Re: USING RAID and other disk drive issues.

louis.avrami..jr (lou2_at_cbnews.att.com) wrote:
: 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.

 Please post the list. Thanks in advance.
: Hope that some of this is helpful.
 

: Lou Avrami ( attmail!lavrami )
Received on Sat Oct 29 1994 - 06:35:56 CET