zfs scrub enable by default

[Alan Somers]

On Wed, Aug 5, 2020 at 10:06 AM John Long via freebsd-fs <
freebsd-fs at freebsd.org> wrote:

> On 05/08/2020 15:24, Alan Somers wrote:
> > On Wed, Aug 5, 2020 at 9:22 AM John Long via freebsd-fs
> > <freebsd-fs at freebsd.org <mailto:freebsd-fs at freebsd.org>> wrote:
> >
> >     On 05/08/2020 13:15, Bob Friesenhahn wrote:
> >      > On Tue, 4 Aug 2020, Karl Denninger wrote:
> >      >
> >      >> Let me give you two allegedly "degenerate" cases that are
> >     actually not
> >      >> degenerate at all.
> >      >>
> >      >> 1. A laptop or workstation.  It is backed up.  It uses ZFS
> because
> >      >> it's faster, and I can establish a filesystem for some project
> very
> >      >> easily and quickly, it's segregated, I can work on it and
> >     destroy it
> >      >> trivially when done.  I can set quotas on that, etc.  If I want
> to
> >      >> move its mountpoint, I can trivially do so. And so on.  Note
> >     that here
> >      >> there is no redundancy at all; no raidZx, no mirroring, etc.  I'm
> >      >> merely using it for convenience.
> >      >
> >      > Did you remember to set copies=2 or copies=3 for zfs filesystems
> >     where
> >      > you hope not to experience data loss?  It needs to be set as soon
> as
> >      > possible since it only applies to new files.  This is a way to
> >     get more
> >      > media redundancy, although the whole drive may fail.
> >
> >     Does copies=n actually create n-1 additional physical copies or is it
> >     copy-on-write, or something else yet?
> >
> >     /jl
> >
> >
> > Yes, copies=3 will actually create 3 physical copies of the data
> > somewhere.  It's basically mirroring at the DMU layer, rather than the
> > block layer. > -Alan
>
> Thanks, I figured that must be the case but I thought it was better ask.
>
> So is it correct that aside from a single disk vdev, it would be a bad
> practice to specify additional copies? How does dedup deal with it?
>
> /jl
>

Yeah.  I wouldn't recommend ever chaning the copies value from the
default.  If dedup is on (also not recommended), then it wouldn't make
sense to use multiple copies anyway.  And if zfs encryption is on, then you
can't use multiple copies, because encryption repurposes some of the block
pointer's fields.
-Alan