Storage question

Matthew Seaman matthew at
Wed Sep 9 13:43:05 UTC 2015

On 2015/09/09 14:16, William A. Mahaffey III wrote:
> I like ZFS in principal (it's one of the things that attracted me to
> FreeBSD about a year ago), but, as someone else noted, it seems to
> require lots of RAM & possibly CPU for best effect. The MythTV box is an
> AMD A4-5000, 1.5 GHz quad-core jaguar, w/ 16 GB of RAM, which isn't
> especially robusto by today's standards, so I am staying w/ UFS.

Actually, ZFS's RAM requirements may not be as gargantuan as all that.
Despite its reputation for gobbling up all that's available and asking
for more, it doesn't have to be that way.  What takes up the space are
the filesystem caches, and how much you need for those depends
absolutely on your usage patterns.  It's certainly possible to run ZFS
on a pretty small-memory machine just so long as you don't expect to
chew through hundreds of files every minute.  Also, 16GB really isn't
considered small except by people dealing with top of the line servers.

If your MythTV usage consists mostly of streaming fairly large files
over the network, then in fact, you won't really win much from caching.
 The files are probably too big to cache entirely in RAM and when you
stream them, you'll read or write them once and then not revisit them
until the next time you watch whatever it was.  So you might as well
turn off the cache for those, reduce the space allocated for ARC and
find you can live happily in a relatively small amount of memory.

ZFS is also not slow for this sort of sequential access -- you'll get
overhead for checksumming and compression (but compression can be an
overall win for IO throughput, simply by reducing the amount of work the
drives have to do) -- but its nothing that any reasonable CPU from the
last 5 or so years can't take in its stride.  You could certainly run
ZFS successfully on the machine you describe, but you'ld have to tune it
carefully and run some decent benchmarks if you wanted to ensure the
best possible results.



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