Re: Informix 7.x and RAID 5

pgreene@jax.jaxnet.com (pgreene) wrote: >We are setting up a data warehouse using Infomix 7.1 on a RS 6000 J30. We have >received conflicting information about using RAID5 storage systems in this environment. >Our database will start at around 30 GB growing to several hundred GB in the future. >Some information that we have says absolutely do not use any RAID system with Informix >7.x and other information says that it is not a problem. If you have experience with this or >knowledge about this I would greatly appreciate your help. Thanks in advance. > RAID is invisible to the database engine. It will work but RAID cannot give you the same performance that mirroring can. You also lose control as to where you can place data and which disks that data will reside on. This can cause data to pile up in one disk even though you specified that the data go to different logical volumes. Also, by using Informix mirroring the database will use split reads and synchronous writes to the disks. Split reads means that if a whole chunk is to be read, one thread will read the first half of the chunk from the primary chunk and a second thread will read the second half of the chunk from the mirror at the same time. To use DSA you definately want to have lots of small chunks. RAID is designed more for a few large devices. Finally, write performance for RAID is not good. If you are going to be doing any kind of OLTP or data manipulation then you should avoid RAID because of the cost of the writes. Ask you hardware vendor for performance statistics comparing RAID 5 to no RAID. Then ask them for a price comparision of the two (for the same amount of writable disk area) and you should be able to come up with a price/performance ratio that will show you the light. Mirroring may be more expensive in the beginning but for performance and high availability it cannot be beat. In the long run I think you will be happier with no RAID. Mike P.S. You should also look at the performance of RAID when a drive crashes. With mirroring the reads and writes continue at their usual pace, with RAID the system slows to a crawl and high performance applications can start to shut themselves down because of a lack of response from the disk subsystem. Ask your vendor for more details!