Re: BTRF - Storage Usage

[Hugo Mills <hugo@carfax.org.uk>]

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On Thu, Sep 27, 2012 at 01:25:58PM +0200, Sébastien Maury wrote:
> Hi,
> 
> Thanks for the quick reply, this clarify me lots of things.
> I've had read the articles you mentioned, but i must admit that your  
> explanations based on my examples makes things even more clearer.
> 
> Also, if i understand things properly, snaphots size aren't included  
> in the "btrfs filesystem show" command output ?
> So, the use, for example, of a "du -sh /.snapshots" is correct to  
> determine the disk usage of my snapshots ?

   "Disk usage of a snapshot" has two different answers:

1) The total size of the files listed in the snapshot, which you can
   get from du.

2) The amount of space that would be freed up by deleting the
   snapshot, which isn't currently available, but probably will be
   soon. (The additional bookkeeping code was part of the qgroups
   patches, which are in 3.6).

> I will see with the people of my company in charge of maintaining  
> distributions to provide us a more recent kernel.
> 
> PS : I use SLES 11 SP2 distribution.

   OK, that one's actually one of the few that does keep proper
backports: https://btrfs.wiki.kernel.org/index.php/Getting_started#Distro_support

   That said, I don't know how good they are at keeping up -- probably
pretty good, but other people here may be able to answer that better.

   Hugo.

> Hugo Mills <hugo@carfax.org.uk> a écrit :
> 
> > On Thu, Sep 27, 2012 at 12:44:27PM +0200, Sébastien Maury wrote:
> >> I've installed a new server using btrfs for my root partition ("/").
> >>
> >> It uses snapper for snapshots management and all seems to work pretty fine.
> >>
> >> My problem is to be able to know the remaining REAL free space in my
> >> partition.
> >
> >    This is in the FAQ:   
> > https://btrfs.wiki.kernel.org/index.php/FAQ#Why_are_there_so_many_ways_to_check_the_amount_of_free_space.3F
> >
> >    Short answer: you can't know in general.
> >
> >    Longer answer -- see below.
> >
> >> Using different commands, i have different results, and i don't know
> >> how to interpret them correctly :
> >
> >> poivron:~ # btrfs filesystem show /dev/sda3
> >> Label: none  uuid: 9e68b667-f9f9-490f-9da1-ae4e91558212
> >>          Total devices 1 FS bytes used 2.58GB
> >>          devid    1 size 131.64GB used 10.04GB path /dev/sda3
> >
> >    You have 131.64 GiB of raw storage in your filesystem. Of that,
> > 10.04 GiB is currently allocated for use by the FS (and it will take
> > more as it needs it).
> >
> >> poivron:~ # btrfs filesystem df /
> >> Data: total=4.01GB, used=2.16GB
> >
> >    4.01 GiB of the 10.04 GiB allocation is assigned for use by data,
> > and 2.16 GiB of that allocation actually contains data.
> >
> >> System, DUP: total=8.00MB, used=4.00KB
> >
> >    16 MiB (=2*8.00 MiB) of the 10.04 GiB allocation is assigned for
> > use as two copies of the system data. There is 4 KiB of system data
> > actually used.
> >
> >> System: total=4.00MB, used=0.00
> >> Metadata, DUP: total=3.00GB, used=429.16MB
> >
> >    6 GiB (=2*3.00 GiB) of your 10.04 GiB allocation is assigned for
> > use as metadata, with two copies (DUP) being kept. 429.16 MiB of the
> > 3.00 GiB is currently in use.
> >
> >> Metadata: total=8.00MB, used=0.00
> >
> >> poivron:~ #  df -hP /
> >> Filesystem      Size  Used Avail Use% Mounted on
> >> /dev/sda3       132G  3.0G  124G   3% /
> >
> >    Plain old df can't handle the truth, so this is at best only a hint
> > at what's actually happening. When "Avail" reaches zero, your FS is
> > probably full. Other than that, you can't necessarily say very much.
> >
> >> ===========
> >>
> >> Please help me understand and interpret those information to know the
> >> most accurately as possible what is my real remaining space, and what
> >> space is used by what.
> >>
> >> Although, i don't really understand the output of the command "btrfs
> >> filesystem df /" : what are exactly "Data", "System DUP", "System
> >> total", "Metadata DUP" and "Metadata total" ?
> >
> >    This should all be covered in the glossary on the website:
> > https://btrfs.wiki.kernel.org/index.php/Glossary
> >
> >    Data is the contents of your files. Metadata is all the other stuff
> > that the FS needs in order to store your files -- directory
> > structures, permissions, locations of the file data, that kind of
> > thing. System is a particular bit of the metadata (the chunk tree)
> > which governs an internal physical/virtual mapping, and which needs to
> > be read before anything else can make any kind of sense.
> >
> >    DUP is a bit like RAID-1: anything stored in a DUP chunk is
> > actually written to two different places on the disk, and can help
> > recovery in the case of physical disk corruption (e.g. bad blocks,
> > head crash).
> >
> >> ==========
> >>
> >> Here are some complementary informations :
> >> poivron:~ # uname -a
> >> Linux poivron 3.0.26-0.7-default #1 SMP Tue Apr 17 10:27:57 UTC 2012
> >> (3829766) x86_64 x86_64 x86_64 GNU/Linux
> >
> >    You [probably(*)] need to upgrade your kernel as soon as possible.
> > btrfs code moves very fast, and 3.0 has significant bugs in it. You
> > should be running the latest released kernel -- right now, that's 3.5,
> > or 3.6-rc7. Next week, it will probably change to 3.6 when Linus makes
> > the next release. Most distributions have a repository somewhere which
> > will give you access to new kernels without too much trouble.
> >
> >    Hugo.
> >
> > (*) Some of the enterprise distributions do have backported btrfs
> > fixes in their apparently older kernels.
> >

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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        --- A clear conscience.  Where did you get this taste ---        
                         for luxuries,  Bernard?                         

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