On 21/02/14 10:40, Sam Skipsey wrote:
> On 21 February 2014 08:34, Winnie Lacesso <[log in to unmask]> wrote:
>> Happy Friday!
>>
>> In spec'ing a new 12-bay (3.5" disks) server, vendor says don't need an
>> 8-port hardware RAID controller & 4-port for our 12 bays; just a 4-port.
>>
>> "As this chassis has a SAS2.0 expander a 4 channel raid controller will
>> work with all your disks so you will only need the one 4 channel raid card
>> (we have used this on larger storage servers with up to 36 disks so it
>> will be more than good enough)."
>>
>> "SAS Expander" is news to me, a bit of lookup seems to confirm what he
>> says (they exist, support many disk connections etc).
>>
>> Does anyone here have practical experience with SAS2.x expanders, are they
>> as wonderful + performant + totally hot-swap as regular N-port hardware
>> RAID controller?

All of the (NAS-style) storage servers we've bought in the last year or 
two have used SAS expanders, rather than discrete SAS or SATA ports for 
each drive. It means we can use smaller, cheaper RAID controllers with 
fewer ports.

As far as performance, hot-swap etc goes, there's no practical 
difference. There is a potential bottle neck between the array and the 
controller as all data is transferred over the 4lane connector, but as 
this equates to 24Gbit/s (3GB/s) it's unlikely to be an issue unless you 
intend to have an array full of SSDs. You can always use some of the 
saved money on the controller to beef up the spec in other areas anyway.

The only note of caution I would add is to go with the recommended 
controller/expander combo provided by your vendor, not everything is as 
100% compatible as one might hope IME.

John

>> A pointer to a less-technical explanatory how-they-work page than
>> wikipedia's page on SAS (sorry!) would be appreciated, if anyone knows of
>> one.
>>
>
> The one line version: A SAS expander does for SAS what an Ethernet
> Switch does for Ethernet. (That is, it lets lots of devices speak SAS
> to your controller without you needing a separate port for each one.
> The Expander lets the controller know which SAS interface is speaking
> at any point in time, so it can respond appropriately.)
>
>> Winnie Lacesso / 55% HPC Storage Admin, 20% Particle Physics, 25% SysOps
>> HH Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL, UK
>> University of Bristol


-- 
John Bland                       [log in to unmask]
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High Energy Physics Division     tel (int): 42911
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