On 2016-02-16 23:49, Duncan wrote:
Christian Völker posted on Tue, 16 Feb 2016 20:25:47 +0100 as excerpted:
sorry for the simple question and I assume every developer here laughs
about this question.
Anyway:
I have read loads of documents but did not find an answer for sure. Even
though I assume I am right.
On a btrfs filesystem created; is it possible to have subvolumes with
data duplication and another subvolume without (resp. with just metadata
duplication)?
I have some large filesystems currently with ext4 and I am thinking of
changing to btrfs. Some of the data is more important than others. So I
want to have data duplication on the important files (sorted in a mount
point) and without for the other subvolume.
So I want to have the advantage of redundancy of important files
combined with the flexibility of the volume manager and shared disk
space.
As Hugo mentions, that's not possible at this point, tho the plan is to
make replication mode a per-subvolume or even per-file property at some
still-future point. Given the rate of progress, however, that future
point is extremely likely to be at least two years out and could well be 5
+ years out -- IOW, it's nothing you could plan for at this point.
However, it's quite possible to do multiple separate btrfs, with each one
having its own replication mode. That is, in fact, what I do here, tho
in my case it's more to keep all my data eggs from being in the same
filesystem basket, in case btrfs decides to eat a filesystem, than it is
for different replication (most of mine are raid1 across partitions on
two devices). If the filesystem goes, being on a different subvolume
from whatever triggered the problem isn't going to help much, while being
on a different filesystem entirely, which might have been mounted
read-only or not even mounted at all at the time, very likely will, and I
prefer not having all my data eggs in the same filesystem basket,
particularly when that filesystem basket is btrfs, which while
stabilizing, isn't yet full stable and mature yet, even if it means a bit
more hands-on administration than would simply shoving everything in the
same basket and hoping the bottom doesn't drop out of it.
Tho that might be just me...
It's not just you, I do so myself, and usually make the same
recommendation to people who ask me about it. This was in fact the
usual recommendation on most old UNIX systems as well, although that was
just as much because of small disks and a lack of logical volume
management as it was about data safety. The big difference these days
is where the splits are made. Traditional UNIX often had /var, /usr,
/home, and / as separate filesystems, often with multiple /home
filesystems on big systems. Most modern Linux distributions can't
handle /usr and /var being on separate filesystems from /, so people
split things differently when the split things at all (the good
installers usually provide options to easily do common splits like a
separate /home, but most make assumptions about how you lay things out,
this is part of the reason I prefer Gentoo, it lets you do things
however you want as long as you can configure it yourself). In my case
for example, I split out stuff that's trivial to recreate (/usr/portage,
/var/lib/layman, /usr/src), stuff that I don't want backed up for other
reasons (I have a separate filesystem I use for local copies of public
VCS repositories for example), and stuff that should just be on a
separate filesystem regardless (either because it needs different
replication, or because it needs to be more isolated from the rest of
the system, back-end storage for the GlusterFS storage cluster I'm
building being an excellent example).
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