Re: Provide a better free space estimate on RAID1

[Roman Mamedov <rm@romanrm.net>]

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On Thu, 6 Feb 2014 22:30:46 -0700
Chris Murphy <lists@colorremedies.com> wrote:

> >From the original post, context is a 2x 1TB raid volume:
> 
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sda2       1.8T  1.1M  1.8T   1% /mnt/p2
> 
> Earlier conventions would have stated Size ~900GB, and Avail ~900GB. But that's not exactly true either, is it?

Much better, and matching the user expectations of how RAID1 should behave,
without a major "gotcha" blowing up into their face the first minute they are
trying it out. In fact next step that I planned would be finding how to adjust
also Size and Used on all my machines to show what you just mentioned. I get it
that btrfs is special and its RAID1 is not the usual RAID1 either, but that's
not a good reason to break the 'df' behavior; do whatever you want with in
'btrfs fi df', but if I'm not mistaken the UNIX 'df' always was about user
data, how much of my data I have already stored on this partition and how much
more can I store. If that's not possible to tell, then try to be reasonably
close to the truth, not deliberately off by 2x.

> On Btrfs ...the amount Avail is likewise not wrong because that space is "not otherwise occupied" which is the definition of available.

That's not the definition of available that's directly useful to anyone, but
rather a filesystem-designer level implementation detail, if anything.

What usually interests me is, I have a 100 GB file, can I fit it on this
filesystem, yes/no? Sure let's find out, just check 'df'. Oh wait, not so fast
let's remember was this btrfs? Is that the one with RAID1 or not?... And what
if I am accessing that partition on a server via a network CIFS/NFS share and
don't even *have a way to find out* any of that.


-- 
With respect,
Roman

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