Question about RAID 1 mirroring

I have two disks of the same capacity, a fast one and a slow one. What happens if I put them in a RAID 1 configuration? Will the slow one drag the fast one down?

Yep. It will be as fast as the slowest drive.

Not quite. For writing, it will indeed be as fast as the slower of the two, but for reading it will be somewhere between the two. This is because you can do two different reads simultaneously. So more read requests will go to the faster drive because it will report itself ready sooner. This is all dependent upon the RAID controller, though. And then there’s the issue of caches…

Right depending on the controller he might get read benefits. RAID1+0 seems the better way to go.

FWIW, of all the cards Ive played with I have yet to see RAID1 do better than a normal drive. I think most controllers just use one primary drive for reading and write to both with no performance boost unless you move to 1+0 or 5.

Performance also depends upon how you measure it. Assuming the card splits reads between the two drives, as Quartz writes, it will end up using the faster drive more than the slower one, but the total number of read requests processed will increase. So, for just reads, average latency will drop, maximum latency will stay the same, and throughput will increase. Write performance will be the same as the slower drive. Mixed read and write performance will be a mess. If the card insists on performing both writes at the same time each write will act to reduce the number of reads the faster drive can be requested to perform. If it allows some amount of splitting of the time, less impact. In general a RAID system is very unlikey to be designed to explicitly cope with different speed disks, and performance will simply be odd. For simple PC level use peak performance matters more than throughput, so you will probably find that overall the system performs little different than a RAID build with two of the slower disk.

Proper, enterprise level RAID systems use identical disks, and some provided the capability of sychronising the rotation of the platters. A very smart controller could work out how to schedule reads across the drives. Serious amounts of cache in the controllers now make this less of an issue, and for RAID-5 performing striped writes from cache removes the read modify write cycle bottleneck. So things change.

During writes, it’s possible that the system will have more trouble than would normally be expected synchronizing writes.
If it gets stuck ‘waiting’ for the second drive to finish writes, that could cause it to actually miss the next window to write on the fast drive, lowering overall write performance.
Let us know how it turns out, if you do it!

Given the low price of disks now, you’re better off buying a 2nd fast one, using that matched pair in the RAID, and keep the other one for other uses in your machine.

You’ve mentioned RAID 1+0 twice, but you don’t get any performance boost until you use at least four drives.

RAID 0+1 requires 4 or more drives [1].
If you’ve only got two drives, you can do RAID 0 or RAID 1 [2].

[1] http://en.wikipedia.org/wiki/Nested_RAID_levels
[2] http://en.wikipedia.org/wiki/RAID