[SATLUG] raid questions

Brad Knowles brad at shub-internet.org
Sat Jan 12 22:04:24 CST 2008


On 1/12/08, Ernest De Leon wrote:

>                      This further becomes a problem if the two RAID sets
>  happen to share the same platter.  It's just not physically possible.

It is physically possible -- I've done it, under Solaris.

However, it is also highly unadvisable.

>                                                                         With
>  that in mind, your read/write speed will be determined by the lowest
>  (slowest) common denominator - the RAID 1 set.  So even though your RAID 0
>  set may be able to read/write at twice the speed or better of the RAID 1
>  set, every RAID 0 read/write command will have to wait until the previous
>  (or concurrent) RAID 1 operation is complete.  (This may actually slow your
>  performance down more than just having everything RAID 1.)

Disks have things like elevator algorithms to sort the incoming data 
stream regardless of where the data requests are coming from, but 
they don't spin at variable speeds.  How "Zone Bit Recording" works 
is that they increase or slow down the writing speed.

But you will definitely slow down both the RAID-0 and RAID-1 
operations, and since a RAID-1 operation can't complete until the 
write operations return on both disks, they will be hurt more than 
the RAID-0 operations which will be brought down to RAID-1 speeds (or 
worse).

-- 
Brad Knowles <brad at shub-internet.org>
LinkedIn Profile: <http://tinyurl.com/y8kpxu>


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