From: Boaz Harrosh [mailto:bharrosh@panasas.com]
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:

quoted

quoted

quoted

On Fri, Jul 23, 2010 at 06:58:03AM -0700, Dan Magenheimer wrote:

quoted

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!

quoted

Anything that need modification of a normal non-shared fs is

utterly

quoted

quoted

quoted

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
  cleancache)

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

Does that help?

Thanks,
Dan

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