Raid 1

[Jonathan Hirschman]

Dan Farrell wrote:
> On Thu, 03 Apr 2008 15:35:03 -0600
> Karl Larsen <k5di at zianet.com> wrote:
>
>   
>> I don't know in depth how the IDE controller works so I couldn't 
>> conclude for myself (nor I had the time to investigate it any further 
>> :-) if setting up both hard drives on the same IDE connector (1x
>> Master, 1x Slave) would be faster than having separate IDE connectors
>> (1x Master, 1x Mater). I picked the latter. I'll wait for some
>> insightful response to this question.
>>     
>
> It is my understanding that putting two drives on the same ide channel
> (1 master, 1 slave) is a poor idea in general, and leads to really
> crummy performance if you RAID them together.
>
> Most IDE Controllers have two channels, each of which has its own cable
> and can support up to 2 drives.  However, each channel can only get the
> most out of 1 drive.
This is correct. PATA, the old style IDE with ribbon cables, is limited 
to one command at a time, and only one device can have access to the IDE 
bus at a time. There are some exceptions, but for the most part, this 
rule of thumb holds true.

Given that, a mirror would mean that two drives would alternate reads 
and writes (and I believe that md gives you "split reads" if you do a 
RAID 1 - meaning that you can do two seeks in parallel as opposed to 
single drive). If you put the drives on two separate IDE channels, 
writes can happen in parallel, and you should see read performance 
become substantially faster (as you can now have two separate reads at a 
time, as opposed to one at a time with a single drive).

Clear as mud? :)

jh

jh