On Fri, Jul 24, 2009 at 06:48:24AM +0200, Marc Cousin wrote:
> > In theory, the latency from random IO should be much closer to zero on a
> > flash drive than on a thrashing hard drive, so I was hoping I might need
> > only 1 or two 64GB or 128GB flash drives to provide decent spool size,
> > perhaps not even raid-ed.
> >
> > In addition, SSD/flash drives should be silent and heat up the room less
> > (although that latter effect will be small--10 watts vs 2 watts for each
> > drive)
> 
> For spooling/despooling there should be no latency problems. You need 
> throughput more than latency, and a standard hard drive will be as good as a 
> SSD or even better if setup correctly.
> 
> All you really need is to be able to read and write big streams at the same 
> time. So the real problem is to help your disk scheduler to be able to read 
> while having a lot of data in the write cache.
> 
> We managed to do that by raising the read ahead on the disk array (with the 
> blockdev command in linux). We have managed to get 300MB read and write at 
> the 
> same time with a disk array (I admit it costed a bit more than 2 intel SSD 
> drives), but we have terabytes of spool capacity.
> 
> If you really want a SSD, I'd use it for the catalog's database if I were 
> you. 
> There, disk latency is often the main source of contention.

For spool, I would worry about the limited write (erase)
cycles of SSD.  Sure, the speed of read/write is enormously
appealing, but with how much my spool gets hit I'd hate to
have to set a really early replacement schedule because my
media can't handle many writes.  Rather than SSD for spool,
RAM-Disk looks like a better way to go.

-John

-- 
       (In this one, Pinky is smart.)
Brain: Pinky, Are you pondering what I'm pondering?
Pinky: Yes I am.
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         John M. Lockard |  U of Michigan - School of Information
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