> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of taemun > > Uhm. Higher RPM = higher linear speed of the head above the platter = > higher throughput. If the bit pitch (ie the size of each bit on the platter) is the
Nope. That's what I originally said, but I was proven wrong. For the data sheet I referenced, all the drive sizes have the same sustained data rate OD, 125 MB/s. Eric posted an explanation for this, which seems entirely believable: The data rate is not being limited by the density of magnetic material on the platter or the rotational speed, but by the head or channel bandwidth to each platter itself. When they run the disks at a higher RPM, they need to stretch the bits longer on the disk surface so as not to exceed the channel bandwidth. When they need to get higher disk capacity, they add more platters. This would logically conclude that you can get a higher maximum disk capacity at a rotational speed which is smaller. In fact, I currently see up to 3T available in 7.2krpm drives ... I see a max 800G in 15krpm... Yes, the higher rpm drives have smaller latency. No, they don't have higher sustained throughput. If anyone wants to look up more drive specs... Here's how to find it on seagate.com: Go to support, Knowledgebase. Under "Support" go to Document Library. Click the drive in question. And then you can find the Data Sheet. The couple of things that are really clear by looking over a bunch of data sheets are: * Higher rpm's means lower latency. (duh.) * Higher rpm's is loosely correlated with higher throughput, but it's not a linear correlation, and not always present. * If you go to a different drive type (SATA vs SAS vs FC) then you can get higher throughput... In no case is the throughput even remotely close to the bus speed, so the improved performance is not *because* of the interface. Presumably the more expensive drive type has a more expensive head or whatever internally. * Larger disk size does not improve sustained throughput at all. Zero. All of this supports what Eric said. The throughput of a drive is not determined by the platter density or rotation speed. It's limited by the head or something else in the data channel accessing the disk. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
