linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 2 Apr 2009 09:51:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rees <drees76@...il.com>
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, 2 Apr 2009 09:33:44 -0700 David Rees <drees76@...il.com> wrote:

> 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.

fdatasync() will still trigger the bad ext3 behaviour.

> 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.

The file layout issue is unrelated to the frequency of fdatasync() -
the block allocation is done at the time of write().

ext3 _should_ handle this case fairly well nowadays - I thought we fixed that.
However it would probably benefit from having the size of the block reservation
window increased - use ioctl(EXT3_IOC_SETRSVSZ).  That way, each file gets a
decent-sized hunk of disk "reserved" for its ongoing appending.  Other
files won't come in and intermingle their blocks with it.
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