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authorKevin Wolf <kwolf@redhat.com>2013-10-29 12:18:58 +0100
committerKevin Wolf <kwolf@redhat.com>2013-10-29 13:10:26 +0100
commitb94a2610573cd9314f244207c8b04cb75e42d7f8 (patch)
tree07be4507b5678548b73b1a822d38e990417a2f76
parent87a5debd3161d24a7d4c685e3c0d8765b5d92a74 (diff)
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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>
-rw-r--r--block.c7
-rw-r--r--block/raw-posix.c9
-rw-r--r--block/raw-win32.c4
-rw-r--r--block/raw_bsd.c1
-rw-r--r--include/block/block_int.h3
5 files changed, 17 insertions, 7 deletions
diff --git a/block.c b/block.c
index 61795fe..58efb5b 100644
--- a/block.c
+++ b/block.c
@@ -2869,9 +2869,10 @@ int64_t bdrv_getlength(BlockDriverState *bs)
if (!drv)
return -ENOMEDIUM;
- if (bdrv_dev_has_removable_media(bs)) {
- if (drv->bdrv_getlength) {
- return drv->bdrv_getlength(bs);
+ if (drv->has_variable_length) {
+ int ret = refresh_total_sectors(bs, bs->total_sectors);
+ if (ret < 0) {
+ return ret;
}
}
return bs->total_sectors * BDRV_SECTOR_SIZE;
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 6f03fbf..f6d48bb 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 676b570..2741e4d 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 0078c1b..2265dcc 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,
diff --git a/include/block/block_int.h b/include/block/block_int.h
index a48731d..1666066 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -156,8 +156,11 @@ struct BlockDriver {
const char *protocol_name;
int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset);
+
int64_t (*bdrv_getlength)(BlockDriverState *bs);
+ bool has_variable_length;
int64_t (*bdrv_get_allocated_file_size)(BlockDriverState *bs);
+
int (*bdrv_write_compressed)(BlockDriverState *bs, int64_t sector_num,
const uint8_t *buf, int nb_sectors);