RAID 4 and RAID 5

RAID 4 and RAID 5

[Vladimir Milovanovic]

Hi all,

I am thrilled that I have found this list, as I didn't know about it.
I have been playing with RAID for a while, and I am enjoying what it can 
do. The one problem I have is differentiating between RAID 4 and RAID 5.

Both are documented in the HOWTO and I have tried to set them up both. 
The /etc/raidtab file does not complain when I define RAID 4 as the 
raid-level argument, but after creating an array with RAID 4 and doing 
cat /proc/mdstat , I see active RAID5, and all the disks. The parity 
algorithm is stated as 0, and at the bottom it states the space 
available on the array and the "level 4".

Setting up RAID 5 is very much the same thing, except it says that the 
parity-algorithm is 2, (I am using left-symmetric) and then, next to the 
space available, it says level 5.

Now here is the crunch : both RAID 4 and RAID 5 give me EXACTLY the same 
results, both in read write performance. I am using many programs for 
benching, hdparm, bonnie, iozone, tiobench and the results are 
consistent and repeatable.

Does anyone know with any degree of certainity whether RAID 4 is 
supported in Linux? What exactly is the status of that? I know that 
there is no point in using RAID 4 over RAID 5 but I want to know so that 
I am able to document it.

Thanks, and nice to see so much discussion about RAID!!! I think I am 
gonna like this place.

Vlad.

Re: RAID 4 and RAID 5

[Neil Brown]

On Tuesday October 15, vlad@webmail.co.za wrote:
> 
> Now here is the crunch : both RAID 4 and RAID 5 give me EXACTLY the same 
> results, both in read write performance. I am using many programs for 
> benching, hdparm, bonnie, iozone, tiobench and the results are 
> consistent and repeatable.
> 
> Does anyone know with any degree of certainity whether RAID 4 is 
> supported in Linux? What exactly is the status of that? I know that 
> there is no point in using RAID 4 over RAID 5 but I want to know so that 
> I am able to document it.

raid4 is definately supported in linux.  It is supported by the raid5
driver so there is a possibility of some confusion.  However it sounds
like you are setting it up correctly.

You would only expect to see a performance difference between raid5
and raid5 when doing lots of small widely distributed random writes.
You would probably also need a fairly large array (at least 5 drives)
to be able to measure much and you would need to be sure that none of
the busses are in a position to cause a bottle neck.  Basically you
need to get the raid4 parity drive overloaded with parity read/updates
before you will see it slow down.

NeilBrown