SUMMARY: How much space is needed for RAID 5?

From: Brian Sherwood <sherwood_at_esu.edu>
Date: Fri, 10 May 1996 09:55:05 -0400 (EDT)

In order to use RAID 5 it seems that I need N+1 disk (All the same disk)

To get 10 Meg I would need 6 2-GIG drives.

Thanks to all that replied.
Here are the replys I have received so far...




>From martin_at_jerry.alf.dec.com Fri May 10 09:50:30 1996
Date: Fri, 10 May 1996 09:02:00 -0400 (EDT)
From: Martin Moore <martin_at_jerry.alf.dec.com>
To: Brian Sherwood <sherwood_at_esu.edu>
Cc: Martin Moore <martin_at_jerry.alf.dec.com>
Subject: Re: How much space is needed for RAID 5?

It depends on the size of the disks. RAID-5 uses one disk in a set for its
redundancy information. So for a capacity of N disks, you need N+1
physical disks.

In your example (10 GB), let's say that you used RZ28 or equivalent (2 GB)
disks. You need the capacity of 5 of these, so you need 6 disks.

Martin

-- 
Martin J. Moore				5555 Windward Parkway West
Digital UNIX Support			Alpharetta GA  30201-7407
Digital Equipment Corporation		1-800-354-9000 x31679
martin_at_alf.dec.com			DECATL::MARTIN
>From TSmith_at_ncc-ccn.ca Fri May 10 09:50:45 1996
Date: Fri, 10 May 1996 12:26:00 -0400
From: "Smith, Tolson (win)" <TSmith_at_ncc-ccn.ca>
To: Brian Sherwood <sherwood_at_esu.edu>
Subject: FW: How much space is needed for RAID 5?
A quick rule is that you will lose 1 disk's worth of space when you create a 
RAID 5 set.  So to get 10 GB of space you would require 6 - 2 GB drives. 
  You will also lose more space depending on which type of file system you 
install on that RAID set.  If you install "ufs" the system will reserve 
approximately 15 % for its self and under "ADVFS" the system will reserve 20 
% for its self.
Tolson Smith        Email: tsmith_at_ncc-ccn.ca
Technical Analyst        Phone: (613) 239-5735
National Capital Commission
Ottawa, Canada
----------
From: alpha-osf-managers-relay
To: alpha-osf-managers
Subject: How much space is needed for RAID 5?
Date: May 10, 1996 08:43
We are considering an Alpha 2000 server and are considering RAID 5.
(looking at DU not VMS)
My question is: what it the formula used to caculate the needed amout
disk space to have X amount of space under RAID 5?
Ex: I want 10 GIGs of disk space under RAID 5.  How many GIGs do I need to
buy?
Also, Any problems having the controller implement the RAID and putting
the ADVFS on top of it?
Brian
 -------------------------------------------------------
Brian Sherwood                       E-mail:  sherwood_at_elwood.esu.edu
Network Administrator                Phone:   717-422-3299
East Stroudsburg University
East Stroudsburg PA 18301
>From john_at_iastate.edu Fri May 10 09:50:52 1996
Date: Fri, 10 May 1996 08:45:40 CDT
From: John Hascall <john_at_iastate.edu>
To: Brian Sherwood <sherwood_at_esu.edu>
Subject: Re: How much space is needed for RAID 5? 
> We are considering an Alpha 2000 server and are considering RAID 5.
> (looking at DU not VMS)
> My question is: what it the formula used to caculate the needed amout
> disk space to have X amount of space under RAID 5?
> Ex: I want 10 GIGs of disk space under RAID 5.  How many GIGs do I need to
> buy?
      RAID 5 uses N+1 disks.  So, one way to get 10GB is 6 2GB disks.
      Alternatively 3 5GB disks (if anyone made such a thing).
> Also, Any problems having the controller implement the RAID and putting
> the ADVFS on top of it?
      We've seen no problems.
John
>From szgyula_at_skysrv.Pha.Jhu.EDU Fri May 10 09:51:02 1996
Date: Fri, 10 May 1996 09:46:49 -0400 (EDT)
From: Gyula Szokoly <szgyula_at_skysrv.Pha.Jhu.EDU>
To: Brian Sherwood <sherwood_at_esu.edu>
Subject: Re: How much space is needed for RAID 5?
> My question is: what it the formula used to caculate the needed amout
> disk space to have X amount of space under RAID 5?
  You have to use the same type of drives, and you loose one of them
for parity. 
  To be more specific: Let's say you have N drives. When you write
one data unit, you write (N-1) data blocks, and one error correction
block, which is the same size as the data blocks. The parity block is
placed on the Nth drive. When you write the next one, the parity block
gets on drive 1, etc. (i.e. the parity block is rotating).
> Ex: I want 10 GIGs of disk space under RAID 5.  How many GIGs do I need to
> buy?
  Well, I wold guess: 
4x4G (will give (4-1)x4=12G)
6x2G (will give (6-1)x2=10G)
11x1G (will give (11-1)x1=10G)
  To tell the truth haven't tried it yet (Dec screwed up the order), so this
is based on pre-sales tech support. Might want to try the Storage Works HotLine
(1-800-DSP-DISK, if memory serves right). They were very helpfull and they
knew what they were talking about.
Gyula
-------------------------------------------------------
Brian Sherwood                       E-mail:  sherwood_at_elwood.esu.edu
Network Administrator                Phone:   717-422-3299
East Stroudsburg University
East Stroudsburg PA 18301

Received on Fri May 10 1996 - 16:31:39 NZST