Re: [patch 00/14] Page cache cleanup in anticipation of Large Blocksize support

Christoph Lameter
2007-06-15T02:04:36Z

On Thu, 14 Jun 2007, Andrew Morton wrote:

> There will be files which should use 64k but which instead end up using 4k.
>
> There will be files which should use 4k but which instead end up using 64k.
>
> Because determining which size to use requires either operator intervention
> or kernel heuristics, both of which will be highly unreliable.
>
> It's better to just make 4k pages go faster.

Initially its quite easy to have a filesystem for your 4k files (basically
the distro you are running) and an archive for video / audio etc files
that has 64k size for data. In the future filesystem may support sizes set
per directory. Basically if things get to slow you can pull the lever.

> > Magical? There is nothing magical about doing transfers in the size that
> > is supported by a device. That is good sense.
>
> By magical heuristics I'm referring to the (required) tricks and guesses
> which the kernel will need to deploy to be able to guess which page-size it
> should use for each file.
>
> Because without such heuristics, none of this new stuff which you're
> proposing would ever get used by 90% of apps on 90% of machines.

In the patchset V3 one f.e. simply formats a volume by specifying the
desired blocksize. If one gets into trouble with fsck and other slowdown
associated with large file I/O then they are going to be quite fast to
format a partition with larger blocksize. Its a know technology in many
Unixes.

The approach essentially gives one freedom to choose a page size. This is
a tradeoff between desired speed, expected file sizes, filesystem behavior
and acceptable fragmentation overhead. If we do this approach then I think
we will see the mkfs.XXX tools to automatically make intelligent choices
on which page size to use. They are all stuck at 4k at the moment.

> > Of course there is. The seeks are reduced since there are an factor
> > of 16 less metadata blocks. fsck does not read files. It just reads
> > metadata structures. And the larger contiguous areas the faster.
>
> Some metadata is contiguous: inode tables, some directories (if they got
> lucky), bitmap tables. But fsck surely reads them in a single swoop
> anyway, so there's no gain there.

The metadata needs to refer to 1/16th of the earlier pages that need to be
tracked. metadata is shrunk significantly.

> Other metadata (indirect blocks) are 100% discontiguous, and reading those
> with a 64k IO into 64k of memory is completely dumb.

The effect of a larger page size is that the filesystem will
place more meta data into a single page instead of spreading it out.
Reading a mass of meta data with a 64k read is an intelligent choice to
make in particular if there is a large series of such reads.