On 1/24/2015 1:12 PM, Dmitri Maziuk wrote:
> On 1/24/2015 9:00 AM, Kern Sibbald wrote:
>
>> Bottom line: contrary to what I previously thought there *might* be some
>> benefits to turning on spooling for disk Volumes, but only if you have
>> really fast spooling disks (or SSD).  This is an interesting topic, and
>> it would be nice to see some real performance measures as often what one
>> expects in performance does not correspond to what computers really deliver.
> How much faster the cache tier is is only part of the equation, but even
> looking at just that: obviously, if your storage is a tape and your
> cache is a disk. Or the storage is spinning rust and cache's an ssd.
> Well, how about a 6G sata disk cache and .5G usb-2 storage? How about a
> 10k rpm "enterprise" cache disk and 5400 rpm storage disks?
>
> That's only cache to storage, there's also clients to cache speed and
> the degree of parallelism. If you only have one client then there is of
> course no point. At some number of clients & the amount of data you
> could keep the spool "full" and have one job despooling as fast as it
> can while all jobs are spooling as fast as they can. Theoretically. --
> Or 2 jobs despooling, or 3, but then you'll bottleneck on the sata
> backplane, or total usb throughput on the mobo.
>

I would add that network bandwidth easily becomes the bottleneck with 
concurrent jobs.

Another question is whether or not attribute spooling is needed. If the 
DB writes are the bottleneck, then of course it is. But what if the DB 
is on fast SSD? In that case direct DB writes could potentially be much 
faster than attribute spooling and de-spooling. I'm not sure it is 
correct to assume that attribute spooling is beneficial. That too 
requires testing.


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