Message-ID: <4C871802.7040203@krogh.cc>
Date: Wed, 08 Sep 2010 06:58:42 +0200
From: Jesper Krogh <jesper@krogh.cc>
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To: Scott Marlowe <scott.marlowe@gmail.com>
CC: Craig James <craig_james@emolecules.com>, pgsql-admin@postgresql.org
Subject: Re: [NOVICE] - SAN/NAS/DAS - Need advises
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On 2010-09-07 22:47, Scott Marlowe wrote:
> Ok, recently I have compared prices a NexSan SASBeast with 42 15K SAS
>>> drives
>>> with a HP MDS600 with 15K SAS drives.
>>>
>>> The first is 8gbit Fibre Channel, the last is 3Gbit DAS SAS. The
>>> fibre channel version is about 20% more expensive pr TB.
>>>
>>> So of course it is a "fraction of the cost of a SAN", but it is a
>>> fairly small one.
>>>        
>> Are you really comparing equal systems?  "8gbit Fibre Channel" means a
>> single Fibre Channel shared by 42 disks, whereas "3GBit DAS SAS" means 42
>> 3gbit channels running in parallel.  It seems like you'd really need some
>> realistic benchmarks that emulate your actual server load before you'd know
>> how these two systems compare.
>>      
> Well, not usually.  Most SAS DAS systems use a single multi-lane cable
> that gives you 4x3GB channels, etc.
>
> However, unless you're doing little than sequentially scanned reports
> of a large size being read, the difference between 8gb and 3gb is not
> going to matter.    There are lots of very hard working transactional
> databases that are lucky to see more than 20 or 40 megabytes a second
> getting trasnferred spread out over 30 or 40 drives.
>
> What really matters here is if the 8gb SAN is as fast as or faster
> than the DAS setup.  For most people measuring the speed of the
> interface is a lot like the famous Tom Lane quote about benchmarking
> jet fighters versus airliners by measuring the amount of runway they
> need.
>    
If you can get 10k tps on the SAN and 10k tps on the DAS
> So to the OP, what are hoping to get from the SAN that you won't get
> from the DAS?  Also, how reliable are these two in comparison to each
> other is kinda of important.  Speed of the interface isn't a real big
> deal for a database server  Size of the battery backed cache in each
> one is  And how each survives the power plug pull test.  If your SAN
> salesman balks at a power on test you don't have to run it, you'll
> know.
>    
All wise words, that I can acknowledge with "hands on" experience.

I was basically only reacting to the "you can do the choice based
on cost alone.. DAS is soo-much-cheaper". In the comparison, I get
an equal amount of disks with same characteristica, in the same
raid-configuration. 1-2GB Battery backed cache on each. So on
paper, I think the systems are directly comparable. In the real
world it is more about "feelings" since I never get to benchmark both of 
them.


Jesper
-- 
Jesper