[SATLUG] raid questions

Ernest De Leon edeleonjr at gmail.com
Sat Jan 12 23:27:47 CST 2008


>
> >                      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.
>

You've done what?  Made the platters or read/write heads move at different
speeds?  or put different raid sets on a single platter?  I was referring to
changing the physical movement of the disk internals, not the data layout.


>  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).


ZBR or ZCAV  only  helps  read/writes stay more consistent by slowing down
read/writes on the inner tracks on the platter and writing faster on outer
tracks.  (Similar to the concept of starting runners further back on the
inner lanes of a track.)  This will not, however, compensate for the delays
caused by the software/hardware RAID controller causing disk contention
trying to balance two different RAID sets on the same physical disks.

Although it's all a matter of semantics and such, the bottom line is that
performance WILL suffer because you have two different RAID sets on the same
physical disks.  I would go for the RAID 1 across both disks.

Ernest



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