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

[Peter Lieven <pl@kamp.de>]

Am 05.03.2014 18:38, schrieb Marcus:
> On Wed, Mar 5, 2014 at 8:53 AM, Peter Lieven <pl@kamp.de> wrote:
>> Am 05.03.2014 16:20, schrieb Marcus:
>>> 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.
>> I do not call FADV_DONTNEED on the whole file, but only
>> on the block that has just been read.
> Yes, I suppose that's one of the advantages of having it integrated
> into the reader.
>
>>> 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.
>> You would normally not run it on the source directly. In my case
>> I run it on a snapshot of an logical volume, but I see your point.
> Totally depends on the situation, just thought it was worth consideration.
Yes, and it was a good remark.
>
>> So you can confirm my oberservations and would be happy if
>> this behaviour could be toggled with a cmdline switch?
> Yes, I've seen the same behavior you mention just with 'cp'. It was
> with a version of the CentOS 6.2 kernel, at least, before we added
> FADV_DONTNEED into the backup scripts.
Ok, Stefan would you be happy with it?
>
>>> * 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.
>> I don't use O_DIRECT. Its an option for the destination file only at the
>> moment. You can set it with -t none as qemu-img argument.
> I just mentioned it because setting it on the source was suggested
> originally and subsequently discussed.
Yes, but it would break readahead and would not work and tmpfs
and many other things we don't see know.

Peter