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Re: [Qemu-block] [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic

From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic limits to block layer
Date: Thu, 16 Jun 2016 13:13:49 +0800
User-agent: Mutt/1.6.1 (2016-04-27)

On Tue, 06/14 15:30, 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>

Reviewed-by: Fam Zheng <address@hidden>

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