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Re: [Qemu-devel] [PATCH v1 01/18] block/io: add bdrv_aio_{preadv, pwrite

From: Pavel Butsykin
Subject: Re: [Qemu-devel] [PATCH v1 01/18] block/io: add bdrv_aio_{preadv, pwritev}
Date: Thu, 24 Nov 2016 13:58:35 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 23.11.2016 17:28, Kevin Wolf wrote:
Am 15.11.2016 um 07:36 hat Pavel Butsykin geschrieben:
It's just byte-based wrappers over bdrv_co_aio_prw_vector(), which provide
  a byte-based interface for AIO read/write.

Signed-off-by: Pavel Butsykin <address@hidden>

I'm in the process to phase out the last users of bdrv_aio_*() so that
this set of interfaces can be removed. I'm doing this because it's an
unnecessary redundancy, we have too many wrapper functions that expose
the same functionality with different syntax. So let's not add new

At first sight, you don't even seem to use bdrv_aio_preadv() for actual
parallelism, but you often have a pattern like this:

     void foo_cb(void *opaque)

     void caller()
         acb = bdrv_aio_preadv(...);

The code will actually become a lot simpler if you use bdrv_co_preadv()
instead because you don't have to have a callback, but you get pure
sequential code.

The part that actually has some parallelism, pcache_readahead_request(),
already creates its own coroutine, so it runs in the background without
using callback-style interfaces.

I used bdrv_co_preadv(), because it conveniently solves the partial
cache hit. To solve the partial cache hit, we need to split a request
into smaller parts, make asynchronous requests and wait for all
requests in one place.

Do you propose to create a coroutine for each part of request? It
seemed to me that bdrv_co_preadv() is a wrapper that allows us to get
rid of the same code.


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