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

[Fam Zheng <famz@redhat.com>]

On Wed, 02/26 16:41, Stefan Hajnoczi wrote:
> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote:
> > I was wondering if it would be a good idea to set the O_DIRECT mode for the source
> > files of a qemu-img convert process if the source is a host_device?
> > 
> > Currently the backup of a host device is polluting the page cache.

Peter, can you give some more detailed explanation of the issue? An example or
benchmark will help a lot. As I understand, one time scanning of a file doesn't
promote the page cache to active list, so it probably won't hurt real hot cache
at all, and will get replaced very soon.

Considering readahead and page cache on metadata, I'm not sure if forcing
O_DIRECT is a good idea.

>    The problem is what to do for image formats.  An image file can be
>    very fragmented so the readahead might not be a win.  Does this mean
>    that for image formats we should tell the kernel access will be
>    random?
> 
>    Furthermore, maybe it's best to do readahead inside QEMU so that even
>    network protocols (nbd, iscsi, etc) can get good performance.  They
>    act like O_DIRECT is always on.

Also, experience with booting a network backed guest can be greatly improved,
because sometimes BIOS and bootloader are simple minded and load a kernel or
initrd by sending thousands of 1 sector requests with iodepth=1, which means
latency of network based IO hurts a lot.

Doing readahead in QEMU makes sense for image formats as well, because we know
where the next data cluster is better than kernel. But again, we are
replicating things from kernel.