> 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 cleancache) - A clustered FS should invoke the "shared_init_fs" cleancache hook to get best performance for some backends. Does that help? Thanks, Dan -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
