linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 2 Apr 2009 09:33:44 -0700
From:	David Rees <drees76@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Janne Grunau <j@...nau.net>,
	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, Apr 2, 2009 at 9:09 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Thu, 2 Apr 2009 13:05:32 +0200 Janne Grunau <j@...nau.net> wrote:
>> MythTV calls fsync every few seconds on ongoing recordings to prevent
>> stalls due to large cache writebacks on ext3.
>
> It should use sync_file_range(SYNC_FILE_RANGE_WRITE).  That will
>
> - have minimum latency.  It tries to avoid blocking at all.
> - avoid writing metadata
> - avoid syncing other unrelated files within ext3
> - avoid waiting for the ext3 commit to complete.

MythTV actually uses fdatasync, not fsync (or at least that's what it
did last time I looked at the source).  Not sure how the behavior of
fdatasync compares to sync_file_range.

Either way - forcing the data to be synced to disk a couple times
every second is a hack and causes fragmentation in filesystems without
delayed allocation.  Fragmentation really goes up if you are recording
multiple shows at once.

-Dave
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