understanding disk space usage

understanding disk space usage

[Vasco Visser]

Hello,

My system is or seems to be running out of disk space but I can't find
out how or why. Might be a BTRFS peculiarity, hence posting on this
list. Most indicators seem to suggest I'm filling up, but I can't
trace the disk usage to files on the FS.

The issue is on my root filesystem on a 28GiB ssd partition (commands
below issued when booted into single user mode):


$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3              28G   26G  2.1G  93% /


$ btrfs --version
btrfs-progs v4.4


$ btrfs fi usage /
Overall:
    Device size:  27.94GiB
    Device allocated:  27.94GiB
    Device unallocated:   1.00MiB
    Device missing:     0.00B
    Used:  25.03GiB
    Free (estimated):   2.37GiB (min: 2.37GiB)
    Data ratio:      1.00
    Metadata ratio:      1.00
    Global reserve: 256.00MiB (used: 0.00B)
Data,single: Size:26.69GiB, Used:24.32GiB
   /dev/sda3  26.69GiB
Metadata,single: Size:1.22GiB, Used:731.45MiB
   /dev/sda3   1.22GiB
System,single: Size:32.00MiB, Used:16.00KiB
   /dev/sda3  32.00MiB
Unallocated:
   /dev/sda3   1.00MiB


$ btrfs fi df /
Data, single: total=26.69GiB, used=24.32GiB
System, single: total=32.00MiB, used=16.00KiB
Metadata, single: total=1.22GiB, used=731.48MiB
GlobalReserve, single: total=256.00MiB, used=0.00B


However:
$ mount -o bind / /mnt
$ sudo du -hs /mnt
9.3G /mnt


Try to balance:
$ btrfs balance start /
ERROR: error during balancing '/': No space left on device


Am I really filling up? What can explain the huge discrepancy with the
output of du (no open file descriptors on deleted files can explain
this in single user mode) and the FS stats?

Any advice on possible causes and how to proceed?


--
Vasco

Re: understanding disk space usage

[Qu Wenruo]

At 02/08/2017 12:44 AM, Vasco Visser wrote:
> Hello,
>
> My system is or seems to be running out of disk space but I can't find
> out how or why. Might be a BTRFS peculiarity, hence posting on this
> list. Most indicators seem to suggest I'm filling up, but I can't
> trace the disk usage to files on the FS.
>
> The issue is on my root filesystem on a 28GiB ssd partition (commands
> below issued when booted into single user mode):
>
>
> $ df -h
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/sda3              28G   26G  2.1G  93% /
>
>
> $ btrfs --version
> btrfs-progs v4.4
>
>
> $ btrfs fi usage /
> Overall:
>     Device size:  27.94GiB
>     Device allocated:  27.94GiB
>     Device unallocated:   1.00MiB

So from chunk level, your fs is already full.

And balance won't success since there is no unallocated space at all.
The first 1M of btrfs is always reserved and won't be allocated, and 1M 
is too small for btrfs to allocate a chunk.

>     Device missing:     0.00B
>     Used:  25.03GiB
>     Free (estimated):   2.37GiB (min: 2.37GiB)
>     Data ratio:      1.00
>     Metadata ratio:      1.00
>     Global reserve: 256.00MiB (used: 0.00B)
> Data,single: Size:26.69GiB, Used:24.32GiB

You still have 2G data space, so you can still write things.

>    /dev/sda3  26.69GiB
> Metadata,single: Size:1.22GiB, Used:731.45MiB

Metadata has has less space when considering "Global reserve".
In fact the used space would be 987M.

But it's still OK for normal write.

>    /dev/sda3   1.22GiB
> System,single: Size:32.00MiB, Used:16.00KiB
>    /dev/sda3  32.00MiB

System chunk can hardly be used up.

> Unallocated:
>    /dev/sda3   1.00MiB
>
>
> $ btrfs fi df /
> Data, single: total=26.69GiB, used=24.32GiB
> System, single: total=32.00MiB, used=16.00KiB
> Metadata, single: total=1.22GiB, used=731.48MiB
> GlobalReserve, single: total=256.00MiB, used=0.00B
>
>
> However:
> $ mount -o bind / /mnt
> $ sudo du -hs /mnt
> 9.3G /mnt
>
>
> Try to balance:
> $ btrfs balance start /
> ERROR: error during balancing '/': No space left on device
>
>
> Am I really filling up? What can explain the huge discrepancy with the
> output of du (no open file descriptors on deleted files can explain
> this in single user mode) and the FS stats?

Just don't believe the vanilla df output for btrfs.

For btrfs, unlike other fs like ext4/xfs, which allocates chunk 
dynamically and has different metadata/data profile, we can only get a 
clear view of the fs from both chunk level(allocated/unallocated) and 
extent level(total/used).

In your case, your fs doesn't have any unallocated space, this make 
balance unable to work at all.

And your data/metadata usage is quite high, although both has small 
available space left, the fs should be writable for some time, but not long.

To proceed, add a larger device to current fs, and do a balance or just 
delete the 28G partition then btrfs will handle the rest well.

Thanks,
Qu

>
> Any advice on possible causes and how to proceed?
>
>
> --
> Vasco
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>

Re: understanding disk space usage

[Vasco Visser]

Thank you for the explanation. What I would still like to know is how
to relate the chunk level abstraction to the file level abstraction.
According to the btrfs output there is 2G of data space is available
and 24G of data space is being used. Does this mean 24G of data used
in files? How do I know which files take up most space? du seems
pretty useless as it reports only 9G of files on the volume.

--
Vasco


On Wed, Feb 8, 2017 at 4:48 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>
>
> At 02/08/2017 12:44 AM, Vasco Visser wrote:
>>
>> Hello,
>>
>> My system is or seems to be running out of disk space but I can't find
>> out how or why. Might be a BTRFS peculiarity, hence posting on this
>> list. Most indicators seem to suggest I'm filling up, but I can't
>> trace the disk usage to files on the FS.
>>
>> The issue is on my root filesystem on a 28GiB ssd partition (commands
>> below issued when booted into single user mode):
>>
>>
>> $ df -h
>> Filesystem            Size  Used Avail Use% Mounted on
>> /dev/sda3              28G   26G  2.1G  93% /
>>
>>
>> $ btrfs --version
>> btrfs-progs v4.4
>>
>>
>> $ btrfs fi usage /
>> Overall:
>>     Device size:  27.94GiB
>>     Device allocated:  27.94GiB
>>     Device unallocated:   1.00MiB
>
>
> So from chunk level, your fs is already full.
>
> And balance won't success since there is no unallocated space at all.
> The first 1M of btrfs is always reserved and won't be allocated, and 1M is
> too small for btrfs to allocate a chunk.
>
>>     Device missing:     0.00B
>>     Used:  25.03GiB
>>     Free (estimated):   2.37GiB (min: 2.37GiB)
>>     Data ratio:      1.00
>>     Metadata ratio:      1.00
>>     Global reserve: 256.00MiB (used: 0.00B)
>> Data,single: Size:26.69GiB, Used:24.32GiB
>
>
> You still have 2G data space, so you can still write things.
>
>>    /dev/sda3  26.69GiB
>> Metadata,single: Size:1.22GiB, Used:731.45MiB
>
>
> Metadata has has less space when considering "Global reserve".
> In fact the used space would be 987M.
>
> But it's still OK for normal write.
>
>>    /dev/sda3   1.22GiB
>> System,single: Size:32.00MiB, Used:16.00KiB
>>    /dev/sda3  32.00MiB
>
>
> System chunk can hardly be used up.
>
>> Unallocated:
>>    /dev/sda3   1.00MiB
>>
>>
>> $ btrfs fi df /
>> Data, single: total=26.69GiB, used=24.32GiB
>> System, single: total=32.00MiB, used=16.00KiB
>> Metadata, single: total=1.22GiB, used=731.48MiB
>> GlobalReserve, single: total=256.00MiB, used=0.00B
>>
>>
>> However:
>> $ mount -o bind / /mnt
>> $ sudo du -hs /mnt
>> 9.3G /mnt
>>
>>
>> Try to balance:
>> $ btrfs balance start /
>> ERROR: error during balancing '/': No space left on device
>>
>>
>> Am I really filling up? What can explain the huge discrepancy with the
>> output of du (no open file descriptors on deleted files can explain
>> this in single user mode) and the FS stats?
>
>
> Just don't believe the vanilla df output for btrfs.
>
> For btrfs, unlike other fs like ext4/xfs, which allocates chunk dynamically
> and has different metadata/data profile, we can only get a clear view of the
> fs from both chunk level(allocated/unallocated) and extent
> level(total/used).
>
> In your case, your fs doesn't have any unallocated space, this make balance
> unable to work at all.
>
> And your data/metadata usage is quite high, although both has small
> available space left, the fs should be writable for some time, but not long.
>
> To proceed, add a larger device to current fs, and do a balance or just
> delete the 28G partition then btrfs will handle the rest well.
>
> Thanks,
> Qu
>
>>
>> Any advice on possible causes and how to proceed?
>>
>>
>> --
>> Vasco
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>
>
>

Re: understanding disk space usage

[Peter Grandi]

>> My system is or seems to be running out of disk space but I
>> can't find out how or why. [ ... ]
>> Filesystem            Size  Used Avail Use% Mounted on
>> /dev/sda3              28G   26G  2.1G  93% /
[ ... ]
> So from chunk level, your fs is already full.  And balance
> won't success since there is no unallocated space at all.

To add to this, 28GiB is a bit too small for Btrfs, because at
that point chunk size is 1GiB. I have the habit of sizing
partitions to an exact number of GiB, and that means that most
of 1GiB will never be used by Btrfs because there is a small
amount of space allocated that is smaller than 1GiB and thus
there will be eventually just less than 1GiB unallocated.
Unfortunately the chunk size is not manually settable.

Example here from 'btrfs fi usage':

Overall:
    Device size:                  88.00GiB
    Device allocated:             86.06GiB
    Device unallocated:            1.94GiB
    Device missing:                  0.00B
    Used:                         80.11GiB
    Free (estimated):              6.26GiB      (min: 5.30GiB)

That means that I should 'btrfs balance' now, because of the
1.94GiB "unallocated", 0.94GiB will never be allocated, and that
leaves just 1GiB "unallocated" which is the minimum for running
'btrfs balance'. I have just done so and this is the result:

Overall:
    Device size:                  88.00GiB
    Device allocated:             82.03GiB
    Device unallocated:            5.97GiB
    Device missing:                  0.00B
    Used:                         80.11GiB
    Free (estimated):              6.26GiB      (min: 3.28GiB)

At some point I had decided to use 'mixedbg' allocation to
reduce this problem and hopefully improve locality, but that
means that metadata and data need to have the same profile, and
I really want metadata to be 'dup' because of checksumming,
and I don't want data to be 'dup' too.

> [ ... ] To proceed, add a larger device to current fs, and do
> a balance or just delete the 28G partition then btrfs will
> handle the rest well.

Usually for this I use a USB stick, with a 1-3GiB partition plus
a bit extra because of that extra bit of space.

https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F
https://btrfs.wiki.kernel.org/index.php/FAQ#Help.21_Btrfs_claims_I.27m_out_of_space.2C_but_it_looks_like_I_should_have_lots_left.21
marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html

Unfortunately if it is a single device volume and metadata is
'dup' to remove the extra temporary device one has first to
convert the metadata to 'single' and then back to 'dup' after
removal.

There are also some additional reasons why space used (rather
than allocated) may be larger than expected, in special but not
wholly infrequent cases. My impression is that the Btrfs design
trades space for performance and reliability.

Re: understanding disk space usage

[Austin S. Hemmelgarn]

On 2017-02-08 09:46, Peter Grandi wrote:
>>> My system is or seems to be running out of disk space but I
>>> can't find out how or why. [ ... ]
>>> Filesystem            Size  Used Avail Use% Mounted on
>>> /dev/sda3              28G   26G  2.1G  93% /
> [ ... ]
>> So from chunk level, your fs is already full.  And balance
>> won't success since there is no unallocated space at all.
>
> To add to this, 28GiB is a bit too small for Btrfs, because at
> that point chunk size is 1GiB. I have the habit of sizing
> partitions to an exact number of GiB, and that means that most
> of 1GiB will never be used by Btrfs because there is a small
> amount of space allocated that is smaller than 1GiB and thus
> there will be eventually just less than 1GiB unallocated.
> Unfortunately the chunk size is not manually settable.
28GB is a perfectly reasonable (if a bit odd) size for a non-mixed-mode 
volume.  The issue isn't total size, it's the difference between total 
size and the amount of data you want to store on it. and how well you 
manage chunk usage.  If you're balancing regularly to compact chunks 
that are less than 50% full, you can get away with as little as 4GB of 
extra space beyond your regular data-set with absolutely zero issues. 
I've run full Linux installations in VM's with BTRFS on 16GB disk images 
before with absolutely zero issues, and have a handful of fairly active 
8GB BTRFS volumes on both of my primary systems that never have any 
issues with free space despite averaging 5GB of space usage.
>
> Example here from 'btrfs fi usage':
>
> Overall:
>     Device size:                  88.00GiB
>     Device allocated:             86.06GiB
>     Device unallocated:            1.94GiB
>     Device missing:                  0.00B
>     Used:                         80.11GiB
>     Free (estimated):              6.26GiB      (min: 5.30GiB)
>
> That means that I should 'btrfs balance' now, because of the
> 1.94GiB "unallocated", 0.94GiB will never be allocated, and that
> leaves just 1GiB "unallocated" which is the minimum for running
> 'btrfs balance'. I have just done so and this is the result:
Actually, that 0.94GB would be used.  BTRFS will create smaller chunks 
if it has to, so if you allocated two data chunks with that 1.94GB of 
space, you would get one 1GB chunk and one 0.94GB chunk.
>
> Overall:
>     Device size:                  88.00GiB
>     Device allocated:             82.03GiB
>     Device unallocated:            5.97GiB
>     Device missing:                  0.00B
>     Used:                         80.11GiB
>     Free (estimated):              6.26GiB      (min: 3.28GiB)
>
> At some point I had decided to use 'mixedbg' allocation to
> reduce this problem and hopefully improve locality, but that
> means that metadata and data need to have the same profile, and
> I really want metadata to be 'dup' because of checksumming,
> and I don't want data to be 'dup' too.
You could also use larger partitions and keep a better handle on free space.
>
>> [ ... ] To proceed, add a larger device to current fs, and do
>> a balance or just delete the 28G partition then btrfs will
>> handle the rest well.
>
> Usually for this I use a USB stick, with a 1-3GiB partition plus
> a bit extra because of that extra bit of space.
If you have a lot of RAM and can guarantee that things won't crash (or 
don't care about the filesystem too much and are just trying to avoid 
having to restore a backup), a ramdisk works well for this too.
>
> https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F
> https://btrfs.wiki.kernel.org/index.php/FAQ#Help.21_Btrfs_claims_I.27m_out_of_space.2C_but_it_looks_like_I_should_have_lots_left.21
> marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html
>
> Unfortunately if it is a single device volume and metadata is
> 'dup' to remove the extra temporary device one has first to
> convert the metadata to 'single' and then back to 'dup' after
> removal.
This shouldn't be needed, if it is then it's a bug that should be 
reported and ideally fixed (there was such a bug when converting from 
multi-device raid profiles to single device, but that got fixed quite a 
few kernel versions ago (I distinctly remember because I wrote the fix)).
>
> There are also some additional reasons why space used (rather
> than allocated) may be larger than expected, in special but not
> wholly infrequent cases. My impression is that the Btrfs design
> trades space for performance and reliability.
In general, yes, but a more accurate statement would be that it offers a 
trade-off between space and convenience.  If you're not going to take 
the time to maintain the filesystem properly, then you will need more 
excess space for it.

Re: understanding disk space usage

[Hugo Mills]

[-- Attachment #1: Type: text/plain, Size: 3399 bytes --]

On Wed, Feb 08, 2017 at 02:46:32PM +0000, Peter Grandi wrote:
> >> My system is or seems to be running out of disk space but I
> >> can't find out how or why. [ ... ]
> >> Filesystem            Size  Used Avail Use% Mounted on
> >> /dev/sda3              28G   26G  2.1G  93% /
> [ ... ]
> > So from chunk level, your fs is already full.  And balance
> > won't success since there is no unallocated space at all.
> 
> To add to this, 28GiB is a bit too small for Btrfs, because at
> that point chunk size is 1GiB. I have the habit of sizing
> partitions to an exact number of GiB, and that means that most
> of 1GiB will never be used by Btrfs because there is a small
> amount of space allocated that is smaller than 1GiB and thus
> there will be eventually just less than 1GiB unallocated.

   Not true -- the last chunk can be smaller than 1 GiB, to use the
available space completely.

   Hugo.

> Unfortunately the chunk size is not manually settable.
> 
> Example here from 'btrfs fi usage':
> 
> Overall:
>     Device size:                  88.00GiB
>     Device allocated:             86.06GiB
>     Device unallocated:            1.94GiB
>     Device missing:                  0.00B
>     Used:                         80.11GiB
>     Free (estimated):              6.26GiB      (min: 5.30GiB)
> 
> That means that I should 'btrfs balance' now, because of the
> 1.94GiB "unallocated", 0.94GiB will never be allocated, and that
> leaves just 1GiB "unallocated" which is the minimum for running
> 'btrfs balance'. I have just done so and this is the result:
> 
> Overall:
>     Device size:                  88.00GiB
>     Device allocated:             82.03GiB
>     Device unallocated:            5.97GiB
>     Device missing:                  0.00B
>     Used:                         80.11GiB
>     Free (estimated):              6.26GiB      (min: 3.28GiB)
> 
> At some point I had decided to use 'mixedbg' allocation to
> reduce this problem and hopefully improve locality, but that
> means that metadata and data need to have the same profile, and
> I really want metadata to be 'dup' because of checksumming,
> and I don't want data to be 'dup' too.
> 
> > [ ... ] To proceed, add a larger device to current fs, and do
> > a balance or just delete the 28G partition then btrfs will
> > handle the rest well.
> 
> Usually for this I use a USB stick, with a 1-3GiB partition plus
> a bit extra because of that extra bit of space.
> 
> https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F
> https://btrfs.wiki.kernel.org/index.php/FAQ#Help.21_Btrfs_claims_I.27m_out_of_space.2C_but_it_looks_like_I_should_have_lots_left.21
> marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html
> 
> Unfortunately if it is a single device volume and metadata is
> 'dup' to remove the extra temporary device one has first to
> convert the metadata to 'single' and then back to 'dup' after
> removal.
> 
> There are also some additional reasons why space used (rather
> than allocated) may be larger than expected, in special but not
> wholly infrequent cases. My impression is that the Btrfs design
> trades space for performance and reliability.

-- 
Hugo Mills             | Alert status chocolate viridian: Authorised
hugo@... carfax.org.uk | personnel only. Dogs must be carried on escalator.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]

Re: understanding disk space usage

[Peter Grandi]

[ ... ]
> The issue isn't total size, it's the difference between total
> size and the amount of data you want to store on it. and how
> well you manage chunk usage. If you're balancing regularly to
> compact chunks that are less than 50% full, [ ... ] BTRFS on
> 16GB disk images before with absolutely zero issues, and have
> a handful of fairly active 8GB BTRFS volumes [ ... ]

Unfortunately balance operations are quite expensive, especially
from inside VMs. On the other hand if the system is not much
disk constrained relatively frequent balances is a good idea
indeed. It is a bit like the advice in the other thread on OLTP
to run frequent data defrags, which are also quite expensive.

Both combined are like running the compactor/cleaner on log
structured (another variants of "COW") filesystems like NILFS2:
running that frequently means tighter space use and better
locality, but is quite expensive too.

>> [ ... ] My impression is that the Btrfs design trades space
>> for performance and reliability.

> In general, yes, but a more accurate statement would be that
> it offers a trade-off between space and convenience. [ ... ]

It is not quite "convenience", it is overhead: whole-volume
operations like compacting, defragmenting (or fscking) tend to
cost significantly in IOPS and also in transfer rate, and on
flash SSDs they also consume lifetime.

Therefore personally I prefer to have quite a bit of unused
space in Btrfs or NILFS2, at a minimum around double at 10-20%
than the 5-10% that I think is the minimum advisable with
conventional designs.

Re: understanding disk space usage

[Qu Wenruo]

At 02/08/2017 05:55 PM, Vasco Visser wrote:
> Thank you for the explanation. What I would still like to know is how
> to relate the chunk level abstraction to the file level abstraction.
> According to the btrfs output there is 2G of data space is available
> and 24G of data space is being used. Does this mean 24G of data used
> in files?

Yes, 24G data is used to store data.
(And space cache, while space cache is relatively small, less than 1M 
for each chunk)

> How do I know which files take up most space? du seems
> pretty useless as it reports only 9G of files on the volume.

Are you using snapshots?

If you are only using 1 subvolume(including snapshots), then it seems 
that btrfs data CoW waste quite a lot of space.

In case of btrfs data CoW, for example you have a 128M file(one extent), 
then you rewrite 64M of it, your data space usage will be 128M + 64M, as 
the first 128M will only be freed after *all* its user get freed.

For single subvolume and little to none reflink usage case, "btrfs fi 
defrag" should help to free some space.

If you have multiple snapshots or a lot of reflinked files, then I'm 
afraid you have to delete some file (including reflink copy or snapshot) 
to free some data.

Thanks,
Qu

>
> --
> Vasco
>
>
> On Wed, Feb 8, 2017 at 4:48 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>>
>>
>> At 02/08/2017 12:44 AM, Vasco Visser wrote:
>>>
>>> Hello,
>>>
>>> My system is or seems to be running out of disk space but I can't find
>>> out how or why. Might be a BTRFS peculiarity, hence posting on this
>>> list. Most indicators seem to suggest I'm filling up, but I can't
>>> trace the disk usage to files on the FS.
>>>
>>> The issue is on my root filesystem on a 28GiB ssd partition (commands
>>> below issued when booted into single user mode):
>>>
>>>
>>> $ df -h
>>> Filesystem            Size  Used Avail Use% Mounted on
>>> /dev/sda3              28G   26G  2.1G  93% /
>>>
>>>
>>> $ btrfs --version
>>> btrfs-progs v4.4
>>>
>>>
>>> $ btrfs fi usage /
>>> Overall:
>>>     Device size:  27.94GiB
>>>     Device allocated:  27.94GiB
>>>     Device unallocated:   1.00MiB
>>
>>
>> So from chunk level, your fs is already full.
>>
>> And balance won't success since there is no unallocated space at all.
>> The first 1M of btrfs is always reserved and won't be allocated, and 1M is
>> too small for btrfs to allocate a chunk.
>>
>>>     Device missing:     0.00B
>>>     Used:  25.03GiB
>>>     Free (estimated):   2.37GiB (min: 2.37GiB)
>>>     Data ratio:      1.00
>>>     Metadata ratio:      1.00
>>>     Global reserve: 256.00MiB (used: 0.00B)
>>> Data,single: Size:26.69GiB, Used:24.32GiB
>>
>>
>> You still have 2G data space, so you can still write things.
>>
>>>    /dev/sda3  26.69GiB
>>> Metadata,single: Size:1.22GiB, Used:731.45MiB
>>
>>
>> Metadata has has less space when considering "Global reserve".
>> In fact the used space would be 987M.
>>
>> But it's still OK for normal write.
>>
>>>    /dev/sda3   1.22GiB
>>> System,single: Size:32.00MiB, Used:16.00KiB
>>>    /dev/sda3  32.00MiB
>>
>>
>> System chunk can hardly be used up.
>>
>>> Unallocated:
>>>    /dev/sda3   1.00MiB
>>>
>>>
>>> $ btrfs fi df /
>>> Data, single: total=26.69GiB, used=24.32GiB
>>> System, single: total=32.00MiB, used=16.00KiB
>>> Metadata, single: total=1.22GiB, used=731.48MiB
>>> GlobalReserve, single: total=256.00MiB, used=0.00B
>>>
>>>
>>> However:
>>> $ mount -o bind / /mnt
>>> $ sudo du -hs /mnt
>>> 9.3G /mnt
>>>
>>>
>>> Try to balance:
>>> $ btrfs balance start /
>>> ERROR: error during balancing '/': No space left on device
>>>
>>>
>>> Am I really filling up? What can explain the huge discrepancy with the
>>> output of du (no open file descriptors on deleted files can explain
>>> this in single user mode) and the FS stats?
>>
>>
>> Just don't believe the vanilla df output for btrfs.
>>
>> For btrfs, unlike other fs like ext4/xfs, which allocates chunk dynamically
>> and has different metadata/data profile, we can only get a clear view of the
>> fs from both chunk level(allocated/unallocated) and extent
>> level(total/used).
>>
>> In your case, your fs doesn't have any unallocated space, this make balance
>> unable to work at all.
>>
>> And your data/metadata usage is quite high, although both has small
>> available space left, the fs should be writable for some time, but not long.
>>
>> To proceed, add a larger device to current fs, and do a balance or just
>> delete the 28G partition then btrfs will handle the rest well.
>>
>> Thanks,
>> Qu
>>
>>>
>>> Any advice on possible causes and how to proceed?
>>>
>>>
>>> --
>>> Vasco
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>
>>>
>>
>>
>
>

Re: understanding disk space usage

[Vasco Visser]

I did have snapshots. Apparently my distro's update utility made these
snapshots without clearly telling me... Everything makes sense now.
Thanks again for the help.

--
Vasco


On Thu, Feb 9, 2017 at 3:53 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>
>
> At 02/08/2017 05:55 PM, Vasco Visser wrote:
>>
>> Thank you for the explanation. What I would still like to know is how
>> to relate the chunk level abstraction to the file level abstraction.
>> According to the btrfs output there is 2G of data space is available
>> and 24G of data space is being used. Does this mean 24G of data used
>> in files?
>
>
> Yes, 24G data is used to store data.
> (And space cache, while space cache is relatively small, less than 1M for
> each chunk)
>
>> How do I know which files take up most space? du seems
>> pretty useless as it reports only 9G of files on the volume.
>
>
> Are you using snapshots?
>
> If you are only using 1 subvolume(including snapshots), then it seems that
> btrfs data CoW waste quite a lot of space.
>
> In case of btrfs data CoW, for example you have a 128M file(one extent),
> then you rewrite 64M of it, your data space usage will be 128M + 64M, as the
> first 128M will only be freed after *all* its user get freed.
>
> For single subvolume and little to none reflink usage case, "btrfs fi
> defrag" should help to free some space.
>
> If you have multiple snapshots or a lot of reflinked files, then I'm afraid
> you have to delete some file (including reflink copy or snapshot) to free
> some data.
>
> Thanks,
> Qu
>
>
>>
>> --
>> Vasco
>>
>>
>> On Wed, Feb 8, 2017 at 4:48 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>>>
>>>
>>>
>>> At 02/08/2017 12:44 AM, Vasco Visser wrote:
>>>>
>>>>
>>>> Hello,
>>>>
>>>> My system is or seems to be running out of disk space but I can't find
>>>> out how or why. Might be a BTRFS peculiarity, hence posting on this
>>>> list. Most indicators seem to suggest I'm filling up, but I can't
>>>> trace the disk usage to files on the FS.
>>>>
>>>> The issue is on my root filesystem on a 28GiB ssd partition (commands
>>>> below issued when booted into single user mode):
>>>>
>>>>
>>>> $ df -h
>>>> Filesystem            Size  Used Avail Use% Mounted on
>>>> /dev/sda3              28G   26G  2.1G  93% /
>>>>
>>>>
>>>> $ btrfs --version
>>>> btrfs-progs v4.4
>>>>
>>>>
>>>> $ btrfs fi usage /
>>>> Overall:
>>>>     Device size:  27.94GiB
>>>>     Device allocated:  27.94GiB
>>>>     Device unallocated:   1.00MiB
>>>
>>>
>>>
>>> So from chunk level, your fs is already full.
>>>
>>> And balance won't success since there is no unallocated space at all.
>>> The first 1M of btrfs is always reserved and won't be allocated, and 1M
>>> is
>>> too small for btrfs to allocate a chunk.
>>>
>>>>     Device missing:     0.00B
>>>>     Used:  25.03GiB
>>>>     Free (estimated):   2.37GiB (min: 2.37GiB)
>>>>     Data ratio:      1.00
>>>>     Metadata ratio:      1.00
>>>>     Global reserve: 256.00MiB (used: 0.00B)
>>>> Data,single: Size:26.69GiB, Used:24.32GiB
>>>
>>>
>>>
>>> You still have 2G data space, so you can still write things.
>>>
>>>>    /dev/sda3  26.69GiB
>>>> Metadata,single: Size:1.22GiB, Used:731.45MiB
>>>
>>>
>>>
>>> Metadata has has less space when considering "Global reserve".
>>> In fact the used space would be 987M.
>>>
>>> But it's still OK for normal write.
>>>
>>>>    /dev/sda3   1.22GiB
>>>> System,single: Size:32.00MiB, Used:16.00KiB
>>>>    /dev/sda3  32.00MiB
>>>
>>>
>>>
>>> System chunk can hardly be used up.
>>>
>>>> Unallocated:
>>>>    /dev/sda3   1.00MiB
>>>>
>>>>
>>>> $ btrfs fi df /
>>>> Data, single: total=26.69GiB, used=24.32GiB
>>>> System, single: total=32.00MiB, used=16.00KiB
>>>> Metadata, single: total=1.22GiB, used=731.48MiB
>>>> GlobalReserve, single: total=256.00MiB, used=0.00B
>>>>
>>>>
>>>> However:
>>>> $ mount -o bind / /mnt
>>>> $ sudo du -hs /mnt
>>>> 9.3G /mnt
>>>>
>>>>
>>>> Try to balance:
>>>> $ btrfs balance start /
>>>> ERROR: error during balancing '/': No space left on device
>>>>
>>>>
>>>> Am I really filling up? What can explain the huge discrepancy with the
>>>> output of du (no open file descriptors on deleted files can explain
>>>> this in single user mode) and the FS stats?
>>>
>>>
>>>
>>> Just don't believe the vanilla df output for btrfs.
>>>
>>> For btrfs, unlike other fs like ext4/xfs, which allocates chunk
>>> dynamically
>>> and has different metadata/data profile, we can only get a clear view of
>>> the
>>> fs from both chunk level(allocated/unallocated) and extent
>>> level(total/used).
>>>
>>> In your case, your fs doesn't have any unallocated space, this make
>>> balance
>>> unable to work at all.
>>>
>>> And your data/metadata usage is quite high, although both has small
>>> available space left, the fs should be writable for some time, but not
>>> long.
>>>
>>> To proceed, add a larger device to current fs, and do a balance or just
>>> delete the 28G partition then btrfs will handle the rest well.
>>>
>>> Thanks,
>>> Qu
>>>
>>>>
>>>> Any advice on possible causes and how to proceed?
>>>>
>>>>
>>>> --
>>>> Vasco
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
>>>> in
>>>> the body of a message to majordomo@vger.kernel.org
>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>>
>>>>
>>>
>>>
>>
>>
>
>

Re: understanding disk space usage

[Austin S. Hemmelgarn]

On 2017-02-08 16:45, Peter Grandi wrote:
> [ ... ]
>> The issue isn't total size, it's the difference between total
>> size and the amount of data you want to store on it. and how
>> well you manage chunk usage. If you're balancing regularly to
>> compact chunks that are less than 50% full, [ ... ] BTRFS on
>> 16GB disk images before with absolutely zero issues, and have
>> a handful of fairly active 8GB BTRFS volumes [ ... ]
>
> Unfortunately balance operations are quite expensive, especially
> from inside VMs. On the other hand if the system is not much
> disk constrained relatively frequent balances is a good idea
> indeed. It is a bit like the advice in the other thread on OLTP
> to run frequent data defrags, which are also quite expensive.
That depends on how and when you do them.  A full balance isn't part of 
regular maintenance, and should never be such.  Regular partial balances 
done to clean up mostly empty chunks absolutely should be part of 
regular maintenance, and are pretty inexpensive in terms of both time 
and resource usage.  Balance with -dusage=20 -musage=20 should run in at 
most a few seconds on most reasonably sized filesystems even on low-end 
systems like a Raspberry Pi, and running that on an at least weekly 
basis will significantly improve the chances that you don't encounter a 
situation like this.
>
> Both combined are like running the compactor/cleaner on log
> structured (another variants of "COW") filesystems like NILFS2:
> running that frequently means tighter space use and better
> locality, but is quite expensive too.
If you run with autodefrag, then you should rarely if ever need to 
actually run a full defrag operation unless you're storing lots of 
database files, VM disk images, or similar stuff.  This goes double on 
an SSD.
>
>>> [ ... ] My impression is that the Btrfs design trades space
>>> for performance and reliability.
>
>> In general, yes, but a more accurate statement would be that
>> it offers a trade-off between space and convenience. [ ... ]
>
> It is not quite "convenience", it is overhead: whole-volume
> operations like compacting, defragmenting (or fscking) tend to
> cost significantly in IOPS and also in transfer rate, and on
> flash SSDs they also consume lifetime.
Overhead is the inverse of convenience.  By over-provisioning to a 
greater degree, you're reducing the need to worry about those 
'expensive' operations, reducing both resource overhead, and management 
overhead.
>
> Therefore personally I prefer to have quite a bit of unused
> space in Btrfs or NILFS2, at a minimum around double at 10-20%
> than the 5-10% that I think is the minimum advisable with
> conventional designs.
I can agree on this point, over-provisioning is mandatory to a much 
greater degree on COW filesystems.

Re: understanding disk space usage

[Adam Borowski]

On Wed, Feb 08, 2017 at 11:48:04AM +0800, Qu Wenruo wrote:
> Just don't believe the vanilla df output for btrfs.
> 
> For btrfs, unlike other fs like ext4/xfs, which allocates chunk dynamically
> and has different metadata/data profile, we can only get a clear view of the
> fs from both chunk level(allocated/unallocated) and extent
> level(total/used).

Actually, df lies on ext4 too.  sysvfs-derived filesystems have statically
allocated inodes, which means that if you try to store small files, at some
point you'll run out of space despite df claiming you have plenty left.


Meow!
-- 
Autotools hint: to do a zx-spectrum build on a pdp11 host, type:
  ./configure --host=zx-spectrum --build=pdp11

Re: understanding disk space usage

[Austin S. Hemmelgarn]

On 2017-02-09 08:25, Adam Borowski wrote:
> On Wed, Feb 08, 2017 at 11:48:04AM +0800, Qu Wenruo wrote:
>> Just don't believe the vanilla df output for btrfs.
>>
>> For btrfs, unlike other fs like ext4/xfs, which allocates chunk dynamically
>> and has different metadata/data profile, we can only get a clear view of the
>> fs from both chunk level(allocated/unallocated) and extent
>> level(total/used).
>
> Actually, df lies on ext4 too.  sysvfs-derived filesystems have statically
> allocated inodes, which means that if you try to store small files, at some
> point you'll run out of space despite df claiming you have plenty left.
Which is why the `-i` option for df exists.

That said, not having any more space is not the same as not being able 
to create more files, and that's been the case since long before Linux 
even existed.  Most people don't think about this though because ext4 
and most other filesystems that use static inode tables allocate insane 
numbers of inodes at mkfs time so it (usually) doesn't ever become an issue.