>>> On Sun, 17 Feb 2008 07:45:26 -0700, "Conway S. Smith" >>> <beolach@xxxxxxxxx> said: [ ... ] beolach> Which part isn't wise? Starting w/ a few drives w/ the beolach> intention of growing; or ending w/ a large array (IOW, beolach> are 14 drives more than I should put in 1 array & expect beolach> to be "safe" from data loss)? Well, that rather depends on what is your intended data setup and access patterns, but the above are all things that may be unwise in many cases. The intended use mentioned below does not require a single array for example. However while doing the above may make sense in *some* situation, I reckon that the number of those situations is rather small. Consider for example the answers to these questions: * Suppose you have a 2+1 array which is full. Now you add a disk and that means that almost all free space is on a single disk. The MD subsystem has two options as to where to add that lump of space, consider why neither is very pleasant. * How fast is doing unaligned writes with a 13+1 or a 12+2 stripe? How often is that going to happen, especially on an array that started as a 2+1? * How long does it take to rebuild parity with a 13+1 array or a 12+2 array in case of s single disk failure? What happens if a disk fails during rebuild? * When you have 13 drives and you add the 14th, how long does that take? What happens if a disk fails during rebuild?? The points made by http://WWW.BAARF.com/ apply too. beolach> [ ... ] media files that would typically be accessed beolach> over the network by MythTV boxes. I'll also be using beolach> it as a sandbox database/web/mail server. [ ... ] most beolach> important stuff backed up, [ ... ] some gaming, which beolach> is where I expect performance to be most noticeable. To me that sounds like something that could well be split across multiple arrays, rather than risking repeatedly extending a single array, and then risking a single large array. beolach> Well, I was reading that LVM2 had a 20%-50% performance beolach> penalty, which in my mind is a really big penalty. But I beolach> think those numbers where from some time ago, has the beolach> situation improved? LVM2 relies on DM, which is not much slower than say 'loop', so it is almost insignificant for most people. But even if the overhead may be very very low, DM/LVM2/EVMS seem to me to have very limited usefulness (e.g. Oracle tablespaces, and there are contrary opinions as to that too). In your stated applications it is hard to see why you'd want to split your arrays into very many block devices or why you'd want to resize them. beolach> And is a 14 drive RAID6 going to already have enough beolach> overhead that the additional overhead isn't very beolach> significant? I'm not sure why you say it's amusing. Consider the questions above. Parity RAID has issues, extending an array has issues, the idea of extending both massively and in several steps a parity RAID looks very amusing to me. beolach> [ ... ] The other reason I wasn't planning on using LVM beolach> was because I was planning on keeping all the drives in beolach> the one RAID. [... ] Good luck :-). - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
