Re: SAN vs Internal Disks

Bryan Murphy wrote:

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