On Sat, 6 Dec 2014, Rohan McLeod <rhn@jeack.com.au> wrote:
[1] http://www.pccasegear.com/index.php?main_page=index&cPath=210_385
I notice all these controllers are SATA III. When I was struggling to understand why my new 4TB HGST_HDN7240_40ALE_640 SATA III HD ; couldn't seem to deliver a much higher data transfer rate on SATA III than on SATA II.; one comment which was illuminating to me was; " there is no such thing as a SATA III rotating hard-drive" which I took to mean as : ' there is no rotating harddrive; whose heads can transfer data on/off the disk; at anything like 6Gb/s ~ 600GB/s' (SATA III max)'. in contrast to say a SATA III SSD, where this is quite possible ! Apologies if this is merely repeating old news !
That's the correct interpretation. The best I've seen for a single disk is about 160MB/s for contiguous reads from the outer tracks - a usage pattern that is quite uncommon IRL. In real world use I've seen systems using all available disk IO capacity while transferring less than 10MB/s because of the seek time. If you had a hardware RAID device that presented itself as a single SATA device with 2 real SATA devices connected then SATA 2 shouldn't be a bottleneck. If your hardware RAID device had 8 disks then for most real world uses of RAID hardware SATA 2 speed wouldn't be a bottleneck. Some SSDs would probably be able to deliver more than 300MB/s, but not the ones I bought. On Sat, 6 Dec 2014, Brett Pemberton <brett.pemberton@gmail.com> wrote:
Keep in mind, with ZFS for example, you can't do like mdadm, and just add a drive to a RAID5/6 array and reshape, growing to use more devices.
Your best bet is to start off with the maximum number of drives you plan to use, and upgrade capacities, instead of adding more devices. Can't comment on BTRFS.
Yes, ZFS doesn't support adding more disks to a RAID array. You can add more RAID arrays to the pool but as you can never remove them you probably don't want to. BTRFS is much better for changing things. It supports things like a "RAID-1" array where the disks have different sizes (as long as no single disk is larger than all the others combined there will be no wasted space). Also it allows removing disks while online. On Sat, 6 Dec 2014, Chris Samuel <chris@csamuel.org> wrote:
Can't comment on BTRFS.
The RAID5/6 code is very experimental and I wouldn't suggest trying to use that functionality for any data you're attached to. Stick to 2 drives, and good backups, for btrfs.
I have servers running BTRFS RAID-1, but so far I haven't even tested BTRFS RAID-5/6. Right at this moment they are merging patches that should make it theoretically usable, but I'd rather have someone else test it first. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/