Trying to make sense of stuff I don't know about...
Matthew Dillon wrote:
AHCI on-motherboard with equivalent capabilities do not appear to be
in wide distribution yet. Most AHCI chips can do NCQ to a single
target (even a single target behind a PM), but not concurrently to
multiple targets behind a port multiplier. Even though SATA bandwidth
constraints might seem to make this a reasonable alternative it
actually isn't because any seek heavy activity to multiple drives
will be serialized and perform EXTREMELY poorly. Linear performance
will be fine. Random performance will be horrible.
Don't use a port multiplier and this goes away. I was hoping to avoid a
PM and using something like the Syba PCI Express SATA II 4 x Ports RAID
Controller seems to be the best solution so far.
http://www.amazon.com/Syba-Express-Ports-Controller-SY-PEX40008/dp/B002R0DZWQ/ref=sr_1_22?ie=UTF8&s=electronics&qid=1258452902&sr=1-22
It should be noted that while hotswap is supported with silicon image
chipsets and port multiplier enclosures (which also use Sili chips in
the enclosure), the hot-swap capability is not anywhere near as robust
as you would find with a more costly commercial SAS setup. SI chips
are very poorly made (this is the same company that went bust under
another name a few years back due to shoddy chipsets), and have a lot
of on-chip hardware bugs, but fortunately OSS driver writers (linux
guys) have been able to work around most of them. So even though the
chipset is a bit shoddy actual operation is quite good. However,
this does mean you generally want to idle all activity on the enclosure
to safely hot swap anything, not just the drive you are pulling out.
I've done a lot of testing and hot-swapping an idle disk while other
drives in the same enclosure are hot is not reliable (for a cheap port
multiplier enclosure using a Sili chip inside, which nearly all do).
What I'm planning to use is an SATA enclosure but I'm pretty sure a port
multiplier is not involved:
http://www.athenapower.us/web_backplane_zoom/bp_sata3141b.html
Also, a disk failure within the enclosure can create major command
sequencing issues for other targets in the enclosure because error
processing has to be serialized. Fine for home use but don't expect
miracles if you have a drive failure.
Another reason to avoid port multipliers.
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