diff options
| author | Kevin Wolf <kwolf@redhat.com> | 2013-10-29 12:18:58 +0100 |
|---|---|---|
| committer | Kevin Wolf <kwolf@redhat.com> | 2013-10-29 13:10:26 +0100 |
| commit | b94a2610573cd9314f244207c8b04cb75e42d7f8 (patch) | |
| tree | 07be4507b5678548b73b1a822d38e990417a2f76 /block | |
| parent | 87a5debd3161d24a7d4c685e3c0d8765b5d92a74 (diff) | |
block: Avoid unecessary drv->bdrv_getlength() calls
The block layer generally keeps the size of an image cached in
bs->total_sectors so that it doesn't have to perform expensive
operations to get the size whenever it needs it.
This doesn't work however when using a backend that can change its size
without qemu being aware of it, i.e. passthrough of removable media like
CD-ROMs or floppy disks. For this reason, the caching is disabled when a
removable device is used.
It is obvious that checking whether the _guest_ device has removable
media isn't the right thing to do when we want to know whether the size
of the host backend can change. To make things worse, non-top-level
BlockDriverStates never have any device attached, which makes qemu
assume they are removable, so drv->bdrv_getlength() is always called on
the protocol layer. In the case of raw-posix, this causes unnecessary
lseek() system calls, which turned out to be rather expensive.
This patch completely changes the logic and disables bs->total_sectors
caching only for certain block driver types, for which a size change is
expected: host_cdrom and host_floppy on POSIX, host_device on win32; also
the raw format in case it sits on top of one of these protocols, but in
the common case the nested bdrv_getlength() call on the protocol driver
will use the cache again and avoid an expensive drv->bdrv_getlength()
call.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'block')
| -rw-r--r-- | block/raw-posix.c | 9 | ||||
| -rw-r--r-- | block/raw-win32.c | 4 | ||||
| -rw-r--r-- | block/raw_bsd.c | 1 |
3 files changed, 10 insertions, 4 deletions
diff --git a/block/raw-posix.c b/block/raw-posix.c index 6f03fbf793..f6d48bbdb2 100644 --- a/block/raw-posix.c +++ b/block/raw-posix.c @@ -1715,7 +1715,8 @@ static BlockDriver bdrv_host_floppy = { .bdrv_aio_flush = raw_aio_flush, .bdrv_truncate = raw_truncate, - .bdrv_getlength = raw_getlength, + .bdrv_getlength = raw_getlength, + .has_variable_length = true, .bdrv_get_allocated_file_size = raw_get_allocated_file_size, @@ -1824,7 +1825,8 @@ static BlockDriver bdrv_host_cdrom = { .bdrv_aio_flush = raw_aio_flush, .bdrv_truncate = raw_truncate, - .bdrv_getlength = raw_getlength, + .bdrv_getlength = raw_getlength, + .has_variable_length = true, .bdrv_get_allocated_file_size = raw_get_allocated_file_size, @@ -1951,7 +1953,8 @@ static BlockDriver bdrv_host_cdrom = { .bdrv_aio_flush = raw_aio_flush, .bdrv_truncate = raw_truncate, - .bdrv_getlength = raw_getlength, + .bdrv_getlength = raw_getlength, + .has_variable_length = true, .bdrv_get_allocated_file_size = raw_get_allocated_file_size, diff --git a/block/raw-win32.c b/block/raw-win32.c index 676b5701db..2741e4de10 100644 --- a/block/raw-win32.c +++ b/block/raw-win32.c @@ -616,7 +616,9 @@ static BlockDriver bdrv_host_device = { .bdrv_aio_writev = raw_aio_writev, .bdrv_aio_flush = raw_aio_flush, - .bdrv_getlength = raw_getlength, + .bdrv_getlength = raw_getlength, + .has_variable_length = true, + .bdrv_get_allocated_file_size = raw_get_allocated_file_size, }; diff --git a/block/raw_bsd.c b/block/raw_bsd.c index 0078c1baeb..2265dcc03f 100644 --- a/block/raw_bsd.c +++ b/block/raw_bsd.c @@ -178,6 +178,7 @@ static BlockDriver bdrv_raw = { .bdrv_co_get_block_status = &raw_co_get_block_status, .bdrv_truncate = &raw_truncate, .bdrv_getlength = &raw_getlength, + .has_variable_length = true, .bdrv_get_info = &raw_get_info, .bdrv_is_inserted = &raw_is_inserted, .bdrv_media_changed = &raw_media_changed, |