hardware for home use large storage

Dan Langille dan at langille.org
Wed Feb 10 05:02:48 UTC 2010

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.


>     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:


>     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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