From: Mikulas Patocka <mpatocka@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Vishal Verma <vishal.l.verma@intel.com>,
Dave Jiang <dave.jiang@intel.com>,
Ira Weiny <ira.weiny@intel.com>,
Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
Eric Sandeen <esandeen@redhat.com>,
Dave Chinner <dchinner@redhat.com>,
"Kani, Toshi" <toshi.kani@hpe.com>,
"Norton, Scott J" <scott.norton@hpe.com>,
"Tadakamadla,
Rajesh (DCIG/CDI/HPS Perf)" <rajesh.tadakamadla@hpe.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: NVFS XFS metadata (was: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache)
Date: Wed, 23 Sep 2020 13:19:42 -0400 (EDT) [thread overview]
Message-ID: <alpine.LRH.2.02.2009230445030.1800@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <20200923024528.GD12096@dread.disaster.area>
On Wed, 23 Sep 2020, Dave Chinner wrote:
> > > dir-test /mnt/test/linux-2.6 63000 1048576
> > > nvfs 6.6s
> > > ext4 dax 8.4s
> > > xfs dax 12.2s
> > >
> > >
> > > dir-test /mnt/test/linux-2.6 63000 1048576 link
> > > nvfs 4.7s
> > > ext4 dax 5.6s
> > > xfs dax 7.8s
> > >
> > > dir-test /mnt/test/linux-2.6 63000 1048576 dir
> > > nvfs 8.2s
> > > ext4 dax 15.1s
> > > xfs dax 11.8s
> > >
> > > Yes, nvfs is faster than both ext4 and XFS on DAX, but it's not a
> > > huge difference - it's not orders of magnitude faster.
> >
> > If I increase the size of the test directory, NVFS is order of magnitude
> > faster:
> >
> > time dir-test /mnt/test/ 2000000 2000000
> > NVFS: 0m29,395s
> > XFS: 1m59,523s
> > EXT4: 1m14,176s
>
> What happened to NVFS there? The runtime went up by a factor of 5,
> even though the number of ops performed only doubled.
This test is from a different machine (K10 Opteron) than the above test
(Skylake Xeon). I borrowed the Xeon for a short time and I no longer have
access to it.
> > time dir-test /mnt/test/ 8000000 8000000
> > NVFS: 2m13,507s
> > XFS: 14m31,261s
> > EXT4: reports "file 1976882 can't be created: No space left on device",
> > (although there are free blocks and inodes)
> > Is it a bug or expected behavior?
>
> Exponential increase in runtime for a workload like this indicates
> the XFS journal is too small to run large scale operations. I'm
> guessing you're just testing on a small device?
In this test, the pmem device had 64GiB.
I've created 1TiB ramdisk, formatted it with XFS and ran dir-test 8000000
on it, however it wasn't much better - it took 14m8,824s.
> In which case, you'd get a 16MB log for XFS, which is tiny and most
> definitely will limit performance of any large scale metadta
> operation. Performance should improve significantly for large scale
> operations with a much larger log, and that should bring the XFS
> runtimes down significantly.
Is there some mkfs.xfs option that can increase log size?
> > If you think that the lack of journaling is show-stopper, I can implement
> > it.
>
> I did not say that. My comments are about the requirement for
> atomicity of object changes, not journalling. Journalling is an
> -implementation that can provide change atomicity-, it is not a
> design constraint for metadata modification algorithms.
>
> Really, you can chose how to do object update however you want. What
> I want to review is the design documentation and a correctness proof
> for whatever mechanism you choose to use. Without that information,
> we have absolutely no chance of reviewing the filesystem
> implementation for correctness. We don't need a proof for something
> that uses journalling (because we all know how that works), but for
> something that uses soft updates we most definitely need the proof
> of correctness for the update algorithm before we can determine if
> the implementation is good...
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
I am thinking about this: I can implement lightweight journaling that will
journal just a few writes - I'll allocate some small per-cpu intent log
for that.
For example, in nvfs_rename, we call nvfs_delete_de and nvfs_finish_add -
these functions are very simple, both of them write just one word - so we
can add these two words to the intent log. The same for setattr requesting
simultaneous uid/gid/mode change - they are small, so they'll fit into the
intent log well.
Regarding verifiability, I can do this - the writes to pmem are wrapped in
a macro nv_store. So, I can modify this macro so that it logs all
modifications. Then I take the log, cut it at random time, reorder the
entries (to simulate reordering in the CPU write-combining buffers),
replay it, run nvfsck on it and mount it. This way, we can verify that no
matter where the crash happened, either an old file or a new file is
present in a directory.
Do you agree with that?
Mikulas
next prev parent reply other threads:[~2020-09-23 17:19 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-15 12:34 [RFC] nvfs: a filesystem for persistent memory Mikulas Patocka
2020-09-15 13:00 ` Matthew Wilcox
2020-09-15 13:24 ` Mikulas Patocka
2020-09-22 10:04 ` Ritesh Harjani
2020-09-15 15:16 ` Dan Williams
2020-09-15 16:58 ` Mikulas Patocka
2020-09-15 17:38 ` Mikulas Patocka
2020-09-16 10:57 ` [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache Mikulas Patocka
2020-09-16 16:21 ` Dan Williams
2020-09-16 17:24 ` Mikulas Patocka
2020-09-16 17:40 ` Dan Williams
2020-09-16 18:06 ` Mikulas Patocka
2020-09-21 16:20 ` NVFS XFS metadata (was: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache) Mikulas Patocka
2020-09-22 5:03 ` Dave Chinner
2020-09-22 16:46 ` Mikulas Patocka
2020-09-22 17:25 ` Matthew Wilcox
2020-09-24 15:00 ` Mikulas Patocka
2020-09-28 15:22 ` Mikulas Patocka
2020-09-23 2:45 ` Dave Chinner
2020-09-23 9:20 ` A bug in ext4 with big directories (was: NVFS XFS metadata) Mikulas Patocka
2020-09-23 9:44 ` Jan Kara
2020-09-23 12:46 ` Mikulas Patocka
2020-09-23 17:19 ` Mikulas Patocka [this message]
2020-09-23 9:57 ` NVFS XFS metadata (was: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache) Jan Kara
2020-09-23 13:11 ` Mikulas Patocka
2020-09-23 15:04 ` Matthew Wilcox
2020-09-22 12:28 ` Matthew Wilcox
2020-09-22 12:39 ` Mikulas Patocka
2020-09-16 18:56 ` [PATCH] pmem: fix __copy_user_flushcache Mikulas Patocka
2020-09-18 1:53 ` Dan Williams
2020-09-18 12:25 ` the "read" syscall sees partial effects of the "write" syscall Mikulas Patocka
2020-09-18 13:13 ` Jan Kara
2020-09-18 18:02 ` Linus Torvalds
2020-09-20 23:41 ` Dave Chinner
2020-09-17 6:50 ` [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache Christoph Hellwig
2020-09-21 16:19 ` [RFC] nvfs: a filesystem for persistent memory Mikulas Patocka
2020-09-21 16:29 ` Dan Williams
2020-09-22 15:43 ` Ira Weiny
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