wrong values in "df" and "btrfs filesystem df"

wrong values in "df" and "btrfs filesystem df"

[Helmut Hullen]

Hallo, linux-btrfs,

First I create an array of 2 disks with

  mkfs.btrfs -d raid0 -m raid1 /dev/sdb1 /dev/sdd1

and mount it at /srv/MM.

Then I fill it with about 1,6 TByte.
And then I add /dev/sde1 via

  btrfs device add /dev/sde1 /srv/MM
  btrfs filesystem balance /srv/MM
    (it run about 20 hours)

Then I work on it, copy some new files, delete some old files - all  
works well. Only

  df /srv/MM
  btrfs filesystem df /srv/MM

show some completely wrong values:

# df /srv/MM

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM

# btrfs filesystem df /srv/MM

Data, RAID0: total=1.67TB, used=1.48TB
System, RAID1: total=16.00MB, used=112.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=3.75GB, used=2.26GB

# btrfs-show

Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
	Total devices 3 FS bytes used 1.48TB
	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
	devid    2 size 1.81TB used 573.77GB path /dev/sde1
	devid    1 size 1.82TB used 570.01GB path /dev/sdd1

Btrfs Btrfs v0.19

------------------------------------------------

"df" shows an "Available" value which isn't related to any real value.  
The sum of "used" and "Available" is far away from the really existent  
disk space. When I copy additional files to "/srv/MM" then "used" still  
shows the right value, and the sum grows (slowly) to the max. available  
space.

In "btrfs filesystem df /srv/MM" the line

  Data, RAID0: total=1.67TB, used=1.48TB

shows a "total" value which isn't related to any existent value; maybe  
it still shows the used space before adding the third partition.
This (wrong) value seems not to change.

Kernel 2.6.38.1
btrfs from november 2010

Viele Gruesse!
Helmut

Re: wrong values in "df" and "btrfs filesystem df"

[Hugo Mills]

[-- Attachment #1: Type: text/plain, Size: 2882 bytes --]

On Sat, Apr 09, 2011 at 08:25:00AM +0200, Helmut Hullen wrote:
> Hallo, linux-btrfs,
> 
> First I create an array of 2 disks with
> 
>   mkfs.btrfs -d raid0 -m raid1 /dev/sdb1 /dev/sdd1
> 
> and mount it at /srv/MM.
> 
> Then I fill it with about 1,6 TByte.
> And then I add /dev/sde1 via
> 
>   btrfs device add /dev/sde1 /srv/MM
>   btrfs filesystem balance /srv/MM
>     (it run about 20 hours)
> 
> Then I work on it, copy some new files, delete some old files - all  
> works well. Only
> 
>   df /srv/MM
>   btrfs filesystem df /srv/MM
> 
> show some completely wrong values:
> 
> # df /srv/MM
> 
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
> 
> # btrfs filesystem df /srv/MM
> 
> Data, RAID0: total=1.67TB, used=1.48TB
> System, RAID1: total=16.00MB, used=112.00KB
> System: total=4.00MB, used=0.00
> Metadata, RAID1: total=3.75GB, used=2.26GB
> 
> # btrfs-show
> 
> Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
> 	Total devices 3 FS bytes used 1.48TB
> 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
> 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
> 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
> 
> Btrfs Btrfs v0.19
> 
> ------------------------------------------------
> 
> "df" shows an "Available" value which isn't related to any real value.  

   I _think_ that value is the amount of space not allocated to any
block group. If that's so, then Available (from df) plus the three
"total" values (from btrfs fi df) should equal the size value from df.

> The sum of "used" and "Available" is far away from the really existent  
> disk space. When I copy additional files to "/srv/MM" then "used" still  
> shows the right value, and the sum grows (slowly) to the max. available  
> space.
> 
> In "btrfs filesystem df /srv/MM" the line
> 
>   Data, RAID0: total=1.67TB, used=1.48TB
> 
> shows a "total" value which isn't related to any existent value; maybe  
> it still shows the used space before adding the third partition.
> This (wrong) value seems not to change.

   It's not wrong -- it simply doesn't mean what you think it does. :)

   The total value in the output of "btrfs fi df" is the total space
allocated to block groups. As the filesystem needs more space, it
will allocate more block groups from the available raw storage pool,
and the number will go up.

   This is explained on the wiki at [1].

   HTH,
   Hugo.

[1] https://btrfs.wiki.kernel.org/index.php/FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
  --- There are three things you should never see being made: laws, ---  
                        standards,  and sausages.                        

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Re: wrong values in "df" and "btrfs filesystem df"

[Stephane Chazelas]

2011-04-09 10:11:41 +0100, Hugo Mills:
[...]
> > # df /srv/MM
> > 
> > Filesystem           1K-blocks      Used Available Use% Mounted on
> > /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
> > 
> > # btrfs filesystem df /srv/MM
> > 
> > Data, RAID0: total=1.67TB, used=1.48TB
> > System, RAID1: total=16.00MB, used=112.00KB
> > System: total=4.00MB, used=0.00
> > Metadata, RAID1: total=3.75GB, used=2.26GB
> > 
> > # btrfs-show
> > 
> > Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
> > 	Total devices 3 FS bytes used 1.48TB
> > 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
> > 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
> > 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
> > 
> > Btrfs Btrfs v0.19
> > 
> > ------------------------------------------------
> > 
> > "df" shows an "Available" value which isn't related to any real value.  
> 
>    I _think_ that value is the amount of space not allocated to any
> block group. If that's so, then Available (from df) plus the three
> "total" values (from btrfs fi df) should equal the size value from df.
[...]

Well,

$ echo $((2898463184 + 1.67*2**30 + 4*2**10 + 16*2**10*2 + 3.75*2**20*2))
4699513214.0799999

I do get the same kind of discrepancy:

$ df -h /mnt
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb              8.2T  3.5T  3.2T  53% /mnt
$ sudo btrfs fi show
Label: none  uuid: ...
        Total devices 3 FS bytes used 3.43TB
        devid    4 size 2.73TB used 1.17TB path /dev/sdc
        devid    3 size 2.73TB used 1.17TB path /dev/sdb
        devid    2 size 2.70TB used 1.14TB path /dev/sda4
$ sudo btrfs fi df /mnt
Data, RAID0: total=3.41TB, used=3.41TB
System, RAID1: total=16.00MB, used=232.00KB
Metadata, RAID1: total=35.25GB, used=20.55GB


$ echo $((3.2 + 3.41 + 2*16/2**20 + 2*35.25/2**10))
6.6788476562500003

-- 
Stephane

Re: wrong values in "df" and "btrfs filesystem df"

[Helmut Hullen]

Hallo, Hugo,

Du meintest am 09.04.11:

>>   df /srv/MM
>>   btrfs filesystem df /srv/MM
>>
>> show some completely wrong values:
>>
>> # df /srv/MM
>>
>> Filesystem           1K-blocks      Used Available Use% Mounted on
>> /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
>>
>> # btrfs filesystem df /srv/MM
>>
>> Data, RAID0: total=3D1.67TB, used=3D1.48TB
>> System, RAID1: total=3D16.00MB, used=3D112.00KB
>> System: total=3D4.00MB, used=3D0.00
>> Metadata, RAID1: total=3D3.75GB, used=3D2.26GB
>>
>> # btrfs-show
>>
>> Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
>> 	Total devices 3 FS bytes used 1.48TB
>> 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
>> 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
>> 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
>>
>> Btrfs Btrfs v0.19
>>
>> ------------------------------------------------
>>
>> "df" shows an "Available" value which isn't related to any real
>> value.

>    I _think_ that value is the amount of space not allocated to any
> block group. If that's so, then Available (from df) plus the three
> "total" values (from btrfs fi df) should equal the size value from
> df.

I'm not convinced - sorry.

"used" plus "available" should be nearly the same value as the "total" =
=20
in "df"; in my example 1.6 TB + 2.9 TB is far away from the total of 5.=
8 =20
TB (here: Tera =3D 10^9).

>    The total value in the output of "btrfs fi df" is the total space
> allocated to block groups. As the filesystem needs more space, it
> will allocate more block groups from the available raw storage pool,
> and the number will go up.

>    This is explained on the wiki at [1].

I've studied the page - the data shown there looks to be consistent. =20
Especially: "used" plus "avail" =3D "size"

-------------------------------------

The data seems (mostly) to be consistent on a new array of disks/=20
partitions. But when I work with "device add" and "device delete" then
there are holes.

Another machine:

/dev/sda about 140 GB
/dev/sdb about 140 GB
/dev/sdc about  70 GB

# mkfs.btrfs -L SCSI -d raid0 -m raid1 /dev/sdb1 /dev/sdc1
# mit 70 GByte gef=FCllt

# df -t btrfs
=46ilesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sdb1    btrfs   214058944  73956996  65522124  54% /mnt/SCSI

# btrfs filesystem show
Label: 'SCSI'  uuid: 1932d11d-021a-4054-8429-db25a0204221
	Total devices 2 FS bytes used 70.43GB
	devid    1 size 136.73GB used 37.03GB path /dev/sdb1
	devid    2 size 67.41GB used 37.01GB path /dev/sdc1

# btrfs filesystem df /mnt/SCSI
Data, RAID0: total=3D72.00GB, used=3D70.33GB
Data: total=3D8.00MB, used=3D0.00
System, RAID1: total=3D8.00MB, used=3D12.00KB
System: total=3D4.00MB, used=3D0.00
Metadata, RAID1: total=3D1.00GB, used=3D104.78MB
Metadata: total=3D8.00MB, used=3D0.00

#------------------------------------------------------------
# btrfs device add /dev/sda1 /mnt/SCSI

# df -t btrfs
=46ilesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sdb1    btrfs   356234160  84177056 200667344  30% /mnt/SCSI

# btrfs filesystem show
Label: 'SCSI'  uuid: 1932d11d-021a-4054-8429-db25a0204221
	Total devices 3 FS bytes used 80.16GB
	devid    1 size 136.73GB used 42.03GB path /dev/sdb1
	devid    3 size 135.59GB used 0.00 path /dev/sda1
	devid    2 size 67.41GB used 42.01GB path /dev/sdc1

# btrfs filesystem df /mnt/SCSI
Data, RAID0: total=3D82.00GB, used=3D80.04GB
Data: total=3D8.00MB, used=3D0.00
System, RAID1: total=3D8.00MB, used=3D12.00KB
System: total=3D4.00MB, used=3D0.00
Metadata, RAID1: total=3D1.00GB, used=3D119.54MB
Metadata: total=3D8.00MB, used=3D0.00

#------------------------------------------------------------
# btrfs fi balance /mnt/SCSI

# df -t btrfs
=46ilesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sdb1    btrfs   356234160  84125344 228211536  27% /mnt/SCSI

# btrfs filesystem show
Label: 'SCSI'  uuid: 1932d11d-021a-4054-8429-db25a0204221
	Total devices 3 FS bytes used 80.14GB
	devid    1 size 136.73GB used 28.28GB path /dev/sdb1
	devid    3 size 135.59GB used 27.00GB path /dev/sda1
	devid    2 size 67.41GB used 29.01GB path /dev/sdc1

# btrfs filesystem df /mnt/SCSI
Data, RAID0: total=3D81.00GB, used=3D80.04GB
Data: total=3D8.00MB, used=3D0.00
System, RAID1: total=3D8.00MB, used=3D12.00KB
System: total=3D4.00MB, used=3D0.00
Metadata, RAID1: total=3D1.25GB, used=3D94.29MB
Metadata: total=3D8.00MB, used=3D0.00

#------------------------------------------------------------
# btrfs device delete /dev/sdc1 /mnt/SCSI
# umount /mnt/SCSI
# mount LABEL=3DSCSI /mnt/SCSI

# df -t btrfs
=46ilesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sdb1    btrfs   285548192 232024076  51758540  82% /mnt/SCSI

# btrfs filesystem show
Label: 'SCSI'  uuid: 1932d11d-021a-4054-8429-db25a0204221
	Total devices 2 FS bytes used 221.04GB
	devid    1 size 136.73GB used 111.51GB path /dev/sdb1
	devid    3 size 135.59GB used 111.51GB path /dev/sda1

# btrfs filesystem df /mnt/SCSI
Data, RAID0: total=3D222.00GB, used=3D220.80GB
System, RAID1: total=3D8.00MB, used=3D24.00KB
System: total=3D4.00MB, used=3D0.00
Metadata, RAID1: total=3D512.00MB, used=3D243.77MB

--------------------------------------------------------------

What "df" shows as "available" immediately after creation is wrong.
What it shows after "device add" is wrong.
What it shows after "balance" is wrong.
What it shows after "device delete" looks ok.

Ok - now I presume that sometimes "df" or/and the "btrfs" commands only=
 =20
show wrong values, but the system works with the right values (there is=
t =20
more free space than shown). But I fear that's not reliable.

--------------------------------

I also write scripts etc; I know since decades it's easy to make =20
mistakes, but sometimes it's hard work to find them.

Viele Gruesse!
Helmut
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Re: wrong values in "df" and "btrfs filesystem df"

[Calvin Walton]

On Sat, 2011-04-09 at 10:11 +0100, Hugo Mills wrote:
> On Sat, Apr 09, 2011 at 08:25:00AM +0200, Helmut Hullen wrote:
> > Hallo, linux-btrfs,
> > 
> > First I create an array of 2 disks with
> > 
> >   mkfs.btrfs -d raid0 -m raid1 /dev/sdb1 /dev/sdd1
> > 
> > and mount it at /srv/MM.
> > 
> > Then I fill it with about 1,6 TByte.
> > And then I add /dev/sde1 via
> > 
> >   btrfs device add /dev/sde1 /srv/MM
> >   btrfs filesystem balance /srv/MM
> >     (it run about 20 hours)
> > 
> > Then I work on it, copy some new files, delete some old files - all  
> > works well. Only
> > 
> >   df /srv/MM
> >   btrfs filesystem df /srv/MM
> > 
> > show some completely wrong values:

>    It's not wrong -- it simply doesn't mean what you think it does. :)
> 
>    The total value in the output of "btrfs fi df" is the total space
> allocated to block groups. As the filesystem needs more space, it
> will allocate more block groups from the available raw storage pool,
> and the number will go up.
> 
>    This is explained on the wiki at [1].

And I just drew up a picture which I think should help explain it a bit,
too: http://www.kepstin.ca/dump/btrfs-alloc.png

If I can figure out how to add images to the btrfs wiki, and find a good
place to put it, do you think this would be a helpful addition?

> [1] https://btrfs.wiki.kernel.org/index.php/FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F

-- 
Calvin Walton <calvin.walton@kepstin.ca>

Re: wrong values in "df" and "btrfs filesystem df"

[Helmut Hullen]

Hallo, Calvin,

Du meintest am 09.04.11:

>>> Then I work on it, copy some new files, delete some old files - all
>>> works well. Only
>>>
>>>   df /srv/MM
>>>   btrfs filesystem df /srv/MM
>>>
>>> show some completely wrong values:

[...]

> And I just drew up a picture which I think should help explain it a
> bit, too: http://www.kepstin.ca/dump/btrfs-alloc.png

Nice picture. But it doesn't solve the problem that I need a reliable  
information about the free/available space. And I prefer asking with  
"df" for this information - "df" should work in the same way for all  
filesystems.

Viele Gruesse!
Helmut

Re: wrong values in "df" and "btrfs filesystem df"

[Calvin Walton]

On Sat, 2011-04-09 at 19:05 +0200, Helmut Hullen wrote:
> >>> Then I work on it, copy some new files, delete some old files - all
> >>> works well. Only
> >>>
> >>>   df /srv/MM
> >>>   btrfs filesystem df /srv/MM
> >>>
> >>> show some completely wrong values:

> > And I just drew up a picture which I think should help explain it a
> > bit, too: http://www.kepstin.ca/dump/btrfs-alloc.png
> 
> Nice picture. But it doesn't solve the problem that I need a reliable  
> information about the free/available space. And I prefer asking with  
> "df" for this information - "df" should work in the same way for all  
> filesystems.

The problem is that the answer to the seemingly simple question: "How
much more data can I put onto this filesystem?" gets pretty hard with
btrfs.

Your case is one of the simpler ones - To calculate the remaining space
for files, you take the unused allocated data space (light blue on my
picture), add the unallocated space (white), divide by the raid mode
redundancy, and subtract some percentage (this is only an estimate, of
course...) of that unallocated space for the additional metadata
overhead.

Now imagine the case where your btrfs filesystem has files stored in
multiple raid modes: e.g. some files are raid5, others raid0.
The amount of data you can write to the filesystem then depends on how
you write the data!

You might be able to fit 64gb if you use raid0, but only 48gb with
raid5; and only 16gb with raid1!

There isn't a single number that btrfs can report which does what you
want.

-- 
Calvin Walton <calvin.walton@kepstin.ca>

Re: wrong values in "df" and "btrfs filesystem df"

[Helmut Hullen]

Hallo, Calvin,

Du meintest am 09.04.11:

>> Nice picture. But it doesn't solve the problem that I need a
>> reliable information about the free/available space. And I prefer
>> asking with "df" for this information - "df" should work in the same
>> way for all filesystems.

> The problem is that the answer to the seemingly simple question: "How
> much more data can I put onto this filesystem?" gets pretty hard with
> btrfs.

> Your case is one of the simpler ones - To calculate the remaining
> space for files, you take the unused allocated data space (light blue
> on my picture), add the unallocated space (white), divide by the raid
> mode redundancy, and subtract some percentage (this is only an
> estimate, of course...) of that unallocated space for the additional
> metadata overhead.

That's simple?

Maybe I'm simple minded. But I expect the same meaning of the shown date  
with

        df <mointpoint>

regardless what kind of filesystem is mounted.
But no test item for an IQ test.

If the value of "available" is unresolvable then btrfs should not show  
any value.


Ok - there's a problem slightly similar to the "available" value of  
compressed partitions.

Viele Gruesse!
Helmut

Re: wrong values in "df" and "btrfs filesystem df"

[Peter Stuge]

Helmut Hullen wrote:
> If the value of "available" is unresolvable then btrfs should not
> show any value.

Disagree strongly. I think a pessimistic estimate would be much
better to show, than no value at all. This may be what is currently
shown.

As for solving this with a high degree of usability, that's not
really possible when constrained to the traditional paradigm that one
filesystem will have completely consistent behavior backing all of
it.

I think the only answer is to have btrfs-specific tools that know
more about the filesystem, and can present the relevant facts.

Taking the example of part fs being raid0 and part being raid5, such
a tool would then list calculated values for both those parts of the
fs. One showing how much could go into the raid0 part, the other how
much could go into the raid5 part.

But for such filesystems, Linux can't do what Helmut would like.

Maybe it would be possible to optimize the reported numbers, to be
what the user actually wants as often as possible. Ie. if there is
only one type of backing storage (sorry, don't know the terms) then
the calculation would be easier to get right, following the simple
formula that was just given. This is all eye candy however,
completely irrelevant IMO as long as the filesystem oopses, or eats
root nodes. :)


//Peter

Re: wrong values in "df" and "btrfs filesystem df"

[Hugo Mills]

[-- Attachment #1: Type: text/plain, Size: 1416 bytes --]

On Sat, Apr 09, 2011 at 09:35:49PM +0200, Peter Stuge wrote:
> But for such filesystems, Linux can't do what Helmut would like.

   Indeed. In the general case, there's no single number that "gets it
right". That said, if data and metadata are the same replication type
for the whole filesystem, you *can* get it pretty much exactly
right(*); if data is the same type for the whole filesystem, you can
get it *nearly* right (since metadata is typically a small fraction of
the data size).

(*) Ignoring the space required for reservations to actually do
anything with the filesystem when it's very close to full...

> Maybe it would be possible to optimize the reported numbers, to be
> what the user actually wants as often as possible. Ie. if there is
> only one type of backing storage (sorry, don't know the terms) then

   RAID level or replication strategy. Although "RAID" gives a
slightly misleading impression of what actually happens in btrfs...

> the calculation would be easier to get right, following the simple
> formula that was just given. This is all eye candy however,
> completely irrelevant IMO as long as the filesystem oopses, or eats
> root nodes. :)
> 
> 
> //Peter

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
   --- Great oxymorons of the world, no.  3: Military Intelligence ---   

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Re: wrong values in "df" and "btrfs filesystem df"

[Miao Xie]

On sat, 9 Apr 2011 10:11:41 +0100, Hugo Mills wrote:
> On Sat, Apr 09, 2011 at 08:25:00AM +0200, Helmut Hullen wrote:
>> Hallo, linux-btrfs,
>>
>> First I create an array of 2 disks with
>>
>>   mkfs.btrfs -d raid0 -m raid1 /dev/sdb1 /dev/sdd1
>>
>> and mount it at /srv/MM.
>>
>> Then I fill it with about 1,6 TByte.
>> And then I add /dev/sde1 via
>>
>>   btrfs device add /dev/sde1 /srv/MM
>>   btrfs filesystem balance /srv/MM
>>     (it run about 20 hours)
>>
>> Then I work on it, copy some new files, delete some old files - all  
>> works well. Only
>>
>>   df /srv/MM
>>   btrfs filesystem df /srv/MM
>>
>> show some completely wrong values:
>>
>> # df /srv/MM
>>
>> Filesystem           1K-blocks      Used Available Use% Mounted on
>> /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
>>
>> # btrfs filesystem df /srv/MM
>>
>> Data, RAID0: total=1.67TB, used=1.48TB
>> System, RAID1: total=16.00MB, used=112.00KB
>> System: total=4.00MB, used=0.00
>> Metadata, RAID1: total=3.75GB, used=2.26GB
>>
>> # btrfs-show
>>
>> Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
>> 	Total devices 3 FS bytes used 1.48TB
>> 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
>> 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
>> 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
>>
>> Btrfs Btrfs v0.19
>>
>> ------------------------------------------------
>>
>> "df" shows an "Available" value which isn't related to any real value.  
> 
>    I _think_ that value is the amount of space not allocated to any
> block group. If that's so, then Available (from df) plus the three
> "total" values (from btrfs fi df) should equal the size value from df.

This value excludes the space that can not be allocated to any block group,
This feature was implemented to fix the bug df command add the disk space, which
can not be allocated to any block group forever, into the "Available" value.
(see the changelog of the commit 6d07bcec969af335d4e35b3921131b7929bd634e)

This implementation just like fake chunk allocation, but the fake allocation
just allocate the space from two of these three disks, doesn't spread the
stripes over all the disks, which has enough space.
 
I will post a patch to fix it soon.

> 
>> The sum of "used" and "Available" is far away from the really existent  
>> disk space. When I copy additional files to "/srv/MM" then "used" still  
>> shows the right value, and the sum grows (slowly) to the max. available  
>> space.
>>
>> In "btrfs filesystem df /srv/MM" the line
>>
>>   Data, RAID0: total=1.67TB, used=1.48TB
>>
>> shows a "total" value which isn't related to any existent value; maybe  
>> it still shows the used space before adding the third partition.
>> This (wrong) value seems not to change.
> 
>    It's not wrong -- it simply doesn't mean what you think it does. :)
> 
>    The total value in the output of "btrfs fi df" is the total space
> allocated to block groups. As the filesystem needs more space, it
> will allocate more block groups from the available raw storage pool,
> and the number will go up.
> 
>    This is explained on the wiki at [1].
> 
>    HTH,
>    Hugo.
> 
> [1] https://btrfs.wiki.kernel.org/index.php/FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F
> 

Re: wrong values in "df" and "btrfs filesystem df"

[Stephane Chazelas]

2011-04-10 18:13:51 +0800, Miao Xie:
[...]
> >> # df /srv/MM
> >>
> >> Filesystem           1K-blocks      Used Available Use% Mounted on
> >> /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
> >>
> >> # btrfs filesystem df /srv/MM
> >>
> >> Data, RAID0: total=1.67TB, used=1.48TB
> >> System, RAID1: total=16.00MB, used=112.00KB
> >> System: total=4.00MB, used=0.00
> >> Metadata, RAID1: total=3.75GB, used=2.26GB
> >>
> >> # btrfs-show
> >>
> >> Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
> >> 	Total devices 3 FS bytes used 1.48TB
> >> 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
> >> 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
> >> 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
> >>
> >> Btrfs Btrfs v0.19
> >>
> >> ------------------------------------------------
> >>
> >> "df" shows an "Available" value which isn't related to any real value.  
> > 
> >    I _think_ that value is the amount of space not allocated to any
> > block group. If that's so, then Available (from df) plus the three
> > "total" values (from btrfs fi df) should equal the size value from df.
> 
> This value excludes the space that can not be allocated to any block group,
> This feature was implemented to fix the bug df command add the disk space, which
> can not be allocated to any block group forever, into the "Available" value.
> (see the changelog of the commit 6d07bcec969af335d4e35b3921131b7929bd634e)
> 
> This implementation just like fake chunk allocation, but the fake allocation
> just allocate the space from two of these three disks, doesn't spread the
> stripes over all the disks, which has enough space.
[...]

Hi Miao,

would you care to expand a bit on that. In Helmut's case above
where all the drives have at least 1.2TB free, how would there
be un-allocatable space?

What's the implication of having disks of differing sizes? Does
that mean that the extra space on larger disks is lost?

Thanks,
Stephane

Re: wrong values in "df" and "btrfs filesystem df"

[Arne Jansen]

On 11.04.2011 09:29, Stephane Chazelas wrote:
> 2011-04-10 18:13:51 +0800, Miao Xie:
> [...]

> 
> What's the implication of having disks of differing sizes? Does
> that mean that the extra space on larger disks is lost?

Yes. Currently the allocator cannot handle different sizes well,
especially when mirroring is involved. I sent a patch for this
to the list some weeks ago (see quasi-round-robin), but it hasn't
been merged yet.

-Arne

> 
> Thanks,
> Stephane

Re: wrong values in "df" and "btrfs filesystem df"

[Helmut Hullen]

Hallo, Stephane,

Du meintest am 11.04.11:

> What's the implication of having disks of differing sizes? Does
> that mean that the extra space on larger disks is lost?

Seems to work.
I've tried:

/dev/sda   140 GByte
/dev/sdb   140 GByte
/dev/sdc    70 GByte

    mkfs.btrfs -d raid0 -m raid1 /dev/sdb1 /dev/sdc1

mounted, more than 140 GByte free

Filled with more than 140 GByte (/dev/sdc1 was full to the brim)

    btrfs device add /dev/sda1 ...
    btrfs filesystem balance ...

Needed many hours, but then more than 210 GByte were usable.

Filled up to about 220 GByte; /dev/sdc1 was again full to the brim

    btrfs device delete /dev/sdc1
    umount
    mount

All looks as expected, only the 2 bigger devices are seen, and they  
contain the expected files.

And that looks good: my major interest in btrfs is working in that way -  
adding a bigger device, deleting a smaller device.

Kernel 2.6.38.1
btrfs from november 2010


Only the values shown with "df" and "btrfs filesystem df" need getting  
used to; maybe "available" has to be seen as "at least available".

Viele Gruesse!
Helmut

Re: wrong values in "df" and "btrfs filesystem df"

[Miao Xie]

On Mon, 11 Apr 2011 08:29:46 +0100, Stephane Chazelas wrote:
> 2011-04-10 18:13:51 +0800, Miao Xie:
> [...]
>>>> # df /srv/MM
>>>>
>>>> Filesystem           1K-blocks      Used Available Use% Mounted on
>>>> /dev/sdd1            5846053400 1593436456 2898463184  36% /srv/MM
>>>>
>>>> # btrfs filesystem df /srv/MM
>>>>
>>>> Data, RAID0: total=1.67TB, used=1.48TB
>>>> System, RAID1: total=16.00MB, used=112.00KB
>>>> System: total=4.00MB, used=0.00
>>>> Metadata, RAID1: total=3.75GB, used=2.26GB
>>>>
>>>> # btrfs-show
>>>>
>>>> Label: MMedia  uuid: 120b036a-883f-46aa-bd9a-cb6a1897c8d2
>>>> 	Total devices 3 FS bytes used 1.48TB
>>>> 	devid    3 size 1.81TB used 573.76GB path /dev/sdb1
>>>> 	devid    2 size 1.81TB used 573.77GB path /dev/sde1
>>>> 	devid    1 size 1.82TB used 570.01GB path /dev/sdd1
>>>>
>>>> Btrfs Btrfs v0.19
>>>>
>>>> ------------------------------------------------
>>>>
>>>> "df" shows an "Available" value which isn't related to any real value.  
>>>
>>>    I _think_ that value is the amount of space not allocated to any
>>> block group. If that's so, then Available (from df) plus the three
>>> "total" values (from btrfs fi df) should equal the size value from df.
>>
>> This value excludes the space that can not be allocated to any block group,
>> This feature was implemented to fix the bug df command add the disk space, which
>> can not be allocated to any block group forever, into the "Available" value.
>> (see the changelog of the commit 6d07bcec969af335d4e35b3921131b7929bd634e)
>>
>> This implementation just like fake chunk allocation, but the fake allocation
>> just allocate the space from two of these three disks, doesn't spread the
>> stripes over all the disks, which has enough space.
> [...]
> 
> Hi Miao,
> 
> would you care to expand a bit on that. In Helmut's case above
> where all the drives have at least 1.2TB free, how would there
> be un-allocatable space?
> 
> What's the implication of having disks of differing sizes? Does
> that mean that the extra space on larger disks is lost?

I'm sorry that I couldn't explain it clearly.

As we know, Btrfs introduced RAID fucntion, and it can allocate some stripes from
different disks to make up a RAID block group. But if there is not enough disk space
to allocate enough stripes, btrfs can't make up a new block group, and the left disk
space can't be used forever. 

For example, If we have two disks, one is 5GB, and the other is 10GB, and we use RAID0
block groups to store the file data. The RAID0 block group needs two stripes which are
on the different disks at least. After all space on the 5GB disk is allocated, there is
about 5GB free space on the 10GB disk, this space can not be used because we have
no free space on the other disk to allocate, and can't make up a new RAID0 block group. 

Beside the two-stripe limit, the chunk allocator will allocate stripes from every disk
as much as possible, to make up a new RAID0 block group. That is if all the disks have
enough free space, the allocator will allocate stripes from all the disks.

In Helmut's case, the chunk allocator will allocate three same-size stripes from those
three disks to make up the new RAID0 block group, every time btrfs allocate new chunks
(block groups), until there is no free space on two disks. So btrfs can use most of the
disk space for RAID0 block group.

But the algorithm of df command doesn't simulate the above allocation correctly, this
simulated allocation just allocates the stripes from two disks, and then, these two disks
have no free space, but the third disk still has 1.2TB free space, df command thinks
this space can be used to make a new RAID0 block group and ignores it. This is a bug,
I think.

BTW: "Available" value is the size of the free space that we may use it to store the file
data. In btrfs filesystem, it is hard to calculate, because the block groups are allocated
dynamically, not all the free space on the disks is allocated to make up data block groups,
some of the space may be allocated to make up data block groups. So we just tell the users
the size of free space maybe they can use to store the file data.

Thanks
Miao

> 
> Thanks,
> Stephane
> 

Re: wrong values in "df" and "btrfs filesystem df"

[Stephane Chazelas]

2011-04-12 15:22:57 +0800, Miao Xie:
[...]
> But the algorithm of df command doesn't simulate the above allocation correctly, this
> simulated allocation just allocates the stripes from two disks, and then, these two disks
> have no free space, but the third disk still has 1.2TB free space, df command thinks
> this space can be used to make a new RAID0 block group and ignores it. This is a bug,
> I think.
[...]

Thanks a lot Miao for the detailed explanation. So, the disk
space is not lost, it's just df not reporting the available
space correctly. That's me relieved.

It explains why I'm getting:

# blockdev --getsize64 /dev/sda4
2967698087424
# blockdev --getsize64 /dev/sdb
3000592982016
# blockdev --getsize64 /dev/sdc
3000592982016
# truncate -s 2967698087424 a
# truncate -s 3000592982016 b
# truncate -s 3000592982016 c
# losetup /dev/loop0 ./a
# losetup /dev/loop1 ./b
# losetup /dev/loop2 ./c
# mkfs.btrfs a b c
# btrfs device scan /dev/loop[0-2]
Scanning for Btrfs filesystems in '/dev/loop0'
Scanning for Btrfs filesystems in '/dev/loop1'
Scanning for Btrfs filesystems in '/dev/loop2'
# mount  /dev/loop0 /mnt/1
# df -k /mnt/1
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/loop0           8758675828        56 5859474304   1% /mnt/1
# echo $(((8758675828 - 5859474304)*2**10))
2968782360576

One disk worth of space lost according to df.

While it should have been more something like
$(((3000592982016-2967698087424)*2)) (about 60GB), or about 0
after the quasi-round-robin allocation patch, right?

Best regards,
Stephane

Re: wrong values in "df" and "btrfs filesystem df"

[Miao Xie]

On tue, 12 Apr 2011 09:17:21 +0100, Stephane Chazelas wrote:
> 2011-04-12 15:22:57 +0800, Miao Xie:
> [...]
>> But the algorithm of df command doesn't simulate the above allocation correctly, this
>> simulated allocation just allocates the stripes from two disks, and then, these two disks
>> have no free space, but the third disk still has 1.2TB free space, df command thinks
>> this space can be used to make a new RAID0 block group and ignores it. This is a bug,
>> I think.
> [...]
> 
> Thanks a lot Miao for the detailed explanation. So, the disk
> space is not lost, it's just df not reporting the available
> space correctly. That's me relieved.
> 
> It explains why I'm getting:
> 
> # blockdev --getsize64 /dev/sda4
> 2967698087424
> # blockdev --getsize64 /dev/sdb
> 3000592982016
> # blockdev --getsize64 /dev/sdc
> 3000592982016
> # truncate -s 2967698087424 a
> # truncate -s 3000592982016 b
> # truncate -s 3000592982016 c
> # losetup /dev/loop0 ./a
> # losetup /dev/loop1 ./b
> # losetup /dev/loop2 ./c
> # mkfs.btrfs a b c
> # btrfs device scan /dev/loop[0-2]
> Scanning for Btrfs filesystems in '/dev/loop0'
> Scanning for Btrfs filesystems in '/dev/loop1'
> Scanning for Btrfs filesystems in '/dev/loop2'
> # mount  /dev/loop0 /mnt/1
> # df -k /mnt/1
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/loop0           8758675828        56 5859474304   1% /mnt/1
> # echo $(((8758675828 - 5859474304)*2**10))
> 2968782360576
> 
> One disk worth of space lost according to df.
> 
> While it should have been more something like
> $(((3000592982016-2967698087424)*2)) (about 60GB), or about 0
> after the quasi-round-robin allocation patch, right?

I think btrfs can utilize the disk space very well(perhaps all the space).
The last 60GB is on the different disks, so btrfs can utilize the space to
make up new RAID0 block groups with two stripes.

> Best regards,
> Stephane
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