I have always used Software RAIDs, either Apple-provided, or via SoftRAID, etc. I am about to re-zhuzh my arrays and decided to do a little research beforehand… and On Apple's site, there is a statement, clear as day…
RAID 1 (mirroring) for disk redundancy
Mirroring improves reliability by writing the same data to two or more disks. If one hard drive mechanism should fail, the second disk still contains all the data. At least two hard drives are required for mirroring. Mirroring does not provide performance improvements. If the hard disks are not the same capacity, the smaller size will be the amount of disk space used on both drives. Disk Utility determines this automatically.
I was always under the impression that, as there are TWO platters for READ operations, that READ performance with a 2-Drive RAID1 mirror would be somewhat improved. This contrasts with write speed, which remains the SAME, as the drives have to both write the same data at commit-time, so as to sustain a mirrored copy of the data. However, when reading I had thought that the array acted similarly to a RAID0, striping the reads accross both drives.
How is RAID1 is supposed to work? If so, is Apple..
- just saying there is no improvemnet to cover their asses, or
- are they actually not implementing RAID 1 to its full extenent? (Does SoftRaid 4.3 claim to do this?)
According to Wikipedia…
In RAID 1 (mirroring without parity or striping), data is written identically to two drives, thereby producing a "mirrored set"; … The array continues to operate as long as at least one drive is functioning. With appropriate operating system support, there can be increased read performance, and only a minimal write performance reduction;…
So it is Possible, but Apple says there's does not see any speed benefit… But in practice, is there any measurable increase in speed for reads with Software Raid 1 (via Disk Utility)?
Best Answer
My guess would be that Apple prefers to have one drive handle only write processes as a backup and read/write from whichever one it considers the main drive. As to why this is, I have absolutely no clue. Seems kind of like a big oversight on their part. I've always kind of gotten the feeling that software RAID is pretty low on Apple's priority list.