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

[Kevin Wolf <kwolf@redhat.com>]

Am 28.02.2014 um 15:35 hat Peter Lieven geschrieben:
> On 27.02.2014 09:57, Stefan Hajnoczi wrote:
> >On Wed, Feb 26, 2014 at 05:01:52PM +0100, Peter Lieven wrote:
> >>On 26.02.2014 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.
> >>>Points to consider:
> >>>
> >>>1. O_DIRECT does not work on Linux tmpfs, you get EINVAL when opening
> >>>    the file.  A fallback is necessary.
> >>>
> >>>2. O_DIRECT has no readahead so performance could actually decrease.
> >>>    The question is, how important is reahead versus polluting page
> >>>    cache?
> >>>
> >>>3. For raw files it would make sense to tell the kernel that access is
> >>>    sequential and data will be used only once.  Then we can get the best
> >>>    of both worlds (avoid polluting page cache but still get readahead).
> >>>    This is done using posix_fadvise(2).
> >>>
> >>>    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.
> >>your comments are regarding qemu-img convert, right?
> >>How would you implement this? A new open flag because
> >>the fadvise had to goto inside the protocol driver.
> >>
> >>I would start with host_devices first and see how it performs there.
> >>
> >>For qemu-img convert I would issue a FADV_DONTNEED after
> >>a write for the bytes that have been written
> >>(i have tested this with Linux and it seems to work quite well).
> >>
> >>Question is, what is the right paramter for reads? Also FADV_DONTNEED?
> >I think so but this should be justified with benchmark results.
> 
> I ran some benchmarks at found that a FADV_DONTNEED issues after
> a read does not hurt regarding to performance. But it avoids buffers
> increasing while I read from a host_device of raw file.

Okay, sounds reasonable.

> As for writing it does only work if I issue a fdatasync after each write, but
> this should be equivalent to O_DIRECT. So I would keep the patch
> to support qemu-img convert sources if they are host_device or file.

Doing an fdatasync() is not an option (and not equivalent to O_DIRECT
at all).

> Here is a proposal for a patch:
> 
> diff --git a/block.c b/block.c
> index 2fd5482..2445433 100644
> --- a/block.c
> +++ b/block.c
> @@ -2626,6 +2626,14 @@ static int bdrv_prwv_co(BlockDriverState *bs, int64_t offset,
>              qemu_aio_wait();
>          }
>      }
> +
> +#ifdef POSIX_FADV_DONTNEED
> +    if (!rwco.ret && bs->open_flags & BDRV_O_SEQUENTIAL &&
> +        bs->drv->bdrv_fadvise && !is_write) {
> +        bs->drv->bdrv_fadvise(bs, offset, qiov->size, POSIX_FADV_DONTNEED);
> +    }
> +#endif
> +

This #ifdef should be in the raw-posix driver. Please try to keep the
qemu interface backend agnostic and leave POSIX_FADV_DONTNEED and
friends as an implementation detail of block drivers.

> diff --git a/include/block/block.h b/include/block/block.h
> index 780f48b..a4dcc3c 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -105,6 +105,9 @@ typedef enum {
>  #define BDRV_O_PROTOCOL    0x8000  /* if no block driver is explicitly given:
>                                        select an appropriate protocol driver,
>                                        ignoring the format layer */
> +#define BDRV_O_SEQUENTIAL 0x10000  /* open device for sequential read/write */
> +
> +
> 
>  #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH)

Why two additional newlines?

BDRV_O_SEQUENTIAL works for me as the external interface.

Kevin