Cleancache: overview
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From: Dan Magenheimer on 23 Jul 2010 13:50 > From: Nitin Gupta [mailto:ngupta(a)vflare.org] > Sent: Friday, July 23, 2010 9:05 AM > To: Dan Magenheimer > Cc: Christoph Hellwig; akpm(a)linux-foundation.org; Chris Mason; > viro(a)zeniv.linux.org.uk; adilger(a)sun.com; tytso(a)mit.edu; > mfasheh(a)suse.com; Joel Becker; matthew(a)wil.cx; linux- > btrfs(a)vger.kernel.org; linux-kernel(a)vger.kernel.org; linux- > fsdevel(a)vger.kernel.org; linux-ext4(a)vger.kernel.org; ocfs2- > devel(a)oss.oracle.com; linux-mm(a)kvack.org; jeremy(a)goop.org; > JBeulich(a)novell.com; Kurt Hackel; npiggin(a)suse.de; Dave Mccracken; > riel(a)redhat.com; avi(a)redhat.com; Konrad Wilk > Subject: Re: [PATCH V3 0/8] Cleancache: overview > > On 07/23/2010 08:14 PM, Dan Magenheimer wrote: > >> From: Christoph Hellwig [mailto:hch(a)infradead.org] > > > >> Also making the ops vector global is just a bad idea. > >> There is nothing making this sort of caching inherently global. > > > > I'm not sure I understand your point, but two very different > > users of cleancache have been provided, and more will be > > discussed at the MM summit next month. > > > > Do you have a suggestion on how to avoid a global ops > > vector while still serving the needs of both existing > > users? > > Maybe introduce cleancache_register(struct cleancache_ops *ops)? > This will allow making cleancache_ops non-global. No value add > but maybe that's cleaner? Oh, OK, that seems reasonable. Dan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Nitin Gupta on 23 Jul 2010 14:40 On 07/23/2010 11:07 PM, Dan Magenheimer wrote: >> From: Dan Magenheimer >> Subject: RE: [PATCH V3 0/8] Cleancache: overview >> >>> From: Christoph Hellwig [mailto:hch(a)infradead.org] >>> Subject: Re: [PATCH V3 0/8] Cleancache: overview >>> >>> 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. >> >> Unless/until all filesystems are 100% built on top of VFS, >> I have to disagree. Abstractions (e.g. VFS) are never perfect. > > After thinking about this some more, I can see a way > to enforce "opt-in" in the cleancache backend without > any changes to non-generic fs code. I think it's a horrible > hack and we can try it, but I expect fs maintainers > would prefer the explicit one-line-patch opt-in. > > 1) Cleancache backend maintains a list of "known working" > filesystems (those that have been tested). Checks against "known working list" indeed looks horrible. Isn't there any way to identify pagecache -> disk I/O boundaries which every filesystem obeys? I'm not yet sure but if this is doable, then we won't require such hacks. > > 2) Nitin's proposed changes pass the *sb as a parameter. > The string name of the filesystem type is available via > sb->s_type->name. This can be compared against > the "known working" list. > sb->s_magic could also be used, or better if we can somehow get rid of these checks :) > Using the sb pointer as a "handle" requires an extra > table search on every cleancache get/put/flush, > and fs/super.c changes are required for fs unmount > notification anyway (e.g. to call cleancache_flush_fs) > so I'd prefer to keep the cleancache_poolid addition > to the sb. I'll assume this is OK since this is in generic > fs code. > I will also try making changes to cleancache so it does not touch any fs specific code. Though IMHO one liners to fs-code should really be acceptable but unfortunately this doesn't seem to be the case. Maybe generic cleancache will have better chances. Thanks, Nitin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Dan Magenheimer on 23 Jul 2010 17:20 > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Boaz Harrosh on 3 Aug 2010 12:30 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" Thanks Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Dan Magenheimer on 3 Aug 2010 13:40
> From: Boaz Harrosh [mailto:bharrosh(a)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: > >>> 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-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |