Quick answer - only one big volume. OK. I will start my thoughts on the subject with single drive example. Which is better for single drive - one volume occupying the whole disk or several files/raw devices defined as DISK volumes? Remember: disk devclass has nothing to do with mount points and allows concurrent access for many sessions. When discussions on this list were about DB volumes the answer was: one volume. Same for disk volumes - one big volume. TSM can write multiple sessions' data simultaneously. It also pipes large quantities of data using transactions so drive queue will be well fed. And if we create more volumes TSM will attempt to write "in parallel" which would force the HDD heads to jump across the disk. Now back to RAID arrays. RAID 1/10 consist of 2xN disks and RAID 5 is N+1 disks. Read rate for RAID 1/10 is either N times HDD speed or 2N times depending on controller. Write rate is N/2 or N times the disks' speed. RAID 5 gives N times read increase. Write rate for RAID 5 is (note) again N times faster than single disk speed but decreases with penalty for recalculation if parity (highly controller dependent). Penalty for stand-alone controllers is somewhat high and increases with number of disks. IBM ESS is using many tricks to mitigate parity penalty thus write speed is close to ideal N x HDD speed. (End of technology overview :-) When large read/write from/to RAID array is performed it is spread to all the disks. Operation on one end of RAID array will transform to many simultaneous operations to the *one end of the disks*! Operation to other end of the array will tranform into operations on *other end of disks*! Thus you will again spend your server's time moving heads of *all* drives back and forth. And if one drive is slower than the others it will slow down the whole operation. That's why all drives in an array should be equal. So for simple RAID arrays (a controller with some disks) the answer is simple as for one drive - make only one big volume. For complex systems like EMC Symmetrix or IBM ESS also take into account other techniques used. For ESS and Symmetrix the generic answer is: make several volumes to spread the load across more internal disk groups. Do not increase the number of volumes far beyond the number of disk groups within the box. Hope you are not bored by this long explanation.
Zlatko Krastev IT Consultant Please respond to "ADSM: Dist Stor Manager" <[EMAIL PROTECTED]> Sent by: "ADSM: Dist Stor Manager" <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] cc: Subject: allocating disk volumes on RAID5 array Since a RAID-5 array shows up as one big filesystem, what's the best strategy for determining how many and of what size disk pool volumes to create for your primary disk storage pool? For the most part I don't think it really matters unlike allocating volumes on individual disks but perhaps I'm not considering something. Thanks..
