[K12OSN] Raid Level Choice

Liam Marshall lsrpm at mts.net
Wed Dec 22 22:04:38 UTC 2004

Les Mikesell wrote:

>On Wed, 2004-12-22 at 13:38, Burke Almquist wrote:
>>>LVM lets you combine physical volumes so you get the same effect
>>>as RAID0.
>>LVM gives you the space benefits but not the speed benefits of using 
>>RAID 0 since it doesn't "stripe" the data across disks. (Assuming that 
>>most of your writes occur to the same partition, /home say)
>I think your files have to be large enough to span across disks (i.e.
>more than one cylinder) before you would see a speed benefit from
>the RAID0 stripe effect.  And the benefit you get from the head on
>the 2nd drive being in the right place to continue reading comes
>at the cost of it not being in the right place for a file someone
>else wants at the same time. I'm not sure how LVM handles the
>separate devices but in general for multiuser use you want as
>many independently-seeking heads as you can get because disk head
>motion is the slowest computer operation. RAID tends to lock them
>all together. 
>  Les Mikesell
>   les at futuresource.com
>K12OSN mailing list
>K12OSN at redhat.com
>For more info see <http://www.k12os.org>
so are you saying there is basically no advantage to hardware raid?

I have effectively shot my wad getting the 4 drives and the raid 
controller.  I was intending it to be both increasing capacity and 
giving the system some sense of disaster recovery through raids ability 
to rebuid or work with alternate mirrored drives

The setup I am currently using has less than 30 GB of hard drive space, 
all of it slow, 5400 rpm old scsi drives.  The faster newer larger SATA 
drives should give me at least the equal to it performance wise.  I can 
function now, I am just out of space and suspecting a drive of being on 
the verge of failure.  each new drive is 80 GB in size, so even in 
something like raid 10 I am getting 160 GB usable space, right?  over 5 
times what I have now.  with raid 5 I would have even more.

What I am looking to do is acheive a perfomance increase, however 
slight, while insuring a better disaster recovery. Right now I got 
nothing except I xcopy the /home directories to my laptops hard drive 
via samba share.  In other words, anything is an improvement over that.  
the controller I bought does hardware raid, so why wouldn;t I use that 
instead of software raid via lvm, which in itself has a performance hit, 
right?  I just don;t know for sure whether to do 5 or 10.  People tell 
me 5 is better, disaster recovery wise, but with a performance hit.  10 
is faster, but no parity stuff is happening, so less disaster recovery

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