Re: [Qemu-devel] [PATCH 0/5] Auto-fragment large transactions at the block layer

[Eric Blake <eblake@redhat.com>]

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On 06/20/2016 05:39 PM, Eric Blake wrote:
> We have max_transfer documented in BlockLimits, but while we
> honor it during pwrite_zeroes, we were blindly ignoring it
> during pwritev and preadv, leading to multiple drivers having
> to implement fragmentation themselves.  This series moves
> fragmentation to the block layer, then fixes the NBD driver to
> use it; if you like this but it needs a v2, you can request that
> I further do other drivers (I know at least iscsi and qcow2 do
> some self-fragmenting and/or error reporting that can be
> simplified by deferring fragmentation to the block layer).

iscsi turned out to be easy (see 6/5), but qcow2 is too tricky (we still
have to fragment in qcow2_alloc_cluster_offset() if cluster allocation
is not contiguous, so there is no one good value of max_transfer to use)

> 
> Prequisite: Kevin's block branch, plus my work on byte-based
> block limits (v2 at the moment):
> https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg04006.html
> 
> Also available as a tag at:
> git fetch git://repo.or.cz/qemu/ericb.git nbd-fragment-v1
> 
> Eric Blake (5):
>   block: Fragment reads to max transfer length
>   block: Fragment writes to max transfer length
>   raw_bsd: Don't advertise flags not supported by protocol layer
>   nbd: Rely on block layer to break up large requests
>   nbd: Drop unused offset parameter
> 
>  include/block/nbd.h |  1 -
>  nbd/nbd-internal.h  |  4 +--
>  block/io.c          | 84 +++++++++++++++++++++++++++++++++++++++--------------
>  block/nbd-client.c  | 78 ++++++++++++++-----------------------------------
>  block/nbd.c         | 12 ++------
>  block/raw_bsd.c     |  6 ++--
>  nbd/common.c        |  3 +-
>  7 files changed, 95 insertions(+), 93 deletions(-)
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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