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Re: [libvirt] [PATCH] storage: Don't unsparsify images when cloning



On 02/07/2012 12:55 PM, Cole Robinson wrote:
> Input to the volume cloning code is a source volume and an XML
> descriptor for the new volume. It is possible for the new volume
> to have a greater size than source volume, at which point libvirt
> will just stick 0s on the end of the new image (for raw format
> anyways).
> 
> Unfortunately a logic error messed up our tracking of the of the
> excess amount that needed to be written: end result is that sparse
> clones were made very much non-sparse, and cloning regular disk
> images could end up excessively sized (though data unaltered).
> 
> Drop the 'remain' variable entriely here since it's redundant, and

s/entriely/entirely/

> track actual allocation directly against the desired 'total'.
> ---
>  src/storage/storage_backend.c |   11 +++--------
>  1 files changed, 3 insertions(+), 8 deletions(-)

Hmm - given that we just added virStorageVolResize, with a flag
VIR_STORAGE_VOL_RESIZE_ALLOCATE, where the default operation is a sparse
resize but the flag requests a non-sparse allocation, I'm wondering if
virStorageVolCreateXMLFrom should also be taught to use a flag that
controls whether or not we attempt to sparsify the output (the default
of sparse is nicer, as it is likely to be faster, but there are some
cases where a sparse file is undesirable because you want to know up
front if you are going to hit ENOSPC errors, rather than overcommitting
due to sparse files that later grow).

> 
> diff --git a/src/storage/storage_backend.c b/src/storage/storage_backend.c
> index 1c22112..caac2f8 100644
> --- a/src/storage/storage_backend.c
> +++ b/src/storage/storage_backend.c
> @@ -127,7 +127,6 @@ virStorageBackendCopyToFD(virStorageVolDefPtr vol,
>      int inputfd = -1;
>      int amtread = -1;
>      int ret = 0;
> -    unsigned long long remain;
>      size_t rbytes = READ_BLOCK_SIZE_DEFAULT;
>      size_t wbytes = 0;
>      int interval;
> @@ -165,13 +164,11 @@ virStorageBackendCopyToFD(virStorageVolDefPtr vol,
>          goto cleanup;
>      }
>  
> -    remain = *total;
> -
>      while (amtread != 0) {
>          int amtleft;
>  
> -        if (remain < rbytes)
> -            rbytes = remain;
> +        if (*total < rbytes)
> +            rbytes = *total;
>  
>          if ((amtread = saferead(inputfd, buf, rbytes)) < 0) {
>              ret = -errno;
> @@ -180,7 +177,7 @@ virStorageBackendCopyToFD(virStorageVolDefPtr vol,
>                                   inputvol->target.path);
>              goto cleanup;
>          }
> -        remain -= amtread;
> +        *total -= amtread;
>  
>          /* Loop over amt read in 512 byte increments, looking for sparse
>           * blocks */
> @@ -225,8 +222,6 @@ virStorageBackendCopyToFD(virStorageVolDefPtr vol,
>      }
>      inputfd = -1;
>  
> -    *total -= remain;

If I'm understanding you correctly, this line was the logic error: it
should have been '*total = remain' instead of using -=.

The only reason for using remain instead of total would have then been
to avoid pointer dereferencing in the loop, but I don't think it's on a
hot enough path to make a difference, so I'm okay with your patch as-is
(although we should still be thinking about a flag that controls whether
the user can request non-sparse clones).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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