Patchwork [v2] qcow2: Implement bdrv_truncate() for growing images

login
register
mail settings
Submitter Stefan Hajnoczi
Date April 28, 2010, 9:24 a.m.
Message ID <1272446655-4810-1-git-send-email-stefanha@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/51153/
State New
Headers show

Comments

Stefan Hajnoczi - April 28, 2010, 9:24 a.m.
This patch adds the ability to grow qcow2 images in-place using
bdrv_truncate().  This enables qemu-img resize command support for
qcow2.

Snapshots are not supported and bdrv_truncate() will return -ENOTSUP.
The notion of resizing an image with snapshots could lead to confusion:
users may expect snapshots to remain unchanged, but this is not possible
with the current qcow2 on-disk format where the header.size field is
global instead of per-snapshot.  Others may expect snapshots to change
size along with the current image data.  I think it is safest to not
support snapshots and perhaps add behavior later if there is a
consensus.

Backing images continue to work.  If the image is now larger than its
backing image, zeroes are read when accessing beyond the end of the
backing image.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
v2:
 * Remove superfluous qcow2_grow_l1_table()-related changes

 block/qcow2.c |   45 +++++++++++++++++++++++++++++++++++++++++----
 block/qcow2.h |    6 ++++++
 2 files changed, 47 insertions(+), 4 deletions(-)
Kevin Wolf - April 28, 2010, 10:15 a.m.
Am 28.04.2010 11:24, schrieb Stefan Hajnoczi:
> This patch adds the ability to grow qcow2 images in-place using
> bdrv_truncate().  This enables qemu-img resize command support for
> qcow2.
> 
> Snapshots are not supported and bdrv_truncate() will return -ENOTSUP.
> The notion of resizing an image with snapshots could lead to confusion:
> users may expect snapshots to remain unchanged, but this is not possible
> with the current qcow2 on-disk format where the header.size field is
> global instead of per-snapshot.  Others may expect snapshots to change
> size along with the current image data.  I think it is safest to not
> support snapshots and perhaps add behavior later if there is a
> consensus.
> 
> Backing images continue to work.  If the image is now larger than its
> backing image, zeroes are read when accessing beyond the end of the
> backing image.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
> v2:
>  * Remove superfluous qcow2_grow_l1_table()-related changes
> 
>  block/qcow2.c |   45 +++++++++++++++++++++++++++++++++++++++++----
>  block/qcow2.h |    6 ++++++
>  2 files changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 4fa3ff9..a65af41 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -140,7 +140,7 @@ static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
>  static int qcow_open(BlockDriverState *bs, int flags)
>  {
>      BDRVQcowState *s = bs->opaque;
> -    int len, i, shift;
> +    int len, i;
>      QCowHeader header;
>      uint64_t ext_end;
>  
> @@ -188,8 +188,7 @@ static int qcow_open(BlockDriverState *bs, int flags)
>  
>      /* read the level 1 table */
>      s->l1_size = header.l1_size;
> -    shift = s->cluster_bits + s->l2_bits;
> -    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
> +    s->l1_vm_state_index = size_to_l1(s, header.size);
>      /* the L1 table must contain at least enough entries to put
>         header.size bytes */
>      if (s->l1_size < s->l1_vm_state_index)
> @@ -851,6 +850,42 @@ static int qcow_make_empty(BlockDriverState *bs)
>      return 0;
>  }
>  
> +static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int ret, new_l1_size;
> +
> +    if (offset & 511) {
> +        return -EINVAL;
> +    }
> +
> +    /* cannot proceed if image has snapshots */
> +    if (s->nb_snapshots) {
> +        return -ENOTSUP;
> +    }
> +
> +    /* shrinking is currently not supported */
> +    if (offset < bs->total_sectors * 512) {
> +        return -ENOTSUP;
> +    }
> +
> +    new_l1_size = size_to_l1(s, offset);
> +    ret = qcow2_grow_l1_table(bs, new_l1_size);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* write updated header.size */
> +    offset = cpu_to_be64(offset);
> +    if (bdrv_pwrite(bs->file, offsetof(QCowHeader, size), &offset,
> +                    sizeof(uint64_t)) != sizeof(uint64_t)) {
> +        return -EIO;
> +    }

Please return the right error code here. Also, bdrv_pwrite doesn't
return short writes, so the code can look like this:

ret = bdrv_pwrite(...);
if (ret < 0) {
    return ret;
}

Otherwise I like the patch. It has actually become simple and easy to
understand.

Kevin

Patch

diff --git a/block/qcow2.c b/block/qcow2.c
index 4fa3ff9..a65af41 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -140,7 +140,7 @@  static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
 static int qcow_open(BlockDriverState *bs, int flags)
 {
     BDRVQcowState *s = bs->opaque;
-    int len, i, shift;
+    int len, i;
     QCowHeader header;
     uint64_t ext_end;
 
@@ -188,8 +188,7 @@  static int qcow_open(BlockDriverState *bs, int flags)
 
     /* read the level 1 table */
     s->l1_size = header.l1_size;
-    shift = s->cluster_bits + s->l2_bits;
-    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
+    s->l1_vm_state_index = size_to_l1(s, header.size);
     /* the L1 table must contain at least enough entries to put
        header.size bytes */
     if (s->l1_size < s->l1_vm_state_index)
@@ -851,6 +850,42 @@  static int qcow_make_empty(BlockDriverState *bs)
     return 0;
 }
 
+static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, new_l1_size;
+
+    if (offset & 511) {
+        return -EINVAL;
+    }
+
+    /* cannot proceed if image has snapshots */
+    if (s->nb_snapshots) {
+        return -ENOTSUP;
+    }
+
+    /* shrinking is currently not supported */
+    if (offset < bs->total_sectors * 512) {
+        return -ENOTSUP;
+    }
+
+    new_l1_size = size_to_l1(s, offset);
+    ret = qcow2_grow_l1_table(bs, new_l1_size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* write updated header.size */
+    offset = cpu_to_be64(offset);
+    if (bdrv_pwrite(bs->file, offsetof(QCowHeader, size), &offset,
+                    sizeof(uint64_t)) != sizeof(uint64_t)) {
+        return -EIO;
+    }
+
+    s->l1_vm_state_index = new_l1_size;
+    return 0;
+}
+
 /* XXX: put compressed sectors first, then all the cluster aligned
    tables to avoid losing bytes in alignment */
 static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
@@ -1050,7 +1085,9 @@  static BlockDriver bdrv_qcow2 = {
     .bdrv_aio_readv	= qcow_aio_readv,
     .bdrv_aio_writev	= qcow_aio_writev,
     .bdrv_aio_flush	= qcow_aio_flush,
-    .bdrv_write_compressed = qcow_write_compressed,
+
+    .bdrv_truncate          = qcow2_truncate,
+    .bdrv_write_compressed  = qcow_write_compressed,
 
     .bdrv_snapshot_create   = qcow2_snapshot_create,
     .bdrv_snapshot_goto     = qcow2_snapshot_goto,
diff --git a/block/qcow2.h b/block/qcow2.h
index 5bd08db..01053b7 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -150,6 +150,12 @@  static inline int size_to_clusters(BDRVQcowState *s, int64_t size)
     return (size + (s->cluster_size - 1)) >> s->cluster_bits;
 }
 
+static inline int size_to_l1(BDRVQcowState *s, int64_t size)
+{
+    int shift = s->cluster_bits + s->l2_bits;
+    return (size + (1ULL << shift) - 1) >> shift;
+}
+
 static inline int64_t align_offset(int64_t offset, int n)
 {
     offset = (offset + n - 1) & ~(n - 1);