Re: SAN vs Internal Disks

[Brian Hurt <bhurt(at)janestcapital(dot)com>]

Bryan Murphy wrote:

> Our database server connects to the san via iSCSI over Gig/E using
> jumbo frames.  File system is XFS (noatime).
>
>
>  
...

> Throughput, however, kinda sucks.  I just can't get the kind of
> throughput to it I was hoping to get.  When our memory cache is blown,
> the database can be downright painful for the next few minutes as
> everything gets paged back into the cache.
>
>  

Remember that Gig/E is bandwidth limited to about 100 Mbyte/sec.  Maybe 
a little faster than that downhill with a tailwind, but not much.  
You're going to get much better bandwidth connecting to a local raid 
card talking to local disks simply due to not having the ethernet as a 
bottleneck.  iSCSI is easy to set up and manage, but it's slow.  This is 
the big advantage Fibre Channel has- serious performance.  You can have 
multiple channels on a single fibre channel card- IIRC, QLogic's cards 
have a default of 4 channels- each pumping 400 Mbyte/sec.  At which 
point the local bus rapidly becomes the bottleneck.  Of course, this 
comes at the cost of a signifigant increase in complexity.

Brian