Re: RAID-5 vs. Disk Mirroring

In article <4e5man$28d_at_newsbf02.news.aol.com>, BBAP <bbap_at_aol.com> wrote:
>Does anyone have experience to share on either RAID-5 or Disk Mirroring.
>We are trying to make a decision on a direction. The concern is which
>method will ensure that data will be there (no loss is permited), while
>not comprimising performance. Cost is not a consideration.
>
>We are looking at HP, SUN, IBM hardware solutions.
>
>Thank you in advance for your answers...
>
>Bernie Lewis
>bbap_at_aol.com

You might get more responses if you posted this question to any/all of the storage newsgroups. Fortunately for you, I just happen to have researched this subject a year or two ago for my employer. I'll forward my analysis to you in case it might help, but remember that the info is two years old and I'm sure some changes have occurred. BUT WAIT! YOU ALSO GET A FREE SET OF GINSU KNIVES!!! Sorry... where's that medication...

Seriously, RAID-5 is a better system than disk mirroring (which is RAID-1, if I remember correctly). You should see better performance out of a RAID-5 system than you would out of a single disk or a RAID-1 system. Think about it. Data transfer rate from a disk is limited by the spindle speed of the disk. If you have one byte spread across 8 disks, each rotating at 7200 RPM, that's an effective spindle speed of almost 60,000 RPM! Of course, it's not quite that simple, but the principle still applies. One added bonus you get with more advanced RAID-5 systems is that the size of a logical drive isn't limited by the size of your physical drive... add more physical drives and your logical drive just gets bigger. RAID-5 is also more expensive, but you said that wasn't an object, so RAID-5 all the way!! Some things to look for:

• Which components are redundant? A truly fault-tolerant system will have a separate power supply for each disk. Doesn't do any good to have replicated disks if they're both hooked to the same dodgy power supply.
• Do all the drives in the system have to be identical? If, five years from now, one of your drives fails and the manufacturer no longer makes that drive, you're screwed. Most systems allow a failed disk to be replaced with the same size or larger disk.
• One of the manufacturers I was looking at explained to me the concept of a "RAID hole" which could manifest itself if you had a power failure on the RAID system. They said that, if a power failure occurred in the middle of a write, the parity information might not be stored correctly and the error wouldn't show up until you had a disk failure and had to do a rebuild. There's more detail about this in the info I'm sending you.
• Does the system allow hot swapping? It's pretty cool to be able to replace a failed drive on the run without powering down the whole system. This was a reasonably new feature a couple of years ago, but is probably commonplace now.
• Even better, does the system have a hot standby disk? If not, if one of the disks fails, your system isn't failsafe until you get a replacement disk in.

Hope all this helps you make your decision.

    Brand

P.S., Where should I send the bill for my consultation fee? :-)

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