From: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> 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> Signed-off-by: Kevin Wolf <kw...@redhat.com> --- block.c | 43 ++++++++++++++++++++++++++++++++++++++----- 1 files changed, 38 insertions(+), 5 deletions(-) diff --git a/block.c b/block.c index b318355..91fecab 100644 --- a/block.c +++ b/block.c @@ -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); -- 1.6.6.1