geom_raid5 inclusion in HEAD?

Arne Wörner arne_woerner at yahoo.com
Tue Nov 6 12:00:24 PST 2007


--- John Nielsen <john at jnielsen.net> wrote:
> I would like to see this get wider testing and eventually be included
> as well. A few questions (some of which I know the answer to but would
> like to have you field for the list):
> 
> Where can I get/test/review the code?
>
http://wgboome.homepage.t-online.de./geom_raid5.tbz
  aka TOS (the original *ah*eh*... maybe B4 would be a nice name, too)
  the most stable version
  but slower than TNG/PP, if there r many requests within a short time,
    that could be combined...
http://wgboome.homepage.t-online.de./geom_raid5-eff.tbz
  aka TNG
  quite memory hungry, but faster than without any "-..."
http://wgboome.homepage.t-online.de./geom_raid5-pp.tbz
  pp = double-plus (I have a little thing with "1984" (by Orwell or so))
  not as memory hungry as "-tng" and possibly not so fast...
  but more memory hungry than without any "-..."
  I like PP's code best (it's the cleanest of all three)...

> Is there still any focus on the "previous" generation code or is "TNG"
> where everything is happening now? Which generation are you talking
> about above?
>
TNG was just for fun, because we wanted to know, if "bcopy" causes a big
performance penalty (that is why TNG has a big memory appetite - we chose a
quick and dirty approach in order to eliminate most bcopy-calls)...

I personally like "-pp" best and I think it should be as stable as TOS...

But the only often tested version is TOS...
The test was: Several GB (from my TV cards) every day for several months...
There were just minor things, that are fixed now... I tested PP in this way,
too, but not so long (today I use *cough* another *cough* OS, that supports my
TV cards better for CO2 output minimization reasons)...

> Does the code in the FreeBSD perforce repository represent the latest
> improvements? Which generation?
>
In the perforce repository is the TOS version...

> How much and what kinds of testing has it already received?
>
Just that real life test...
I did a consitency test with TOS according to Pawel's recommendation:
1. create this: gmirror (graid5 (3 disks), graid3 (3 disks))
2. write some random data with raidtest(I dont know if it can do?) or with
UFS+dd
3. wait for the gmirror device to enter state "SYNC-ED" (or how it is
called)...
4. compare contents of the graid5 and graid3 device (they should be equal)...

> What is the relationship between the FreeNAS project and
> the geom_raid5 developer(s)?
>
Hmm... I dont know... I never saw one of them... And I am not a developer in
the FreeNAS project...

> What version(s) of the code is/are in FreeNAS?
>
I think, they use TOS, because they like to run on low memory boxes...

> Has the
> FreeNAS project or anyone else done any (preferrably thorough and
> repeatable) testing for correctness, stability and performance?
>
They did some HOWTOs (removing a disk and then running in "degraded" mode and
such things)...
But no real long-term test or correctness test...
I wonder myself, what would happen, if it writes a lot in "degraded" mode...

> Are the
> methods and/or results publicly available?
>
They have a knowledge base with some performance graphs, but I dont remember
the URL...

> Just who is (/are) the developer(s) behind this code?
>
I wanted to see, if I can still write programs... And so I tried to write a
RAID5 kernel module and took the structure from gstripe and gmirror (Pawel J.'s
sources)...
Then I sent some emails to the -geom list and then "fluffles" told me, that I
uploaded a non-working version of graid5... Since then "fluffles" is graid5's
testing officer and makes recommendations about more time efficient
algorithms...
I personally would find it cool, if graid5 would become part of FreeBSD, but if
FreeBSD thinks, that ZFS suffices, it would be OK for me...

Psychologically I personally would say, that I have ambivalent feelings about
this, because some silly error in graid5 might cause some unhappiness... We
have to judge, if the possible use of graid5 exceeds the risks plus the
effort...

> Where can I get more information? Can the information on your blog be
> considered authoritative?
>
I dont remember what it says...

> What is the status of the code? Are there any known outstanding issues
> or poorly tested functionality/configurations?
>
Spare disk management, automatic disorder alarms, periodic consistency checks
(linux's md (raid5) does that from time to time) are currently not
implemented... Every admin would have to do this with his own scripts...

I would say correctness has not been tested as much as necessary...
Pawel's test with gmirror on top of a graid5 device and some trustworthy device
would be quite good for the first...
But maybe fluffles did that test already... I was somehow not so interested in
graid5 in the last weeks...

> Are there any threads on the freebsd-geom or other mailing lists that
> would be enlightening? Has a call for testing and/or code review been
> made on that list recently?
> 
Not recently... And: "No"...
IIRC they believed, that it is too much code and too complex code... Maybe that
was the polite form of "chaotic/un-maintain-able code"... :-))

-Arne

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