hardware for home use large storage
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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