On Fri, Sep 3, 2010 at 12:08 AM, Jeremy Chadwick <free...@jdc.parodius.com> wrote: > On Thu, Sep 02, 2010 at 04:56:04PM -0400, Zaphod Beeblebrox wrote:
>> With 1.5T disks, I find that the 4 to 1 multipliers have a small >> effect on speed. The 4 drives I have on the multipler are saturated >> at 100% a little bit more than the drives directly connected. >> Essentially you have 3 gigabit for 4 drives instead of 3 gigabit for 1 >> drive. > > 1:4 SATA replicators impose a bottleneck on the overall bandwidth > available between the replicator and the disks attached, as you stated. > Diagram: > > ICH10 > |||___ (SATA300) Port 0, Disk 0 > ||____ (SATA300) Port 1, Disk 1 > |_____ (SATA300) Port 2, eSATA Replicator > ||||________ (SATA300) Port 0, Disk 2 > |||_________ (SATA300) Port 1, Disk 3 > ||__________ (SATA300) Port 2, Disk 4 > |___________ (SATA300) Port 3, Disk 5 > > If Disks 2 through 5 are decent disks (pushing 100MB/sec), essentially > you have 100*4 = 400MB/sec worth of bandwidth being shoved across a > 300MB/sec link. That's making the assumption the disks attached are > magnetic and not SSD, and not taking into consideration protocol > overhead. > A better choice is a SATA multilane HBA, which are usually PCIe-based > with a single connector on the back of the HBA which splits out to > multiple disks (usually 4, but sometimes more). That's just connector-foo. The cards are still very expensive. Many ZFS loads don't saturate disks ... or don't saturate them consistently. I just built several systems with one port per disk --- and those cards tended towards $100/port. 1:4 replicators are less than $10/port and the six port motherboards don't seem to add any cost (4 or 6 SATA ports seem standard now). My point is: if you're building a database server and speed is all you care about, then one port per disk makes sense. If you are building a pile of disk and you're on a budget, port replicators are a good solution. _______________________________________________ freebsd-stable@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"