Storage question

Quartz quartz at
Wed Sep 9 21:13:20 UTC 2015

> 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,

Err..... this is just a home fileserver/mediabox, right? That's PLENTY 
of horsepower. ZFS has a reputation for being ram hungry, but that's 
mainly the ARC trying to hold a copy of a bazillion files being used by 
dozens of users or you turn on live dedupe. Likewise, CPU usage only 
gets high if you set max compression. You're very unlikely to be pushing 
it like that.

> I am quite amenable to running ZFS, I just don't want to have to
> abandon it & return to UFS if my system proves inadequate for the
> task,

Unless this machine needs to serve dozen of streams to dozens of users, 
you'll be fine, really.

>If I go to ZFS, I (*think* I) use it
> for the whole drives,

That's an issue of minor debate.

Currently ZFS has no capability to shrink the size of a pool once 
created. If a drive dies, the replacement MUST be precisely equal or 
greater in size- even one sector smaller won't work. Since you can't 
guarantee exact sector count (especially between different 
manufacturers) you're kinda stuck only buying bigger drives.... although 
you're probably going to be doing that anyway, so to most people it's a 
moot point.

There are several ways around this by playing with fake partitions and 
GEOM and stuff, I can mail you off list if you're interested.

>& slice it up into
> 'partitions/slices/whatever' to do the install, right ?

Not "partitions" in the traditional sense of the word. A ZFS pool is 
divided up into "datasets", which are kind of a cross between a 
partition and a directory. Datasets can have their own individual quotas 
and compression and backup copies and whatever.

The 10.x installer can set all this up for you if you use the 'root on 
zfs' wizard. You can look at what it does and then learn how to do it 
manually on 9.x

>I have heard that filling your zpool is a *BAD* thing

ZFS is a copy-on-write fs, which means it never directly overwrites 
things. If you completely fill up the drive there's no space left to 
write any changes (which includes file deletions!) and you get stuck. 
This isn't as big a deal as it sounds though since there's a bunch of 
tricks available to prevent it, ie; quotas, reservations, dummy 
datasets, etc. (I think setting 'refreservation=1G' is the current 
preferred way these days).

> could you amplify on that point about no compression w/ MythTV

As others said, video files are already compressed. However, the power 
of ZFS is that with datasets you can selectively enable compression as 
you want. I'd STRONGLY suggest leaving compression enabled for the 
system files, and just disabling it for the directories that hold the 
video data. Make sure you enable this option *BEFORE* you start writing 
files though, ZFS can only compress stuff as it's writing it, not 

(It's almost a moot point anyway, since the lightweight lz compression 
option is so fast it hardly uses enough CPU to be noticeable. You could 
just enable it globally and be fine.)

More information about the freebsd-questions mailing list