Commit 51762288 authored by Stefan Hajnoczi's avatar Stefan Hajnoczi Committed by Kevin Wolf
Browse files

block: Cache total_sectors to reduce bdrv_getlength calls 



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: default avatarStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 4899d10d

block.c

+38 −5
Original line number Diff line number Diff line
@@ -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);