From owner-freebsd-fs  Thu Nov  4 15:27:48 1999
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To: Greg Lehey <grog@lemis.com>
Cc: freebsd-fs@FreeBSD.ORG
Subject: Re: feature list journalled fs 
In-Reply-To: Message from Greg Lehey <grog@mojave.sitaranetworks.com> 
   of "Thu, 04 Nov 1999 16:13:17 EST." <19991104161317.49512@mojave.sitaranetworks.com> 
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Date: Fri, 05 Nov 1999 00:26:14 +0100
From: Mattias Pantzare <pantzer@ludd.luth.se>
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> On Thursday,  4 November 1999 at  1:20:36 +0100, Mattias Pantzare wrote=
:
> >>   I am under the impression that you can only enlarge a vinum volume=
 if it
> >> in a RAID 0 configuration (concatenation). Obviously, it would be ve=
ry
> >> difficult to enlarge a RAID 1 or RAID 5 configuration as it would re=
quire
> >> restriping the data across all disks; I'm not familiar with any prod=
uct,
> >> hardware or software, that can do this.
> >
> > Solaris DiskSuite almost extends RAID 5 configruations. You can add d=
isks to a
> > RAID 5 set, but the extra disks will only hold data, no parity.
> =

> That's normal for RAID-5.  Only one disk in any stripe contains the
> parity information.

Disk, not stripe.

> =

> > I think that it is a strange mix of RAID 5 and concatenation. All
> > data is still parity protected. It might not be as fast as a true
> > RAID 5, but it can be very usful.
> =

> What's the difference?


If you have 3 disks and 3 stripes and number sectors from 1 to 6:

Disk 1   Disk 2   Disk 3 =

  1        2        P    =

  P        3        4   =

  5        P        6 =


Then add a new disk:

Disk 1   Disk 2   Disk 3   New Disk
  1        2        P        7
  P        3        4        8
  5        P        6        9


All you have to do is recalculate the new parity data when you write new =
data =

if you zero the new disk before using it.

Disk accesses will not be spread out as in normal RAID5, but you still ge=
t =

parity protection.



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