[SATLUG] raid questions

Ernest De Leon edeleonjr at gmail.com
Sat Jan 12 17:58:01 CST 2008


I'm guessing he didn't cite resources because he supposed it would be
apparent what is happening, but not everyone knows exactly what is
happening.  Just stop for a minute and think about how the two different
RAIDs work.  RAID 0 stores equal pieces of a dataset across the drives in
that RAID array.  In the case of a 2 drive array, that is half of the data
set on each.  RAID 1 mirrors the data across all members of the array.
Whether hardware or software driven, the process is something like this:
The OS sends a write operation to disk, the RAID controller (hardware or
software) dispatches the write command to each device in the array.  The
drive then writes the data and the controller in turn informs the OS that
the write has completed.  This process for RAID 1 is much slower compared to
RAID 0, but you are buying redundancy, not speed.

Now here comes the part where you see what is happening.  Assume that a RAID
0 read/write operating is going on at the same time that RAID1 operations
are (as in your case with different RAID sets on the same physical disks.)
The physical drive cannot be spinning platters or moving read/write heads at
different speeds internally to compensate for the different RAID sets
striped across it.  This further becomes a problem if the two RAID sets
happen to share the same platter.  It's just not physically possible.  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.)  I lack a better
way to explain it than this, but it is essentially physical disk
contention.  This is the same reason that most enterprise storage admins
will implement a RAID10 for applications that require blistering speeds AND
redundancy.  This configuration, however, spans many disks where two RAID 0
arrays are mirrored (this creating the RAID 1 array of RAID 0 arrays or RAID
10.)

Hopefully this will be helpful.

Ernest

On Jan 12, 2008 3:39 PM, Chris Lemire <good_bye300 at yahoo.com> wrote:

> It's not a possibility to get another drive right now. I need to pay for
> several other things. He didn't site any sources. Is there any books or
> legitimate information that says raid 0 and raid 1 together are not good?
> He's saying that would cause too much disk usage, but is that more disk
> usage than having both partitions in raid 1?
>
> "Todd W. Bucy" <toddwbucy at grandecom.net> wrote: Chris Lemire wrote:
> > I'm being told that my raid setup is a bad idea. Is this true?
> >
> > http://forums.fedoraforum.org/showthread.php?p=941593#post941592
> >
> >
> I can't answer to whether or not it will "chew up" you hard drives but
> he may be about sluggish performance when putting a raid 0 and raid 1 on
> the same drive.  IF you can you might want to consider getting another
> 320 gig and going raid 5.
>
> Todd
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