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

[Peter Lieven <pl@kamp.de>]

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:

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