Re: high throughput storage server?

[David Brown <david.brown@hesbynett.no>]

(Sorry for the mixup in sending this by direct email instead of posting 
to the list.)

On 17/02/11 00:32, Stan Hoeppner wrote:
> David Brown put forth on 2/15/2011 7:39 AM:
>
>> This brings up an important point - no matter what sort of system you get (home
>> made, mdadm raid, or whatever) you will want to do some tests and drills at
>> replacing failed drives.  Also make sure everything is well documented, and well
>> labelled.  When mdadm sends you an email telling you drive sdx has failed, you
>> want to be /very/ sure you know which drive is sdx before you take it out!
>
> This is one of the many reasons I recommended an enterprise class vendor
> solution.  The Nexsan unit can be configured for SMTP and/or SNMP and/or pager
> notification.  When a drive is taken offline the drive slot is identified in the
> GUI.  Additionally, the backplane board has power and activity LEDs next to each
> drive.  When you slide the chassis out of the rack (while still fully
> operating), and pull the cover, you will see a distinct blink pattern of the
> LEDs next to the failed drive.  This is fully described in the documentation,
> but even without reading such it'll be crystal clear which drive is down.  There
> is zero guess work.
>
> The drive replacement testing scenario you describe is unnecessary with the
> Nexsan products as well as any enterprise disk array.
>

I'd still like to do a test - you don't want to be surprised at the 
wrong moment.  The test lets you know everything is working fine, and 
gives you a feel of how long it will take, and how easy or difficult it is.

But I agree there is a lot of benefit in the sort of clear indications 
of problems that you get with that sort of hardware rather a home made 
system.


>> You also want to consider your raid setup carefully.  RAID 10 has been mentioned
>> here several times - it is often a good choice, but not necessarily.  RAID 10
>> gives you fast recovery, and can at best survive a loss of half your disks - but
>> at worst a loss of two disks will bring down the whole set.  It is also very
>> inefficient in space.  If you use SSDs, it may not be worth double the price to
>> have RAID 10.  If you use hard disks, it may not be sufficient safety.
>
> RAID level space/cost efficiency from a TCO standpoint is largely irrelevant
> today due to the low price of mech drives.  Using the SATABeast as an example,
> the cost per TB of a 20TB RAID 10 is roughly $1600/TB and a 20TB RAID 6 is about
> $1200/TB.  Given all the advantages of RAID 10 over RAID 6 the 33% premium is
> more than worth it.
>

 >

I don't think it is fair to give general rules like that.  In this 
particular case, that might be how the sums work out.  But in other 
cases, using RAID 10 instead of RAID 6 might mean stepping up in chassis 
or controller size and costs.  Also remember that RAID 10 is not better 
than RAID 6 in every way - a RAID 6 array will survive any two failed 
drives, while with RAID 10 an unlucky pairing of failed drives will 
bring down the whole raid.  Different applications require different 
balances here.