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block: Cache total_sectors to reduce bdrv_getlength calls

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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.kiszka@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 <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2010-04-19 16:56:41 +01:00 committed by Kevin Wolf
parent 4899d10d14
commit 51762288b4
1 changed files with 38 additions and 5 deletions
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43
block.c
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@ -352,6 +352,26 @@ static BlockDriver *find_image_format(const char *filename)
return drv;
}
/**
* Set the current 'total_sectors' value
*/
static int refresh_total_sectors(BlockDriverState *bs, int64_t hint)
{
BlockDriver *drv = bs->drv;
/* query actual device if possible, otherwise just trust the hint */
if (drv->bdrv_getlength) {
int64_t length = drv->bdrv_getlength(bs);
if (length < 0) {
return length;
}
hint = length >> BDRV_SECTOR_BITS;
}
bs->total_sectors = hint;
return 0;
}
/*
* Common part for opening disk images and files
*/
@ -363,6 +383,7 @@ 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 +437,12 @@ 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;
ret = refresh_total_sectors(bs, bs->total_sectors);
if (ret < 0) {
goto free_and_fail;
}
#ifndef _WIN32
if (bs->is_temporary) {
unlink(filename);
@ -959,13 +983,18 @@ 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) {
ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
}
return ret;
}
/**
@ -976,8 +1005,12 @@ 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->growable || !drv->bdrv_getlength) {
return bs->total_sectors * BDRV_SECTOR_SIZE;
}
return drv->bdrv_getlength(bs);