> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of James > > I’m trying to select the appropriate disk spindle speed for a proposal and > would welcome any experience and opinions (e.g. has anyone actively > chosen 10k/15k drives for a new ZFS build and, if so, why?).
There is nothing special about ZFS in relation to spindle speed. If you get higher rpm's, then you get higher iops, and the same is true for EXT3, NTFS, HFS+, ZFS, etc. One characteristic people often overlook is: When you get a disk with higher capacity (say, 2T versus 600G) then you get more empty space and hence typically lower fragmentation in the drive. Also, the platter density is typically higher, so if the two drives have equal RPM's, typically the higher capacity drive can perform faster sustained sequential operations. Even if you use slow drives, assuming you have them in some sort of raid configuration, they quickly add up sequential speed to reach the bus speed. So if you expect to do large sequential operations, go for the lower rpm disks. But if you expect to do lots of small operations, then twice the rpm's literally means twice the performance. So for small random operations, go for the higher rpm disks. > ** My understanding is that ZFS will adjust the amount of data accepted into > each “transaction” (TXG) to ensure it can be written to disk in 5s. Async > data > will stay in ARC, Sync data will also go to ZIL or, if overthreshold, will go > to disk > and pointer to ZIL(on low latency SLOG) – ie. all writes apart from sync > writes ZFS will aggregate small random writes into larger sequential writes. So you don't have to worry too much about rpm's and iops during writes. But of course there's nothing you can do about the random reads. So if you do random reads, you do indeed want higher rpm's. Your understanding (or terminology) of arc is not correct. Arc and l2arc are read cache. The terminology for the context you're describing would be the write buffer. Async writes will be stored in the ram write buffer and optimized for sequential disk blocks before writing to disk. Whenever there are sync writes, they will be written to the ZIL (hopefully you have a dedicated ZIL log device) immediately, and then they will join the write buffer with all the other async writes. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
