Date: Wed, 10 Feb 2010 00:02:49 -0500 From: Dan Langille <dan@langille.org> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: FreeBSD Stable <freebsd-stable@freebsd.org> Subject: Re: hardware for home use large storage Message-ID: <4B723DF9.3070105@langille.org> In-Reply-To: <201002091949.o19JntPo009017@apollo.backplane.com> References: <4B6F9A8D.4050907@langille.org> <alpine.OSX.2.00.1002090103520.982@hotlap.local> <4B71490B.6030602@langille.org> <b269bc571002091004r67167bcobd54cd70f4256dad@mail.gmail.com> <4B71AED5.4030002@wensing.org> <201002091949.o19JntPo009017@apollo.backplane.com>
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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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