to be avoided for current windows and obsolete linux
> when writeback caching is on.)
> 
> I can certainly imagine (c) might be the case, although when I use strace to
> watch the IO to the block device, I see pretty regular fdatasyncs being
> issued by the guests, interleaved with the writes, so I'm not sure how
> likely the problem would be in practice. Perhaps my test guests were
> unrepresentatively well-behaved.
> 
> However, the potentially unlimited time-window for loss of incorrectly
> unsynced data is also something one could imagine fixing at the qemu level.
> Perhaps I should be implementing something like
> cache=writeback,flushtimeout=N which, upon a write being issued to the block
> device, starts an N second timer if it isn't already running. The timer is
> destroyed on flush, and if it expires before it's destroyed, a gratuitous
> flush is sent. Do you think this is worth doing? Just a simple 'while sleep
> 10; do sync; done' on the host even!
> 
> We've used cache=none and cache=writethrough, and whilst performance is fine
> with a single guest accessing a disk, when we chop the disks up with LVM and
> run a even a small handful of guests, the constant seeking to serve tiny
> synchronous IOs leads to truly abysmal throughput---we've seen less than
> 700kB/s streaming write rates within guests when the backing store is
> capable of 100MB/s.
> 
> With cache=writeback, there's still IO contention between guests, but the
> write granularity is a bit coarser, so the host's elevator seems to get a
> bit more of a chance to help us out and we can at least squeeze out 5-10MB/s
> from two or three concurrently running guests, getting a total of 20-30% of
> the performance of the underlying block device rather than a total of around
> 5%.

Hi Chris,

Are you using CFQ in the host? What is the host kernel version? I am not sure
what is the problem here but you might want to play with IO controller and put
these guests in individual cgroups and see if you get better throughput even
with cache=writethrough.

If the problem is that if sync writes from different guests get intermixed
resulting in more seeks, IO controller might help as these writes will now
go on different group service trees and in CFQ, we try to service requests
from one service tree at a time for a period before we switch the service
tree.

The issue will be that all the logic is in CFQ and it works at leaf nodes
of storage stack and not at LVM nodes. So first you might want to try it with
single partitioned disk. If it helps, then it might help with LVM
configuration also (IO control working at leaf nodes).

Thanks
Vivek

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