dodecahedron wrote:what exactly is RAID5?
RAID 5 stripes data and parity across three or more discs. It allows for failure of a disc without data loss and automatic reconstruction of the failed disc when it is replaced by a new one. It is a good compromise between performance, data integrity, and disc usage.
dodecahedron wrote:why afraid?
what's wrong with RAID5?
There isn't anything inherently "wrong" with RAID 5. However, many people tend to automatically think "RAID" means faster performance relative to a single, plain-old disc. After all, a RAID array includes multiple discs that can be read from and written to in parallel. It must be faster, because the disc activity can be spread across multiple discs simultaneously, right? Wrong. It isn't that simple.
For example, a RAID 5 setup can improve read performance, but write performance is actually slower than for a single plain-old disc. Before a write can be made, depending on stripe sizes, the size of the write and the alignment of the write, the existing data must be read from multiple discs in the array, the parity recalculated, and the data written back to multiple discs.
The write penalty can be reduced by properly tuning the array to the application, configuring the stripe sizes, and adjusting cache settings. But this can be time consuming and difficult to do, and thus often isn't done right. The ops people at my job set up a server for a project of mine, and performance was terrible. My stinky little IDE desktop was running rings around a rack-mount server with SCSI drives. As it turned out, ops had configured a RAID 5 array and not tuned it properly. Once I pointed that out and they did their magic, the server beat my desktop (as it should have). This isn't a snip at ops, since they do a good job, but just points out that RAID 5 is not a slam-dunk.
Anyway, that was the basis for my "fear". I didn't want MonteLDS thinking RAID (in the guise of RAID 5) was the no-brainer answer to the problem of getting high performance for his video storage needs.