On Tue, 25 Jun 2013, Craig Sanders <cas@taz.net.au> wrote:
I was pointing out that there is a small but noticable performance benefit with ZFS RAID-Z arrays if you have a power-of-two number of *data* disks - e.g. 2, 4, or 8 data disks.
with 5 SAS ports, you can have 4 data disk (yes, a power of 2) and 1 parity for RAID-Z1. Or you can have three data disks (NOT a power of 2) and 2 parity disks for RAID-Z3.
hence, if you want RAIDZ2 with four data disks, then 4 SAS data + 1 SAS parity + 1 SATA parity would achieve that.
Does RAID-Z allow you to designate parity disks? I thought it did more of a RAID-5 thing than a RAID-4 thing. Anyway I wouldn't recommend that on a HP server as SATA disks are really slow. It seems as if HP was deliberately making them slow to drive sales of expensive SAS disks. So in this case I'm looking at a RAID-Z2 with 4 disks and one spare disk.
Performance doesn't matter. I'm looking at replacing a system with a pair of 80G IDE disks in a software RAID-1 which is giving more than enough performance.
RAID-1 typically performs much better than RAID-5/6 or RAID-Z, especially on writes, and switching to RAID5/6/Z can result in a surprisingly unpleasant drop in performance.
(OTOH, ancient IDE drives probably aren't very fast....but i'd still expect a RAID-1 of them to get better write speed than even modern non-SSD disks in RAID5/RAIDZ)
I expect that an array of 4*10,000rpm SAS disks running RAID-Z2 with 12G of RAM for cache can handily beat a system with 2*7200rpm (or maybe 5400rpm) IDE disks and hardly any spare RAM for cache.
they don't do things like that, and they're a lot cheaper (it's hard to beat the M1015, an IBM card using the LSI SAS2008 controller - 8 SAS 6Gbps ports for under $100 new on ebay).
smart raid cards are pointless if you intend to run software-raid like mdadm, LVM, ZFS or btrfs anyway - you want just dumb ports or JBOD for those, not hardware raid.
Yes. However the server in question was bought some years before ZFS on Linux was available and it worked quite well as a hardware RAID-6 device with battery backed write-back cache. I wouldn't recommend that anyone buy a server of similar specs today, but for some years it worked well for it's intended purpose. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/