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Re: [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic limits to blo
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic limits to block layer |
Date: |
Wed, 15 Jun 2016 15:38:19 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0 |
On 14/06/2016 23:30, Eric Blake wrote:
> We were basing the advertisement of maximum discard and transfer
> length off of UINT32_MAX, but since the rest of the block layer
> has signed int limits on a transaction, nothing could ever reach
> that maximum, and we risk overflowing an int once things are
> converted to byte-based rather than sector-based limits. What's
> more, we DO have a much smaller limit: both the current kernel
> and qemu-nbd have a hard limit of 32M on a read or write
> transaction, and while they may also permit up to a full 32 bits
> on a discard transaction, the upstream NBD protocol is proposing
> wording that without any explicit advertisement otherwise,
> clients should limit ALL requests to the same limits as read and
> write, even though the other requests do not actually require as
> many bytes across the wire. So the better limit to tell the
> block layer is 32M for both values.
>
> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v2: new patch
> ---
> block/nbd.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/block/nbd.c b/block/nbd.c
> index 6015e8b..bf67c8a 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -362,8 +362,8 @@ static int nbd_co_flush(BlockDriverState *bs)
>
> static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
> {
> - bs->bl.max_discard = UINT32_MAX >> BDRV_SECTOR_BITS;
> - bs->bl.max_transfer_length = UINT32_MAX >> BDRV_SECTOR_BITS;
> + bs->bl.max_discard = NBD_MAX_SECTORS;
> + bs->bl.max_transfer_length = NBD_MAX_SECTORS;
> }
>
> static int nbd_co_discard(BlockDriverState *bs, int64_t sector_num,
>
Acked-by: Paolo Bonzini <address@hidden>
Cc: address@hidden
- Re: [Qemu-devel] [PATCH v2 04/17] nbd: Allow larger requests, (continued)
- [Qemu-devel] [PATCH v2 01/17] block: Tighter assertions on bdrv_aligned_pwritev(), Eric Blake, 2016/06/14
- [Qemu-devel] [PATCH v2 02/17] block: Document supported flags during bdrv_aligned_preadv(), Eric Blake, 2016/06/14
- [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic limits to block layer, Eric Blake, 2016/06/14
- [Qemu-devel] [PATCH v2 03/17] block: Fix harmless off-by-one in bdrv_aligned_preadv(), Eric Blake, 2016/06/14
- [Qemu-devel] [PATCH v2 13/17] block: Set default request_alignment during bdrv_refresh_limits(), Eric Blake, 2016/06/14
- [Qemu-devel] [PATCH v2 11/17] raw-win32: Set request_alignment during .bdrv_refresh_limits(), Eric Blake, 2016/06/14