Re: [Qemu-devel] [PATCH 7/7] block/raw-posix: set max_write_zeroes to INT_MAX for regular files

[Kevin Wolf <kwolf@redhat.com>]

Am 02.02.2015 um 15:12 hat Peter Lieven geschrieben:
> Am 02.02.2015 um 15:04 schrieb Kevin Wolf:
> >Am 02.02.2015 um 14:55 hat Peter Lieven geschrieben:
> >>Am 02.02.2015 um 14:23 schrieb Kevin Wolf:
> >>>Am 30.01.2015 um 09:42 hat Denis V. Lunev geschrieben:
> >>>>fallocate() works fine and could handle properly with arbitrary size
> >>>>requests. There is no sense to reduce the amount of space to fallocate.
> >>>>The bigger is the size, the better is the performance as the amount of
> >>>>journal updates is reduced.
> >>>>
> >>>>The patch changes behavior for both generic filesystem and XFS codepaths,
> >>>>which are different in handle_aiocb_write_zeroes. The implementation
> >>>>of fallocate and xfsctl(XFS_IOC_ZERO_RANGE) for XFS are exactly the same
> >>>>thus the change is fine for both ways.
> >>>>
> >>>>Signed-off-by: Denis V. Lunev <den@openvz.org>
> >>>>Reviewed-by: Max Reitz <mreitz@redhat.com>
> >>>>CC: Kevin Wolf <kwolf@redhat.com>
> >>>>CC: Stefan Hajnoczi <stefanha@redhat.com>
> >>>>CC: Peter Lieven <pl@kamp.de>
> >>>>CC: Fam Zheng <famz@redhat.com>
> >>>>---
> >>>>  block/raw-posix.c | 17 +++++++++++++++++
> >>>>  1 file changed, 17 insertions(+)
> >>>>
> >>>>diff --git a/block/raw-posix.c b/block/raw-posix.c
> >>>>index 7b42f37..933c778 100644
> >>>>--- a/block/raw-posix.c
> >>>>+++ b/block/raw-posix.c
> >>>>@@ -293,6 +293,20 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
> >>>>      }
> >>>>  }
> >>>>+static void raw_probe_max_write_zeroes(BlockDriverState *bs)
> >>>>+{
> >>>>+    BDRVRawState *s = bs->opaque;
> >>>>+    struct stat st;
> >>>>+
> >>>>+    if (fstat(s->fd, &st) < 0) {
> >>>>+        return; /* no problem, keep default value */
> >>>>+    }
> >>>>+    if (!S_ISREG(st.st_mode) || !s->discard_zeroes) {
> >>>>+        return;
> >>>>+    }
> >>>>+    bs->bl.max_write_zeroes = INT_MAX;
> >>>>+}
> >>>Peter, do you remember why INT_MAX isn't actually the default? I think
> >>>the most reasonable behaviour would be that a limitation is only used if
> >>>a block driver requests it, and otherwise unlimited is assumed.
> >>The default (0) actually means unlimited or undefined. We introduced
> >>that limit of 16MB in bdrv_co_write_zeroes to create only reasonable
> >>sized requests because there is no guarantee that write zeroes is a
> >>fast operation. We should set INT_MAX only if we know that write
> >>zeroes of an arbitrary size is always fast.
> >Well, splitting it up doesn't make it any faster. I think we can assume
> >that drv->bdrv_co_write_zeroes() wants to know the full request size
> >unless the driver has explicitly set bs->bl.max_write_zeroes.
> 
> You mean sth like this:

Yes, I think that's what I meant.

Kevin

> diff --git a/block.c b/block.c
> index 61412e9..8272ef9 100644
> --- a/block.c
> +++ b/block.c
> @@ -3192,10 +3192,7 @@ int coroutine_fn bdrv_co_copy_on_readv(BlockDriverState *bs,
>                              BDRV_REQ_COPY_ON_READ);
>  }
> 
> -/* if no limit is specified in the BlockLimits use a default
> - * of 32768 512-byte sectors (16 MiB) per request.
> - */
> -#define MAX_WRITE_ZEROES_DEFAULT 32768
> +#define MAX_WRITE_ZEROES_BOUNCE_BUFFER 32768
> 
>  static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>      int64_t sector_num, int nb_sectors, BdrvRequestFlags flags)
> @@ -3206,7 +3203,7 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>      int ret = 0;
> 
>      int max_write_zeroes = bs->bl.max_write_zeroes ?
> -                           bs->bl.max_write_zeroes : MAX_WRITE_ZEROES_DEFAULT;
> +                           bs->bl.max_write_zeroes : INT_MAX;
> 
>      while (nb_sectors > 0 && !ret) {
>          int num = nb_sectors;
> @@ -3242,7 +3239,7 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>          if (ret == -ENOTSUP) {
>              /* Fall back to bounce buffer if write zeroes is unsupported */
>              int max_xfer_len = MIN_NON_ZERO(bs->bl.max_transfer_length,
> - MAX_WRITE_ZEROES_DEFAULT);
> + MAX_WRITE_ZEROES_BOUNCE_BUFFER);
>              num = MIN(num, max_xfer_len);
>              iov.iov_len = num * BDRV_SECTOR_SIZE;
>              if (iov.iov_base == NULL) {
> @@ -5099,11 +5096,6 @@ static void coroutine_fn bdrv_discard_co_entry(void *opaque)
>      rwco->ret = bdrv_co_discard(rwco->bs, rwco->sector_num, rwco->nb_sectors);
>  }
> 
> -/* if no limit is specified in the BlockLimits use a default
> - * of 32768 512-byte sectors (16 MiB) per request.
> - */
> -#define MAX_DISCARD_DEFAULT 32768
> -
>  int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
>                                   int nb_sectors)
>  {
> @@ -5128,7 +5120,7 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
>          return 0;
>      }
> 
> -    max_discard = bs->bl.max_discard ?  bs->bl.max_discard : MAX_DISCARD_DEFAULT;
> +    max_discard = bs->bl.max_discard ?  bs->bl.max_discard : INT_MAX;
>      while (nb_sectors > 0) {
>          int ret;
>          int num = nb_sectors;
> 
> 
> 
> Peter