block.c | 2 ++ block/io.c | 7 ++++++- include/block/block-common.h | 1 + qapi/block-core.json | 3 ++- qemu-nbd.c | 2 +- qemu-options.hx | 2 +- storage-daemon/qemu-storage-daemon.c | 2 +- 7 files changed, 14 insertions(+), 5 deletions(-)
When we for example have a sparse qcow2 image and discard: unmap is enabled,
there can be a lot of fragmentation in the image after some time. Surely on VM's
that do a lot of writes/deletes.
This causes the qcow2 image to grow even over 110% of its virtual size,
because the free gaps in the image get to small to allocate new
continuous clusters. So it allocates new space as the end of the image.
Disabling discard is not an option, as discard is needed to keep the
incremental backup size as low as possible. Without discard, the
incremental backups would become large, as qemu thinks it's just dirty
blocks but it doesn't know the blocks are empty/useless.
So we need to avoid fragmentation but also 'empty' the useless blocks in
the image to have a small incremental backup.
There are multiple ways to properly resolve this. One way is to not
discard the blocks on a discard request, but just zero them. This causes
the allocation the still exist, and results in no gaps.
This should also cause a perfectly continuous image when using full
preallocation.
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
---
block.c | 2 ++
block/io.c | 7 ++++++-
include/block/block-common.h | 1 +
qapi/block-core.json | 3 ++-
qemu-nbd.c | 2 +-
qemu-options.hx | 2 +-
storage-daemon/qemu-storage-daemon.c | 2 +-
7 files changed, 14 insertions(+), 5 deletions(-)
diff --git a/block.c b/block.c
index 5ec1a3897e..ed21d115dd 100644
--- a/block.c
+++ b/block.c
@@ -1146,6 +1146,8 @@ int bdrv_parse_discard_flags(const char *mode, int *flags)
/* do nothing */
} else if (!strcmp(mode, "on") || !strcmp(mode, "unmap")) {
*flags |= BDRV_O_UNMAP;
+ } else if (!strcmp(mode, "zeroes")) {
+ *flags |= BDRV_O_UNMAP_ZERO;
} else {
return -1;
}
diff --git a/block/io.c b/block/io.c
index 6fa1993374..dc7592e938 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2988,10 +2988,15 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset,
}
/* Do nothing if disabled. */
- if (!(bs->open_flags & BDRV_O_UNMAP)) {
+ if (!(bs->open_flags & BDRV_O_UNMAP) &&
+ !(bs->open_flags & BDRV_O_UNMAP_ZERO)) {
return 0;
}
+ if (bs->open_flags & BDRV_O_UNMAP_ZERO) {
+ return bdrv_co_pwrite_zeroes(child, offset, bytes, 0);
+ }
+
if (!bs->drv->bdrv_co_pdiscard && !bs->drv->bdrv_aio_pdiscard) {
return 0;
}
diff --git a/include/block/block-common.h b/include/block/block-common.h
index b5122ef8ab..079ee390c3 100644
--- a/include/block/block-common.h
+++ b/include/block/block-common.h
@@ -179,6 +179,7 @@ typedef enum {
#define BDRV_O_AUTO_RDONLY 0x20000 /* degrade to read-only if opening
read-write fails */
#define BDRV_O_IO_URING 0x40000 /* use io_uring instead of the thread pool */
+#define BDRV_O_UNMAP_ZERO 0x80000 /* rewrite guest unmap to zero */
#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_NO_FLUSH)
diff --git a/qapi/block-core.json b/qapi/block-core.json
index c05ad0c07e..0f91d1a6b6 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2971,11 +2971,12 @@
#
# @ignore: Ignore the request
# @unmap: Forward as an unmap request
+# @zeroes: Zero the clusters instead of unmapping (since 8.1)
#
# Since: 2.9
##
{ 'enum': 'BlockdevDiscardOptions',
- 'data': [ 'ignore', 'unmap' ] }
+ 'data': [ 'ignore', 'unmap', 'zeroes' ] }
##
# @BlockdevDetectZeroesOptions:
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 6ff45308a9..6c0b326db4 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -148,7 +148,7 @@ static void usage(const char *name)
" valid options are: 'none', 'writeback' (default),\n"
" 'writethrough', 'directsync' and 'unsafe'\n"
" --aio=MODE set AIO mode (native, io_uring or threads)\n"
-" --discard=MODE set discard mode (ignore, unmap)\n"
+" --discard=MODE set discard mode (ignore, unmap, zeroes)\n"
" --detect-zeroes=MODE set detect-zeroes mode (off, on, unmap)\n"
" --image-opts treat FILE as a full set of image options\n"
"\n"
diff --git a/qemu-options.hx b/qemu-options.hx
index b5efa648ba..7e9d383499 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1209,7 +1209,7 @@ SRST
ERST
DEF("blockdev", HAS_ARG, QEMU_OPTION_blockdev,
- "-blockdev [driver=]driver[,node-name=N][,discard=ignore|unmap]\n"
+ "-blockdev [driver=]driver[,node-name=N][,discard=ignore|unmap|zeroes]\n"
" [,cache.direct=on|off][,cache.no-flush=on|off]\n"
" [,read-only=on|off][,auto-read-only=on|off]\n"
" [,force-share=on|off][,detect-zeroes=on|off|unmap]\n"
diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
index 0e9354faa6..08e8b1b3d9 100644
--- a/storage-daemon/qemu-storage-daemon.c
+++ b/storage-daemon/qemu-storage-daemon.c
@@ -86,7 +86,7 @@ static void help(void)
" specify tracing options\n"
" -V, --version output version information and exit\n"
"\n"
-" --blockdev [driver=]<driver>[,node-name=<N>][,discard=ignore|unmap]\n"
+" --blockdev [driver=]<driver>[,node-name=<N>][,discard=ignore|unmap|zeroes]\n"
" [,cache.direct=on|off][,cache.no-flush=on|off]\n"
" [,read-only=on|off][,auto-read-only=on|off]\n"
" [,force-share=on|off][,detect-zeroes=on|off|unmap]\n"
--
2.40.1Uploaded a new patch which might be better/cleaner :)
"[PATCH] qcow2: add discard-no-unref option"
That patch only applies to qcow2 and also passes unmap further down the
storage stack.
On 10/05/2023 16:27, Jean-Louis Dupond wrote:
> When we for example have a sparse qcow2 image and discard: unmap is enabled,
> there can be a lot of fragmentation in the image after some time. Surely on VM's
> that do a lot of writes/deletes.
> This causes the qcow2 image to grow even over 110% of its virtual size,
> because the free gaps in the image get to small to allocate new
> continuous clusters. So it allocates new space as the end of the image.
>
> Disabling discard is not an option, as discard is needed to keep the
> incremental backup size as low as possible. Without discard, the
> incremental backups would become large, as qemu thinks it's just dirty
> blocks but it doesn't know the blocks are empty/useless.
> So we need to avoid fragmentation but also 'empty' the useless blocks in
> the image to have a small incremental backup.
>
> There are multiple ways to properly resolve this. One way is to not
> discard the blocks on a discard request, but just zero them. This causes
> the allocation the still exist, and results in no gaps.
> This should also cause a perfectly continuous image when using full
> preallocation.
>
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
> Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
> ---
> block.c | 2 ++
> block/io.c | 7 ++++++-
> include/block/block-common.h | 1 +
> qapi/block-core.json | 3 ++-
> qemu-nbd.c | 2 +-
> qemu-options.hx | 2 +-
> storage-daemon/qemu-storage-daemon.c | 2 +-
> 7 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/block.c b/block.c
> index 5ec1a3897e..ed21d115dd 100644
> --- a/block.c
> +++ b/block.c
> @@ -1146,6 +1146,8 @@ int bdrv_parse_discard_flags(const char *mode, int *flags)
> /* do nothing */
> } else if (!strcmp(mode, "on") || !strcmp(mode, "unmap")) {
> *flags |= BDRV_O_UNMAP;
> + } else if (!strcmp(mode, "zeroes")) {
> + *flags |= BDRV_O_UNMAP_ZERO;
> } else {
> return -1;
> }
> diff --git a/block/io.c b/block/io.c
> index 6fa1993374..dc7592e938 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2988,10 +2988,15 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset,
> }
>
> /* Do nothing if disabled. */
> - if (!(bs->open_flags & BDRV_O_UNMAP)) {
> + if (!(bs->open_flags & BDRV_O_UNMAP) &&
> + !(bs->open_flags & BDRV_O_UNMAP_ZERO)) {
> return 0;
> }
>
> + if (bs->open_flags & BDRV_O_UNMAP_ZERO) {
> + return bdrv_co_pwrite_zeroes(child, offset, bytes, 0);
> + }
> +
> if (!bs->drv->bdrv_co_pdiscard && !bs->drv->bdrv_aio_pdiscard) {
> return 0;
> }
> diff --git a/include/block/block-common.h b/include/block/block-common.h
> index b5122ef8ab..079ee390c3 100644
> --- a/include/block/block-common.h
> +++ b/include/block/block-common.h
> @@ -179,6 +179,7 @@ typedef enum {
> #define BDRV_O_AUTO_RDONLY 0x20000 /* degrade to read-only if opening
> read-write fails */
> #define BDRV_O_IO_URING 0x40000 /* use io_uring instead of the thread pool */
> +#define BDRV_O_UNMAP_ZERO 0x80000 /* rewrite guest unmap to zero */
>
> #define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_NO_FLUSH)
>
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index c05ad0c07e..0f91d1a6b6 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -2971,11 +2971,12 @@
> #
> # @ignore: Ignore the request
> # @unmap: Forward as an unmap request
> +# @zeroes: Zero the clusters instead of unmapping (since 8.1)
> #
> # Since: 2.9
> ##
> { 'enum': 'BlockdevDiscardOptions',
> - 'data': [ 'ignore', 'unmap' ] }
> + 'data': [ 'ignore', 'unmap', 'zeroes' ] }
>
> ##
> # @BlockdevDetectZeroesOptions:
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 6ff45308a9..6c0b326db4 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -148,7 +148,7 @@ static void usage(const char *name)
> " valid options are: 'none', 'writeback' (default),\n"
> " 'writethrough', 'directsync' and 'unsafe'\n"
> " --aio=MODE set AIO mode (native, io_uring or threads)\n"
> -" --discard=MODE set discard mode (ignore, unmap)\n"
> +" --discard=MODE set discard mode (ignore, unmap, zeroes)\n"
> " --detect-zeroes=MODE set detect-zeroes mode (off, on, unmap)\n"
> " --image-opts treat FILE as a full set of image options\n"
> "\n"
> diff --git a/qemu-options.hx b/qemu-options.hx
> index b5efa648ba..7e9d383499 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -1209,7 +1209,7 @@ SRST
> ERST
>
> DEF("blockdev", HAS_ARG, QEMU_OPTION_blockdev,
> - "-blockdev [driver=]driver[,node-name=N][,discard=ignore|unmap]\n"
> + "-blockdev [driver=]driver[,node-name=N][,discard=ignore|unmap|zeroes]\n"
> " [,cache.direct=on|off][,cache.no-flush=on|off]\n"
> " [,read-only=on|off][,auto-read-only=on|off]\n"
> " [,force-share=on|off][,detect-zeroes=on|off|unmap]\n"
> diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
> index 0e9354faa6..08e8b1b3d9 100644
> --- a/storage-daemon/qemu-storage-daemon.c
> +++ b/storage-daemon/qemu-storage-daemon.c
> @@ -86,7 +86,7 @@ static void help(void)
> " specify tracing options\n"
> " -V, --version output version information and exit\n"
> "\n"
> -" --blockdev [driver=]<driver>[,node-name=<N>][,discard=ignore|unmap]\n"
> +" --blockdev [driver=]<driver>[,node-name=<N>][,discard=ignore|unmap|zeroes]\n"
> " [,cache.direct=on|off][,cache.no-flush=on|off]\n"
> " [,read-only=on|off][,auto-read-only=on|off]\n"
> " [,force-share=on|off][,detect-zeroes=on|off|unmap]\n"
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