Re: Is stability a joke?

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 12 Sep 2016 08:20:20 -0400
schrieb "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> On 2016-09-11 09:02, Hugo Mills wrote:
> > On Sun, Sep 11, 2016 at 02:39:14PM +0200, Waxhead wrote:  
> >> Martin Steigerwald wrote:  
>  [...]  
>  [...]  
>  [...]  
>  [...]  
> >> That is exactly the same reason I don't edit the wiki myself. I
> >> could of course get it started and hopefully someone will correct
> >> what I write, but I feel that if I start this off I don't have deep
> >> enough knowledge to do a proper start. Perhaps I will change my
> >> mind about this.  
> >
> >    Given that nobody else has done it yet, what are the odds that
> > someone else will step up to do it now? I would say that you should
> > at least try. Yes, you don't have as much knowledge as some others,
> > but if you keep working at it, you'll gain that knowledge. Yes,
> > you'll probably get it wrong to start with, but you probably won't
> > get it *very* wrong. You'll probably get it horribly wrong at some
> > point, but even the more knowledgable people you're deferring to
> > didn't identify the problems with parity RAID until Zygo and Austin
> > and Chris (and others) put in the work to pin down the exact
> > issues.  
> FWIW, here's a list of what I personally consider stable (as in, I'm 
> willing to bet against reduced uptime to use this stuff on production 
> systems at work and personal systems at home):
> 1. Single device mode, including DUP data profiles on single device 
> without mixed-bg.
> 2. Multi-device raid0, raid1, and raid10 profiles with symmetrical 
> devices (all devices are the same size).
> 3. Multi-device single profiles with asymmetrical devices.
> 4. Small numbers (max double digit) of snapshots, taken at infrequent 
> intervals (no more than once an hour).  I use single snapshots
> regularly to get stable images of the filesystem for backups, and I
> keep hourly ones of my home directory for about 48 hours.
> 5. Subvolumes used to isolate parts of a filesystem from snapshots.
> I use this regularly to isolate areas of my filesystems from backups.
> 6. Non-incremental send/receive (no clone source, no parent's, no 
> deduplication).  I use this regularly for cloning virtual machines.
> 7. Checksumming and scrubs using any of the profiles I've listed
> above. 8. Defragmentation, including autodefrag.
> 9. All of the compat_features, including no-holes and skinny-metadata.
> 
> Things I consider stable enough that I'm willing to use them on my 
> personal systems but not systems at work:
> 1. In-line data compression with compress=lzo.  I use this on my
> laptop and home server system.  I've never had any issues with it
> myself, but I know that other people have, and it does seem to make
> other things more likely to have issues.
> 2. Batch deduplication.  I only use this on the back-end filesystems
> for my personal storage cluster, and only because I have multiple
> copies as a result of GlusterFS on top of BTRFS.  I've not had any
> significant issues with it, and I don't remember any reports of data
> loss resulting from it, but it's something that people should not be
> using if they don't understand all the implications.

I could at least add one "don't do it":

Don't use BFQ patches (it's an IO scheduler) if you're using btrfs.
Some people like to use it especially for running VMs and desktops
because it provides very good interactivity while maintaining very good
throughput. But it completely destroyed my btrfs beyond repair at least
twice, either while actually using a VM (in VirtualBox) or during high
IO loads. I now stick to the deadline scheduler instead which provides
very good interactivity for me, too, and the corruptions didn't occur
again so far.

The story with BFQ has always been the same: System suddenly freezes
during moderate to high IO until all processes stop working (no process
shows D state, tho). Only hard reboot possible. After rebooting, access
to some (unrelated) files may fail with "errno=-17 Object already
exists" which cannot be repaired. If it affects files needed during
boot, you are screwed because file system goes RO.

-- 
Regards,
Kai

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