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RAID isn't, and never was, intended for performance gains. In fact RAID almost always is slower than a single drive.
RAID is an acronym for Redundant Array of Independant (it used to be Drives now it's) Devices. Somebody, ok several somebodies, tried to stripe across tape drives (which created the devices change) - until two things happened.
1) faster larger capacity tape drives came out, almost to the Moore's Law timeframe, and it keeps happening!
2) disks kept getting faster/cheaper/larger capacity with no end in sight, tracked Moore's Law as well.
Oh, they also figured out that if you loose two tapes in a stripe set you were hosed. too easy to do with all of the media handling and such. The devices overlap the placement (A mystical way of saying - writing stuff such that you can survive at least one drive failure, sometimes two but not *those* two!) of the data to protect you in case of a failure, i.e. Redundant. All of the versions of RAID (except Zero) protect data first, and then try to minimize the overhead (that's read - slow) introduced by preparing your data to be split across several spindles, and computing a checksum and *then* writing everything out. Reading is no better, the data has to be retrieved from several spindles and reconsituted (and reconstructed if you have had a failure). And, as I mentioned earlier, if you are reading something to change it and write it back, then all of the stripe has to be read to recompute the parity for that stripe - and then the stripe gets written. The infamous "read/modify/write" necessary in every RAID except 0 and 1. 0 doesn't protect, 1 does. All other forms of RAID are doing this somewhere somehow. Now *is* the time to pay attention to the man behind the curtain!). Different RAID levels (2,3,4,5,6, and their derivatives, 7, 10, 50, 0&1) all have different read and/or write characteristics, each has a worst and each a best feature compared to all of the others. None of them is the best, but one is the best for your requirement.
Spindle sycronization was tried several years ago, have one drive 180 degrees out from the other, do RAID-1, and the smart controller would know which drive had the data closer and use it for reading. You needed drives that supported spindle sync, and a wire between the *two* you wanted to sync. Not the three or four or five. Two. Writing? Oh, that's different! You have to write to *both* drives, slower. The controller won't post a write complete successful until it does the second drive. RAID-1 is slower than a single drive because of this. Keeping the drive spindles synced was problematic and became a show stopper. I'm not sure if it has resurfaced (things that sound good always come back until somebody remembers why they went away last time!) or not.
Suggest you do a google search on RAID performance, both reading and writing, compared to a single drive.
Yes, RAID-0, an oxymoron, splits data across several spindles without the overhead of protection. Remember the objective was/is to protect data. So, RAID 10 was created! Stripes across mirrors. The overhead with the mirrors is still underneath it all. And then you can get into the dark magic of *tuning*! Stripe sizes, block transfer, caching controller, total cache amount, cache block size, cache expiration time... Unless you'd rather be spending your time *creating* stuff to store on the storage - or working on your SAAB!
Anytime you have data that you don't want to loose, and you have it on a disk that spins, you had better protect it. Original location plus another one. RAID (of *any* variant save zero) is a crutch that gives you a second chance when the primary storage experiences a failure. All of the different versions of RAID have either an upside or a downside. They are all different, so the choices need to be made depending upon your particular needs.
That said, with the cost per GB of drives it's difficult to skip the easy and automatic primary protection of your data. It's like not having a spare in your car... umm, you do have a spare in your car don't you? :)
, Thu, 26 Oct 2006 02:04:50
, Wed, 25 Oct 2006 20:42:54
, Thu, 26 Oct 2006 10:53:32, Wed, 25 Oct 2006 15:40:47
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