Re: testing raid 0

[Greg Oster <oster@cs.usask.ca>]

On misc@openbsd.org Aaron Crandall <acrandal@wgz.com> said:
> RAID 5 is another beast altogether.  The disks are being written to in
> stripes.  The stripes are across all the disks (you need at least 3 disks)
> and a parity stripe is written to a spare disk. 

Stripes go across the disks (you don't write a stripe to a single disk).  
In RAID 5, the parity bits are distributed across all the disks in the array 
(as opposed to RAID 4, where the parity bits are found only on a single drive).

> The way it is setup, any
> 1 disk can crash and the array can still keep going along, and by
> replacing that disk the data can be fully recovered.  So for RAID 5 you
> get the multi-disk writing advantage of RAID 0 

You get a 'multi-disk writing advantage', but it's certainly not the same one 
you get with RAID 0 in terms of performance.

> and a very good fault
> recovery of RAID 1, but you will need at least 3 disks, and one is
> sacrificed to the parity stripes.  As you add more disks you can have
> more parity disks as well so that multiple drives can die/be changed
> at the same time without losing data.
>         RAID 5 is difficult to run in software and is hardware intensive
> for IDE (because of the number of IDE controllers needed), so unless you have
> a SCSI controller and many matched SCSI disks it's difficult to come up
> with RAID 5.  When you do have it!  Wow.
> 
> RAID 10 (or 1/0 or 0+1) is a combination of RAID 0 and RAID 1 where you
> have two sets of two disks.  So, with say 4 10Gig drives, you pair them
> and have two RAID 1 arrays.  Then you put those arrays in a RAID 0 set so
> they read/write at twice the speed.  If any one disk dies, it can be
> reaplced and re-mirrored off its RAID 1 buddy.  This is easier to do than
> RAID 5, but not as fast and doesn't keep as much space per disk.

RAID 0+1 (or whatever you want to call it) should be faster than RAID 5 for 
the same amount of storage (not same number of disks :) ).  RAID 5 writes are 
quite expensive.

> RAID 4 (yes, Network Appliance filers use it) is very specialized and even
> faster than RAID 5. 

Ummm...  I think this is the first time I've ever seen that claim :)  RAID 4 
has the inherent problem that all parity bits go to the same drive, which 
typically creates a bottleneck at that drive.  With hardware raid you might be 
able to lessen that a bit, but RAID 5 will typically be faster.

> It relies on writing to the same byte on eight disks
> at the same time and a parity byte to a last disk.  All the disks have to
> be matched.

For those interested, here are a few places to find out more about RAID:
  http://www.acnc.com/raid.html
  http://www.twincom.com/raid.html
  

Later...

Greg Oster