Re: Reducing memory requirements for high extent xfs files

[Michael Nishimoto <miken@agami.com>]

David Chinner wrote:
> On Wed, May 30, 2007 at 09:49:38AM -0700, Michael Nishimoto wrote:
>  > Hello,
>  >
>  > Has anyone done any work or had thoughts on changes required
>  > to reduce the total memory footprint of high extent xfs files?
> 
> We changed the way we do memory allocation to avoid needing
> large contiguous chunks of memory a bit over a year ago;
> that solved the main OOM problem we were getting reported
> with highly fragmented files.
> 
>  > Obviously, it is important to reduce fragmentation as files
>  > are generated and to regularly defrag files, but both of these
>  > alternatives are not complete solutions.
>  >
>  > To reduce memory consumption, xfs could bring in extents
>  > from disk as needed (or just before needed) and could free
>  > up mappings when certain extent ranges have not been recently
>  > accessed.  A solution should become more aggressive about
>  > reclaiming extent mapping memory as free memory becomes limited.
> 
> Yes, it could, but that's a pretty major overhaul of the extent
> interface which currently assumes everywhere that the entire
> extent tree is in core.
> 
> Can you describe the problem you are seeing that leads you to
> ask this question? What's the problem you need to solve?
> 
> Cheers,
> 
> Dave.
> --
> Dave Chinner
> Principal Engineer
> SGI Australian Software Group

I realize that this work won't be trivial which is why I asked if anyone
has thought about all relevant issues.

When using NFS over XFS, slowly growing files (can be ascii log files)
tend to fragment quite a bit.  One system had several hundred files
which required more than one page to store the extents.  Quite a few
files had extent counts greater than 10k, and one file had 120k extents.
Besides the memory consumption, latency to return the first byte of the
file can get noticeable.

   Michael