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.

"Bottleneck: by solving it you create another."

You yourself mention no interleaving as a possible benefit for faster 
restores. Especially if you have fewer external drives you need to go 
fetch and hook up and "update slots". And yes, you'd also have fewer 
volume-to-job rows in the database and associated joins. Doesn't matter 
if you have one client with a few thousand files and all your backups 
fit on one wd mybook that's always connected. Might make a lot of 
difference on the "enterprise" side of the scale.

Dima


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