graid3 - requirements or manpage wrong?

[Eirik Øverby]

On 24. Nov 2004, at 18:11, Pawel Jakub Dawidek wrote:

> On Wed, Nov 24, 2004 at 10:54:07AM +0100, Eirik ?verby wrote:
> +> to the best of my ability I have been investigating the 'real' 
> +> requirements of a raid-3 array, and cannot see that the following 
> text 
> +> from graid3(8) cannot possibly be correct - and if it is, then the 
> +> implementation must be wrong or incomplete (emphasis added):
> +> 
> +> label      Create a RAID3 device.  The last given component will 
> contain
> +>                parity data, all the rest - regular data.  ***Number 
> of 
> +> compo-
> +>                nents has to be equal to 3, 5, 9, 17, etc. (2^n + 
> 1).***
> +> 
> +> I might be wrong, but I cannot see how a raid-3 array should 
> require 
> +> (2^n + 1) drives - I am fairly certain I have seen raid-3 arrays 
> +> consisting of four drives, for example. This is also what I had 
> hoped to 
> +> accomplish.
>
> This requirement is because we want sectorsize to be power of 2
> (UFS needs it).
> In RAID3 we want to send every I/O request to all components at once,
> that's why we need sector size to be N*512, where N is a power of 2 
> value
> AND because graid3 uses one parity component we need N+1 providers.

OK I see, makes sense. So it's not really a raid3 issue, but an 
implementation issue.
The only problem then is - gvinum being in a completely unusable state 
(for raid5 anyway), what are my alternatives? I have four 160gb IDE 
drives, and I want capacity+redundancy. Performance is a non-issue, 
really. What do I do - in software?

/Eirik

>
>
> -- 
> Pawel Jakub Dawidek                       http://www.FreeBSD.org
> pjd at FreeBSD.org                           http://garage.freebsd.pl
> FreeBSD committer                         Am I Evil? Yes, I Am!
On 24. Nov 2004, at 18:11, Pawel Jakub Dawidek wrote:

On Wed, Nov 24, 2004 at 10:54:07AM +0100, Eirik ?verby wrote:
+> to the best of my ability I have been investigating the 'real' 
+> requirements of a raid-3 array, and cannot see that the following 
text 
+> from graid3(8) cannot possibly be correct - and if it is, then the 
+> implementation must be wrong or incomplete (emphasis added):
+> 
+> label      Create a RAID3 device.  The last given component will 
contain
+>                parity data, all the rest - regular data.  ***Number 
of 
+> compo-
+>                nents has to be equal to 3, 5, 9, 17, etc. (2^n + 
1).***
+> 
+> I might be wrong, but I cannot see how a raid-3 array should require 
+> (2^n + 1) drives - I am fairly certain I have seen raid-3 arrays 
+> consisting of four drives, for example. This is also what I had 
hoped to 
+> accomplish.

This requirement is because we want sectorsize to be power of 2
(UFS needs it).
In RAID3 we want to send every I/O request to all components at once,
that's why we need sector size to be N*512, where N is a power of 2 
value
AND because graid3 uses one parity component we need N+1 providers.

OK I see, makes sense. So it's not really a raid3 issue, but an 
implementation issue.
The only problem then is - gvinum being in a completely unusable state 
(for raid5 anyway), what are my alternatives? I have four 160gb IDE 
drives, and I want capacity+redundancy. Performance is a non-issue, 
really. What do I do - in software?

/Eirik



-- 
Pawel Jakub Dawidek                       http://www.FreeBSD.org
pjd at FreeBSD.org                           http://garage.freebsd.pl
FreeBSD committer                         Am I Evil? Yes, I Am!