Messages in this thread | | | Date | Thu, 2 Apr 2009 09:51:39 -0700 | From | Andrew Morton <> | Subject | Re: Linux 2.6.29 |
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On Thu, 2 Apr 2009 09:33:44 -0700 David Rees <drees76@gmail.com> wrote:
> On Thu, Apr 2, 2009 at 9:09 AM, Andrew Morton <akpm@linux-foundation.org> wrote: > > On Thu, 2 Apr 2009 13:05:32 +0200 Janne Grunau <j@jannau.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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