Re: Adding a disk to a RAID array

Elizabeth, it appears your understanding of how RAID arrays work is
incomplete. Note that for a RAID 5 array all data on the virtual disk
it creates is spread out over all three drives at a rather fine grained
level and stored redundantly on all three drives such that it can
be recovered if only two of the drives are working. Adding a single
drive to this sort of a configuration breaks all the redundancy and
would require that it be redistributed so that any three drives working
will allow the data to come off properly. I would be surprised if there
are controllers allowing this on RAID 5. I would be exceptionally
surprised if this could be done while the array is in use. From a system
access standpoint you would be better off to add three more disks as
a second RAID 5 array than trying to shoehorn in a fourth drive into
a three drive RAID 5 array. (For starters you would have to use a new
drive that is the same size as the drives in the array. And the added
space would be different from the size of the new disk because of how
the redundancy information would have to be redistributed.)

{^_^}
----- Original Message ----- 
From: "Elizabeth R Martin" <[email protected]>
To: <[email protected]>
Sent: Friday, March 14, 2003 8:08 AM
Subject: Re: Adding a disk to a RAID array


> Lulani,
>
> You are correct in that the OS sees only 1 drive /dev/sda because the
hardware
> raid controller card takes care of all the RAID stuff.  I am rung RAID 5
> currently with 3 drives, I want to add a 4th, it has been sitting in a
cabinet
> for when needed.  The drive is then partitioned according to the
information I
> posted sda1, sda2, etc.  Once I get the new hard drive installed in the
BIOS of
> the controller card, it will create the /dev/sda seen by the OS as a
larger
> drive with the end of the drive not being used.  I know that I have to
some how
> edit fdisk manually so that sda4, the extended partition, ending # shows
the new
> end of the partition.  I then have to manually edit fdisk again so that
the
> ending # on sda6 shows the end of the partition and then restore the data.
Like
> I said I remember manually doing this once before a long time ago but the
Unix
> Guru who helped me then is no longer here.  This is something that I will
have
> to plan downtime for since this is a critical system and I don't want to
make to
> many mistakes with the changes.
>
> Hope this information clears up any questions.
>
> Elizabeth Martin