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. > > 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.
So you can confirm my oberservations and would be happy if this behaviour could be toggled with a cmdline switch? > > * 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. Peter > > 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 >>