Re: USING RAID and other disk drive issues.

As I understand it, and from what I have observed from my own project, RAID does create a big logical drive out of the several drives which make up the RAID array. If you place all of your database objects on a single RAID array, I/O balancing is a moot point from a practical standpoint. Perhaps there is some gain by placing a data tablespace on one disk in the RAID array, and the corresponding index tablespace on another disk within the array, but I haven't seen a measurable difference in my application.

RAID Levels 1-5 do provide protection against single drive failures. If two or more drives fail, then the array is kaput. RAID 6, which I've seen mentioned only a couple of times commercially, guards against failures of two disks within the array. RAID 0, or disk striping, does not provide any data redundancy. RAID levels 2,3,4 use a parity disk to keep track of the restoral data. This parity disk can become an I/O bottleneck. RAID 5 stripes the parity across all the disks, along with the data. RAID 1 is disk mirroring, just a one-to-one backup. I guess the points that I'm trying to make are that 1)RAID is not completely bulletproof (it certainly doesn't take the place of backups, which I've heard some say) and 2) the RAID level that is used, and how it is implemented, can have a great deal of impact on performance.

I know one experienced UNIX/ORACLE DBA who said that he has observed RAID implementations (RAID 5 mostly) impact performance anywhere from 25% to 50%. I don't know if my own experiences are typical, but in our benchmarking we placed all of our database objects in a single array. With that setup, our "queries from hell" took several minutes to complete! We were completely I/O bound. Our present configuration is two RAID ranks, one for data, the other for indexes, with rollback on a separate disk (mirrored with Volume Manager) and redo logs on another disk, also mirrored. Our bad queries now complete in under one minute (and we keep hacking away at it!).

An excellent reference for RAID is a boook called "The RAIDBook: A Source Book for RAID Technology", published by as organization called the RAID Advisory Board. This board is made up of 40+ manufacturers of RAID products, including several computer industry heavyweights. You can order the book by calling (507)931-0967, or MCI Mail them at 470-6032.

I know of several other sources of RAID information, mainly periodicals. If interested, please E-mail me. If several people are interested, I'll post the list.

Hope that some of this is helpful.

Lou Avrami ( attmail!lavrami ) Received on Tue Oct 25 1994 - 21:04:54 CET