hardware for home use large storage

[Steve Polyack]

On 2/10/2010 12:02 AM, Dan Langille wrote:
> 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 
>

Dan, I can personally vouch for these cards under FreeBSD.  We have 3 of 
them in one system, with almost every port connected to a port 
multiplier (SiI5xxx PMs).  Using the siis(4) driver on 8.0-RELEASE 
provides very good performance, and supports both NCQ and FIS-based 
switching (an essential for decent port-multiplier performance).

One thing to consider, however, is that the card is only single-lane 
PCI-Express.  The bandwidth available is only 2.5Gb/s (~312MB/sec, 
slightly less than that of the SATA-2 link spec), so if you have 4 
high-performance drives connected, you may hit a bottleneck at the 
bus.   I'd be particularly interested if anyone can find any similar 
Silicon Image SATA controllers with a PCI-E 4x or 8x interface ;)

>
>>
>>     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).
>
>
I haven't had such bad experience as the above, but it is certainly a 
concern.  Using ZFS we simply 'offline' the device, pull, replace with a 
new one, glabel, and zfs replace.  It seems to work fine as long as 
nothing is accessing the device you are replacing (otherwise you will 
get a kernel panic a few minutes down the line).  mav at FreeBSD.org has 
also committed a large patch set to 9-CURRENT which implements "proper" 
SATA/AHCI hot-plug support and error-recovery through CAM.

-Steve Polyack