Hello, I am going to be building a server for a small office that will act as a database and fileserver. I'm trying to figure out the best hd configuration that is still affordable. All other things being equal, do you think it would be better to have one 4 HD Raid 10 array, or two Raid 1 arrays (one for OS and programs, the other for database and files)?
Seperating OS from Data is only useful is both volumes have I/O traffic at the same time. Normally a server caches enough so only mutations and rarely accessed data will cause I/O.
Unless such is the case for you, i think one RAID array will provide highest performance. In cases where you have a shortage of RAM, or have many parallel access (more than one operation at a time) you may notice some slowness due to the disks. The only real solution would be the use of an SSD in this scenario. But unless your server has many users a simple RAID0+1 pool will do.
Just make sure the system had plenty of RAM, in fact harddrives in database servers are often only used to write changes to disk; the actual data is cached in RAM allowing faster access by many orders of magnitude.