The BlockDriver bdrv_getlength function is called from the I/O code path
when checking that the request falls within the device. Unfortunately
this involves an lseek system call in the raw protocol; every read or
write request will incur this lseek cost.
Jan Kiszka <jan.kis...@siemens.com> identified this issue and its
latency overhead. This patch caches device length in the existing
total_sectors variable so lseek calls can be avoided for fixed size
devices.
Growable devices fall back to the full bdrv_getlength code path because
I have not added logic to detect extending the size of the device in a
write.
Signed-off-by: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com>
---
block.c | 28 ++++++++++++++++++++++------
1 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/block.c b/block.c
index def3400..d5a3ba7 100644
--- a/block.c
+++ b/block.c
@@ -363,6 +363,7 @@ static int bdrv_open_common(BlockDriverState *bs, const
char *filename,
assert(drv != NULL);
bs->file = NULL;
+ bs->total_sectors = 0;
bs->is_temporary = 0;
bs->encrypted = 0;
bs->valid_key = 0;
@@ -416,9 +417,7 @@ static int bdrv_open_common(BlockDriverState *bs, const
char *filename,
}
bs->keep_read_only = bs->read_only = !(open_flags & BDRV_O_RDWR);
- if (drv->bdrv_getlength) {
- bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
- }
+ bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
#ifndef _WIN32
if (bs->is_temporary) {
unlink(filename);
@@ -959,13 +958,26 @@ int bdrv_pwrite(BlockDriverState *bs, int64_t offset,
int bdrv_truncate(BlockDriverState *bs, int64_t offset)
{
BlockDriver *drv = bs->drv;
+ int ret;
if (!drv)
return -ENOMEDIUM;
if (!drv->bdrv_truncate)
return -ENOTSUP;
if (bs->read_only)
return -EACCES;
- return drv->bdrv_truncate(bs, offset);
+ ret = drv->bdrv_truncate(bs, offset);
+ if (ret < 0) {
+ return ret;
+ }
+
+ /* refresh total sectors */
+ if (drv->bdrv_getlength) {
+ bs->total_sectors = 0; /* discard cached value */
+ bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
+ } else {
+ bs->total_sectors = offset >> BDRV_SECTOR_BITS;
+ }
+ return ret;
}
/**
@@ -976,8 +988,12 @@ int64_t bdrv_getlength(BlockDriverState *bs)
BlockDriver *drv = bs->drv;
if (!drv)
return -ENOMEDIUM;
- if (!drv->bdrv_getlength) {
- /* legacy mode */
+
+ /* Fixed size devices use the total_sectors value for speed instead of
+ issuing a length query (like lseek) on each call. Also, legacy block
+ drivers don't provide a bdrv_getlength function and must use
+ total_sectors. */
+ if ((bs->total_sectors && !bs->growable) || !drv->bdrv_getlength) {
return bs->total_sectors * BDRV_SECTOR_SIZE;
}
return drv->bdrv_getlength(bs);
--
1.7.0
