On Tuesday, 14 January 2003 at  9:16:22 -0500, Bill Moran wrote:
> Ion-Mihai Tetcu wrote:
>> Hi,
>> I have to setup a database (MySQL) server this days.
>> The machine will be a AthlonXP 2000+, 1G RAM, 2x Cheetah X15 36LP series
>> drive of 18GB.
>> And I could spare a 20G ATA100 IDE drive.
>> I have no experience with vinum, so i want to ask you if it is a good
>> ideea to add the IDE drive. I was thinking to use it as a backup drive
>> in case the SCSI controller fail. I'm courios about what impact will
>> have on the performance of the system and as the server will have to
>> enter in production in a few days I do not have the time to play with.
> I'm not going to take the time to research the specs on each of
> those drives.  However, if the ATA drive is slower than the others,
> and you configure the three in a striping configuration, the total
> speed of the array will be limited by the slowest drive. 

For some definition of total speed.  A better way to look at it is at
the total bandwidth.  It will obviously be lower if one of the disks
is slower.

> (The same would be true of a mirroring config, but you don't
> generally do that with 3 drives) 

No, with mirroring the slowest drive would slow down the others on
writes, because they always go to both disks.  That's not the case
with striping.

> If you config them as concatenated, or with different partitions,
> the speed will depend on which drive is being accessed.

Yes, that's the case with striping too.  You shouldn't make more than
one disk access per I/O request, otherwise latency kills you.

> So the answer is: whether or not it's a good idea depends on how you
> configure it.  If you configure the ATA as temp space, and the
> Cheetahs as database storage space, you'll probably see better
> performance than if both were on one drive, since writing to both
> ATA & SCSI simultaneously should go faster than trying to write temp
> and data to one drive at once.  However, even this depends on your
> motherboard and controller, so your actual performance is going to
> depend.  Unless someone else has used your exact config and done
> benchmarks, there's no way to be 100% sure.

Note that there's no a priori reason to believe that the ATA drive is
going to be slower than the SCSI drives.

My own take on this: it should work, and performance shouldn't be
markedly different from a 3 SCSI disk config.  How you actually use it
depends very much on your intended use, though.

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