Storage considerations.

Flash storage devices are small, low-power and cheap in 
small sizes. Read speeds are relatively high, and with
good quality devices write speeds seem reasonably high,
at least initially. A fast flash device seems to work
very well, either as microSD or USB, until all the free
space has been used and it starts garbage collection. 

That leads to strangely erratic write behavior. Worse
yet, it appears that while the card is busy making space
for a write, read activity either slows down or stops,
sometimes for tens of seconds. Much mischief ensues.

Mechanical hard disks are much slower to read and no
faster than new flash to write. They cost more and
require more power to run. The smallest mechanical 
disk is far bigger than many hobby machines need. An 
inexpensive hard disk costs more than the Pi itself, 
by the time one buys the disk and a powered hub to 
run it we've gone from a $100 server to a $200 server.  

I've been running flash-only RPI2 servers for a little
over four years now. They're lightly loaded and seem
to work very well. Power consumption is only a couple
of watts per server. 


A test Pi3 running arm64 worked well
with a 128 GB microSD for slightly over one year in what
I'd call rather heavy use, mostly running OS build/install.
Suddenly it slowed dramatically, in particular when
writing to microSD. At the same time gstat began reporting
wild fluctuations in queue length and storage percent busy.
It appears this is a warning that garbage colletion is now
required before writes and makes the card close to unusable.

When all the dust settles, there's no single choice for
storage. For a personal mail and webserver, microSD or 
USB flash work fine. If one wants to run intensive test
or development work, I'd suggest a mechanical hard disk or 
maybe an SSD. A hard disk and powered hub needed to run it 
will roughly double the total cost and power consumption.  

20200501