linux-kernel - Re: [GIT PULL] Ext3 latency fixes

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Message-ID: <20090405002013.GB7553@mit.edu>
Date:	Sat, 4 Apr 2009 20:20:13 -0400
From:	Theodore Tso <tytso@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 latency fixes

On Sat, Apr 04, 2009 at 03:13:13PM -0700, Linus Torvalds wrote:
> And yes, anticipatory seems to be quite noticeably better than cfq here. 
> With cfq I got a few two-second delays on 'ftruncate()' too (probably 
> because of your new serialization code?), and the longest fsync() delay 
> was over 7 seconds. That was definitely solidly in the "painful" category.

What is going on with in your "ftruncate()" case?  The synchronization
code I added will do call filemap_flush() on close if the file
descriptor had been previously truncated down to zero, either because
it was opened with O_TRUNCATE, or if ftruncate(fd, 0) was explicitly
called.  But it won't actually call fsync() or do anything special on
the actual ftrucate() call; it just sets a flag indicating that the
file in question should be flushed on close.

This is to make the right thing happen for applications which try to
edit a file in place via:

     fd = open("foo", O_RDWR);
     len = read(fd, buf, MAXBUF);
     <modify buf>
     ftruncate(fd, 0);
     write(fd, buf, len);
     close(fd);

Otherwise, given the lack of fsync(fd) in the above sequence, a crash
may leave he file "foo" truncated or only partially written out.

        	     	  		     - Ted
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