Re: RAID

Several people wrote: >> What is the story with RAID and informix? >It's possible. >> Is it a good Idea? >_I_ don't like it, but that doesn't mean it's not a good idea. :-) When properly implemented, it can be a very good idea. The trick is to create several physically independent LUNs (Logical UNits). By physically independent, I mean the following: Take a RAID cabinet with 50 drives in it. Many people will stripe across all 50 drives, and then split that up into subdisks for use with Informix. But this can cause disk contention. The better way would be to create five logical disks (LUNs) each striped across ten drives. Each LUN then controls its own disks, and there is no disk contention between LUNs. The striping has accomplished something very similar to round-robin fragmentation across those disks. But because you have five physically independent and independently addressable LUNs, you can fragment by expression across the LUNs. (Five total expressions). Or, you can place high-access table A on LUN 1, high access table B on LUN 2, etc. Of course, you don't HAVE to do 5 LUNs, 10 disks each, it was just an easy example. You can fudge the numbers to fit your particular situation. By using RAID intelligently in combination with Informix, you can get the "best of both worlds." The other trick is that it's better to have many small disks instead of few large disks, even in a RAID solution. > >> Is mirroring the db spaces with Informix better/ faster? > >Neither. There is some debate about whether hardware or Informix >mirroring is better or faster. > That's not entirely true either. Typically hardware mirroring will be somewhat faster than Informix mirroring. Although Informix mirroring gives the DBA somewhat more control, and will generate alarms when there are mirroring problems. (In hardware mirroring, these will typically go to the root user, and will be transparent to Informix). If you're deciding between Informix mirroring and mirroring with a software disk manager like Veritas, then it's a coin toss from a performance perspective. >> There are companies that claim that their RAID works well with Informix is >> this true? > >Possibly. Yes. >If you use RAID, you have no control over data placement within the >"slice" and there is definitely a performance hit with RAID-5. So, if >you want optimum tuning capability, you should work at the >individual disk level and mirror either at the Informix or the hardware >level. That's not neccessarily true. If RAID-5 is done _in hardware_, then there is minimal performance hit. We use a Sun RSM-2000 (aka A3000) with hardware RAID 5, and it performs almost identically to RAID-0, and still faster than individual disks. Software RAID-5, you'll definitely get killed in performance, though. And while it's true that you have no control over data placement within a RAID LUN, if you create the LUNs themselves in an intelligent manner, you'll be fine. > >Above a certain point, however, it becomes difficult to cost-justify >carrying twice as much disk as you actually need.... :-) This, too, depends. The trick here is to put a number on how much down time costs you, per hour, and an estimate of how long it takes to replace a disk and restore. Then you find out that disk is cheap, even if you're using EMC or IBM disk (which tends to be very expensive), by comparison. If you have more questions on this, I'd be glad to discuss my experiences in more detail outside of the group. My e-mail is tgirsch@iname.com, and my reward for posting that address will be a ton of Spam mail. - Tom Girsch Database Systems Manager Arch Communications Group