Re: [PATCH] block: file-posix: Fail unmap with NO_FALLBACK on block device

[Kevin Wolf]

Am 15.06.2020 um 21:32 hat Nir Soffer geschrieben:
> We can zero 2.3 g/s:
> 
> # time blkdiscard -z test-lv
> 
> real 0m43.902s
> user 0m0.002s
> sys 0m0.130s

> We can write 445m/s:
> 
> # dd if=/dev/zero bs=2M count=51200 of=test-lv oflag=direct conv=fsync
> 107374182400 bytes (107 GB, 100 GiB) copied, 241.257 s, 445 MB/s

So using FALLOC_FL_PUNCH_HOLE _is_ faster after all. What might not be
faster is zeroing out the whole device and then overwriting a
considerable part of it again.

I think this means that we shouldn't fail write_zeroes at the file-posix
level even if BDRV_REQ_NO_FALLBACK is given. Instead, qemu-img convert
is where I see a fix.

Certainly qemu-img could be cleverer and zero out more selectively. The
idea of doing a blk_make_zero() first seems to have caused some
problems, though of course its introduction was also justified with
performance, so improving one case might hurt another if we're not
careful.

However, when Peter Lieven introduced this (commit 5a37b60a61c), we
didn't use write_zeroes yet during the regular copy loop (we do since
commit 690c7301600). So chances are that blk_make_zero() doesn't
actually help any more now.

Can you run another test with the patch below? I think it should perform
the same as yours. Eric, Peter, do you think this would have a negative
effect for NBD and/or iscsi?

The other option would be providing an option and making it Someone
Else's Problem.

Kevin


diff --git a/qemu-img.c b/qemu-img.c
index d7e846e607..bdb9f6aa46 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2084,15 +2084,6 @@ static int convert_do_copy(ImgConvertState *s)
         s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
     }

-    if (!s->has_zero_init && !s->target_has_backing &&
-        bdrv_can_write_zeroes_with_unmap(blk_bs(s->target)))
-    {
-        ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | 
BDRV_REQ_NO_FALLBACK);
-        if (ret == 0) {
-            s->has_zero_init = true;
-        }
-    }
-
     /* Allocate buffer for copied data. For compressed images, only one cluster
      * can be copied at a time. */
     if (s->compressed) {