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 <p...@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 > >
