RAID Question - Why no parity without striping

[DocMelvis]

Can anyone explain why there is no RAID level that provides redundancy by parity but avoids the fragmentation and some of the other negatives associated with striping? Is striping a pre-requisite for use of parity? Given the RAID levels available today, striping does appear to be a prerequisite, but in theory, is there a reason why parity cannot be implemented without striping?

[jlreich]

Welcome to the http://www.pcguide.com/ubb/pcgubb.gif forums.

If you don't want to use striping then there is no reason for a parity block. You would simply use mirroring. :)

[DocMelvis]

Mirroring takes away 50% of my total drive capacity. I still want to be able to use parity so that I need to scrifice only one disk's worth of capacity.

[jlreich]

I see your point. ;)

If my understanding of the way parity blocks are used to rebuild an array when a drive fails, it can't be done.

When a drive fails the remaining drives with half of the stripe and with the parity blocks are both needed to rebuild the array. With out the other half of the stripe the parity blocks are useless. Without the parity the other half of the stripe is useless.

If you have a single drive with parity on another drive, and the the single drive fails, there is no way to recover the data from just the parity blocks. There just isn't enough data to do it. Which is why mirroring is need.

If you want to save disk space then I recommend not using raid at all, or just using raid 0, and doing backups of data either manually or using software to do it automatically. Or you can make compressed images of your entire system, or certain partitions, to DVD or to an external hard drive.

If this question is in regard to an actual need, give us more details of your system and what your backup requirements are and we can help come up with a good solution for your situation.

There are many ways to go about data backup. :)

[Paul Komski]

RAID always has some Redundancy and striping is fundamental to all RAID levels bar mirroring. Striping is done for two main reasons - to allow for increased volume sizes and/or for performance. if large volumes are desired this can be achieved by spanning or JBOD - which is not technically RAID IMHO because there is no Redundacy. Such spanning can be done both in hardware and in software using dynamic disks. Striping is not the same as file fragmentation.