On 6/30/19 10:08 AM, Maxim Levitsky wrote: > Regular block devices (/dev/sda*, /dev/nvme*, etc) interface is not limited
The regular block device interface is
or
Regular block devices interfaces are
> by the underlying storage limits, but rather the kernel block layer
> takes care to split the requests that are too large/fragmented.
>
> Doing so allows us to have less overhead in qemu.
>
> Signed-off-by: Maxim Levitsky <mlevi...@redhat.com>
> ---
> block/file-posix.c | 16 +++++++---------
> 1 file changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index ab05b51a66..66dad34f8a 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1038,15 +1038,13 @@ static void raw_reopen_abort(BDRVReopenState *state)
> s->reopen_state = NULL;
> }
>
> -static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
> +static int sg_get_max_transfer_length(BlockDriverState *bs, int fd)
> {
> #ifdef BLKSECTGET
> int max_bytes = 0;
> - short max_sectors = 0;
> - if (bs->sg && ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
> +
> + if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
> return max_bytes;
> - } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
> - return max_sectors << BDRV_SECTOR_BITS;
> } else {
> return -errno;
> }
> @@ -1055,7 +1053,7 @@ static int
> hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
> #endif
> }
>
> -static int hdev_get_max_segments(const struct stat *st)
> +static int sg_get_max_segments(const struct stat *st)
> {
> #ifdef CONFIG_LINUX
> char buf[32];
> @@ -1106,12 +1104,12 @@ static void raw_refresh_limits(BlockDriverState *bs,
> Error **errp)
> struct stat st;
>
> if (!fstat(s->fd, &st)) {
> - if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) {
> - int ret = hdev_get_max_transfer_length(bs, s->fd);
Is it worth delaying the fstat()...
> + if (bs->sg) {
> + int ret = sg_get_max_transfer_length(bs, s->fd);
> if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
> bs->bl.max_transfer = pow2floor(ret);
> }
> - ret = hdev_get_max_segments(&st);
> + ret = sg_get_max_segments(&st);
...until inside the if (bs->sg) condition, to avoid wasted work for
other scenarios?
> if (ret > 0) {
> bs->bl.max_transfer = MIN(bs->bl.max_transfer,
> ret * getpagesize());
>
Reviewed-by: Eric Blake <ebl...@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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