linux-kernel - Re: Status of buffered write path (deadlock fixes)

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Date:	Wed, 13 Dec 2006 08:49:10 -0500
From:	Peter Staubach <staubach@...hat.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Mark Fasheh <mark.fasheh@...cle.com>,
	Linux Memory Management <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Andrew Morton <akpm@...gle.com>
Subject: Re: Status of buffered write path (deadlock fixes)

Trond Myklebust wrote:
> On Wed, 2006-12-13 at 12:56 +1100, Nick Piggin wrote:
>   
>> Note that these pages should be *really* rare. Definitely even for normal
>> filesystems I think RMW would use too much bandwidth if it were required
>> for any significant number of writes.
>>     
>
> If file "foo" exists on the server, and contains data, then something
> like
>
> fd = open("foo", O_WRONLY);
> write(fd, "1", 1);
>
> should never need to trigger a read. That's a fairly common workload
> when you think about it (happens all the time in apps that do random
> write).

I have to admit that I've only been paying attention with one eye, but
why doesn't this require a read?  If "foo" is non-zero in size, then
how does the client determine how much data in the buffer to write to
the server?

Isn't RMW required for any i/o which is either not buffer aligned or
a multiple of the buffer size?

    Thanx...

       ps
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