RE: [PATCH V3 0/8] Cleancache: overview

From: Dan Magenheimer
Date: Tue Aug 03 2010 - 13:38:48 EST

> From: Boaz Harrosh [mailto:bharrosh@xxxxxxxxxxx]
> Sent: Tuesday, August 03, 2010 10:23 AM
> To: Dan Magenheimer
> Subject: Re: [PATCH V3 0/8] Cleancache: overview
> On 07/24/2010 12:17 AM, Dan Magenheimer wrote:
> >>> On Fri, Jul 23, 2010 at 06:58:03AM -0700, Dan Magenheimer wrote:
> >>>> CHRISTOPH AND ANDREW, if you disagree and your concerns have
> >>>> not been resolved, please speak up.
> >>
> >> Hi Christoph --
> >>
> >> Thanks very much for the quick (instantaneous?) reply!
> >>
> >>> Anything that need modification of a normal non-shared fs is
> utterly
> >>> broken and you'll get a clear NAK, so the propsal before is a good
> >>> one.
> >>
> >> No, the per-fs opt-in is very sensible; and its design is
> >> very minimal.
> >
> > Not to belabor the point, but maybe the right way to think about
> > this is:
> >
> > Cleancache is a new optional feature provided by the VFS layer
> > that potentially dramatically increases page cache effectiveness
> > for many workloads in many environments at a negligible cost.
> >
> > Filesystems that are well-behaved and conform to certain restrictions
> > can utilize cleancache simply by making a call to cleancache_init_fs
> > at mount time. Unusual, misbehaving, or poorly layered filesystems
> > must either add additional hooks and/or undergo extensive additional
> > testing... or should just not enable the optional cleancache.
> OK, So I maintain a filesystem in Kernel. How do I know if my FS
> is not "Unusual, misbehaving, or poorly layered"

A reasonable question. I'm not a FS expert so this may not be
a complete answer, but please consider it a start:

- The FS should be block-device-based (e.g. a ram-based FS
such as tmpfs should not enable cleancache)

- To ensure coherency/correctness, the FS must ensure that all
file removal or truncation operations either go through VFS
or add hooks to do the equivalent "flush" operations (e.g.
I started looking at FS-cache-based net FS's and was concerned
there might be problems, dunno for sure)

- To ensure coherency/correctness, inode numbers must be unique
(e.g. no emulating 64-bit inode space on 32-bit inode numbers)

- The FS must call the VFS superblock alloc and deactivate routines
or add hooks to do the equivalent cleancache calls done there.

- To maximize performance, all pages fetched from the FS should
go through the do_mpage_readpage routine or the FS should add
hooks to do the equivalent (e.g. btrfs requires a hook for this)

- Currently, the FS blocksize must be the same as PAGESIZE. This
is not an architectural restriction, but no backends currently
support anything different (e.g. hugetlbfs? should not enable

- A clustered FS should invoke the "shared_init_fs" cleancache
hook to get best performance for some backends.

Does that help?

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