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I hear more and more people say; take a whole enclosure and place everything in a RAID 5 instead of by application look for the best performance. So what is best?

Such a simple question and unfortunatley there is not a simple answer. It is all dependant on how the different applications access the data. Large sequential operations will tend to starve small block IO on the same set of disks and trying to operate at the same time. You need to determine minimum performance threshholds for each application on the array and make sure it can meet them under worst case scenarios (rebuilding a failed disk and processing IO). RAID is a protection scheme. Some view it as a way to get bigger LUNs. Disk IO is based on the speed of the disk and is a measure of how many disks and the type of RAID and it's overhead. In reference as to how you layout RAID on a shelf of disk or across multiple shelves will depend on many things.

First of all...RAID 5 with enough drives in a raid group can satisfy quite a few performance needs. It really depends on you applications and their performance requirements. It is usually better to have LUNs span as many drives as possible, however you should not put together applications with different IO profiles. Also remember, with RAID 5 your MTBF (Mean Time Between Failure) can be quite large, even more so with RAID 6. But, your MTBDF (Mean Time Before Drive Failure) can be very short (HDD MTBF / # Drives in use) It would be wise to have a supply of spares at all times. Today the vendors are coming with 500 GB or even 1 TB disks and with this, rebuilding times take a lot of time. It's also important to note that a RAID 6 has 2 parity stripes which are great for recovering data if a drive is lost but the trade off is even longer re-build times. With drives this large in a 15 bay RAID enclosure running in a 7X24 environment the rebuild times can be in weeks and not in days. From this perspective, it is not advisable to put all the disks in one large RAID 5 or RAID 6 group. Your choices then for RAID 5 vs RAID 6, SATA vs SAS and 500 Gb or 1 Tb disks or some mix become clearer. These are also impacted by your overall data protection strategy. Backups, snaps, disaster recovery methods, etc. along with budget all determine what you really need to do. So here we are.. Money and performance needs. I would try to find a sweet spot and decide from there. If money is not the real problem, take a closer look at what your applications need. Are your applications generating sequential I/O's or just random I/O's? And what about read and write ratio? A RAID 5 config has a penalty of 4 or 8 etc. when writing data where in a RAID10 setup this is only 2, but then again: RAID10 costs twice as much, and RAID 5 only costs 1 drive extra. I'd suggest talking to the financial Division as well as talking to a performance guru.

So what is the best advise for RAID groups? There is non! As simple as it is. But keep in mind that there are good default choices for RAID groups like 3+1 or 4+1 for RAID 5 and or do RAID 6 with 6+2, 8+2 10+2 anyway two 4+1 RAID 5's equal the same overhead as 8+2 RAID 6 but will have better performance. It woud be nice for disk vendors to bring out low capacity drives again so you could design your capacity and performance a little more granular. Give this page also a visit for understanding RAID solutions.

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