Re: [Qemu-devel] qemu-img convert cache mode for source

[Marcus <shadowsor@gmail.com>]

I think this is a more generic sysadmin problem.  I've seen the same
thing in the past with simply snapshotting a logical volume or zfs
zvol and copying it off somewhere. Page cache bloats, the system
starts swapping. To avoid it, we wrote a small C program that calls
FADV_DONTNEED on a file, and fork off a process to call it on the
source file every X seconds in our backup scripts.

It's a little strange to me to have qemu-img do this, just like it
would be strange if 'cp' did it, but I can see it as a very useful
shortcut if it's an optional flag.  qemu-img to me is just an admin
tool, and the admin should decide if they want their tool's reads
cached.  Some additional things that come to mind:

* If you are running qemu-img on a running VM's source file,
FADV_DONTNEED may ruin the cache you wanted if the VM is not running
cache=none.

* O_DIRECT I think will cause unexpected problems, for example the
zfsonlinux guys (and tmpfs as mentioned) don't yet support it. If it
is used, there has to be a fallback or a way to turn it off.

On Wed, Mar 5, 2014 at 7:44 AM, Peter Lieven <pl@kamp.de> wrote:
> Am 04.03.2014 10:24, schrieb Stefan Hajnoczi:
>> On Mon, Mar 03, 2014 at 01:20:21PM +0100, Peter Lieven wrote:
>>> On 03.03.2014 13:03, Stefan Hajnoczi wrote:
>>>> So what is the actual performance problem you are trying to solve and
>>>> what benchmark output are you getting when you compare with
>>>> FADV_DONTNEED against without FADV_DONTNEED?
>>> I found the performance to be identical. For the problem see below please.
>>>> I think there's a danger that the discussion will go around in circles.
>>>> Please post the performance results that kicked off this whole effort
>>>> and let's focus on the data.  That way it's much easier to evaluate what
>>>> changes to QEMU are a win and which are not necessary.
>>> I found that under memory pressure situations the increasing buffers
>>> leads to vserver memory being swapped out. This caused trouble
>>> especially in overcommit scenarios (where all memory is backed by
>>> swap).
>> I think the general idea is qemu-img should not impact running guests,
>> even on a heavily loaded machine.  But again, this needs to be discussed
>> using concrete benchmarks with configurations and results posted to the
>> list.
> Sure, this is why I started to look at this. I found that under high memory
> pressure a backup (local storage -> NFS) causes swapping. I started to
> use libnfs as destination to avoid influence of the kernel NFS client. But
> I saw that the buffers still increase while a backup is running. With the
> proposed patch I sent recently
>
> [PATCH] block: introduce BDRV_O_SEQUENTIAL
>
> I don't see this behaviour while I have not yet observed a performance penalty.
>
> Peter
>>
>> Stefan
>
>