installed new hard drive - now can't find it

[Ajai Khattri]

On Thu, 29 Apr 2004, Dana Holland wrote:

> I don't know.  We're using Raid 5.  I'm really not sure Dell knows
> either.  They literally had us spend the entire day yesterday installing
> and reinstalling a rpm because they said we were missing a command -
> they kept telling us to enter "fastcmd" at the prompt.  I finally got
> frustrated with them and went to google - and there I find that fastcmd
> is the prompt you get when you enter the afacli command.  All the
> processes ran after that.


You should read up on RAID levels. Normally you would select the RAID type
in the RAID card's BIOS. Anyway, here is the definition of RAID 5 (from
the RAID how-to on tldp.org):


"* RAID-5

    * This is perhaps the most useful RAID mode when one wishes to combine
a larger number of physical disks, and still maintain some redundancy.
RAID-5 can be used on three or more disks, with zero or more spare-disks.
The resulting RAID-5 device size will be (N-1)*S, just like RAID-4. The
big difference between RAID-5 and -4 is, that the parity information is
distributed evenly among the participating drives, avoiding the bottleneck
problem in RAID-4.
    * If one of the disks fail, all data are still intact, thanks to the
parity information. If spare disks are available, reconstruction will
begin immediately after the device failure. If two disks fail
simultaneously, all data are lost. RAID-5 can survive one disk failure,
but not two or more.
    * Both read and write performance usually increase, but can be hard to
predict how much. Reads are similar to RAID-0 reads, writes can be either
rather expensive (requiring read-in prior to write, in order to be able to
calculate the correct parity information), or similar to RAID-1 writes.
The write efficiency depends heavily on the amount of memory in the
machine, and the usage pattern of the array. Heavily scattered writes are
bound to be more expensive."


So, if your RAID array is configured to appear as a single volume, the
size should have increased by (N-1)*S where N is the number of drives in
the array and S is the size of the drive (all drives are usually the same
size). When you are in the card's BIOS and have added the drive, the card
will then start building that drive with striped parity data automatically
(I have used RAID 0+1 with Adaptec cards and this is the what they do for
me).


--
Aj.
Sys. Admin / Developer