RE: SV: RAID-6 support in kernel?

RE: SV: RAID-6 support in kernel?

[Dale Stephenson]

Kasper Dupont wrote:
> Roy Sigurd Karlsbakk wrote:
> > 
> > Below is a (patented?)
> 
> No problem for me I live in Europe.
> 
> > version that works. This is from the linux-raid list
> > 
> > >  A1   A2  (P1) (PA)
> > > (P2) (PB)  B2   B1
> 
> Nice, looks like it works.
> 
> > >  C4   C3  (PC) (P3)
> > > (PD) (P4)  D3   D4
> 
> In this encoding the roles of disk one and two are
> switched, and three and four are also switched. Are
> there any reason for this?
> 
I liked spreading the row-based and column-based parity across all disks.
Would that matter?  It could, depending on implementation.

Consider an access pattern to the RAID which writes full stripes, and only
the first two lines (repeating) are used as an access pattern.  This might
go something like this:

write A1 & A2 (Reads P1 & P2, calculates P1, PA, P2, writes A1, A2, P1, PA,
P2)
write B2 & B1 (Reads P1 & P2, calculates P1, P2, PB, writes P1, P2, PB, B2,
B1)

Repeat this a bunch of times, and you'll find you are writing twice as many
blocks to Drives 1 & 3 as you are to Drives 2 & 4.  However, if you add the
C and D rows to the scheme, the full-stripe writes spread the column parity
writes evenly across all drives.

Of course, for a 4 drive setup there's no reason to use RAID 6 at all (RAID
10 will withstand any two drive failure if you only use 4 drives), but
that's the reasoning.  I think the best way to deal with the read-modify
write problem for RAID 6 is to use a small chunk size and deal with NxN
chunks as a unit.  But YMMV.

Dale Stephenson
steph@snapserver.com

Re: SV: RAID-6 support in kernel?

[Roy Sigurd Karlsbakk]

> Of course, for a 4 drive setup there's no reason to use RAID 6 at all (RAID
> 10 will withstand any two drive failure if you only use 4 drives), but
> that's the reasoning.  I think the best way to deal with the read-modify
> write problem for RAID 6 is to use a small chunk size and deal with NxN
> chunks as a unit.  But YMMV.

RAID10 will _not_ withstand any two-drive fail in a 4-drive scenario. If D1 
and D3 fail, you're fscked

D1 D2
D3 D4

-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.

Re: SV: RAID-6 support in kernel?

[Roy Sigurd Karlsbakk]

> > > > > RAID-6 layout: http://www.acnc.com/04_01_06.html
> > >
>  > > If it is supposed to survive two arbitrary disk failures something is
> > > wrong with that figure. They store 12 logical sectors in 20 physical
> > > sectors across 4 drives. With two lost disks there are 10 physical
> > > sectors left from which we want to reconstruct 12 logical sectors.
> > > That is impossible.
> >
> > Might be the diagram is wrong.

> Could be the case, so until I find another description I will
> still not know how RAID-6 works.

Below is a (patented?) version that works. This is from the linux-raid list

>  A1   A2  (P1) (PA)
> (P2) (PB)  B2   B1
>  C4   C3  (PC) (P3)
> (PD) (P4)  D3   D4
>
> Disclaimer: I took that from Patent 6,353,895.  If you look it up you'll see
> a lot of different schemes and discussion of XOR-based RAID 6, in language
> disguised as English.  You'll also see that I'm listed as the inventor.
> That's four companies back for me, but my current employer unknowingly
> has some rights to it, so I hope it will see the light of day sometime.
>
> Dale Stephenson
> steph@snapserver.com

-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.
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Re: SV: RAID-6 support in kernel?

[Kasper Dupont]

Roy Sigurd Karlsbakk wrote:
> 
> Below is a (patented?)

No problem for me I live in Europe.

> version that works. This is from the linux-raid list
> 
> >  A1   A2  (P1) (PA)
> > (P2) (PB)  B2   B1

Nice, looks like it works.

> >  C4   C3  (PC) (P3)
> > (PD) (P4)  D3   D4

In this encoding the roles of disk one and two are
switched, and three and four are also switched. Are
there any reason for this?

-- 
Kasper Dupont -- der bruger for meget tid på usenet.
For sending spam use mailto:razor-report@daimi.au.dk
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Re: SV: RAID-6 support in kernel?

[Roy Sigurd Karlsbakk]

> No problem for me I live in Europe.

fine :-)
Norge kaller :-)


There's some talk abuot different raid levels (including 6) here:
http://www.pdl.cmu.edu/RAIDframe/raidframebook.pdf

Hope this helps

roy

-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.