Understanding "btrfs filesystem usage"
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
In detail:
root@diaspora:~# btrfs filesystem usage /local
Overall:
Device size: 883.51GiB
Device allocated: 825.51GiB
Device unallocated: 58.00GiB
Device missing: 0.00B
Used: 494.45GiB
Free (estimated): 387.94GiB (min: 387.94GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 320.00MiB (used: 0.00B)Data,single: Size:823.47GiB, Used:493.53GiB
/dev/sda4 823.47GiB
Metadata,single: Size:2.01GiB, Used:938.67MiB
/dev/sda4 2.01GiB
System,single: Size:32.00MiB, Used:128.00KiB
/dev/sda4 32.00MiB
Unallocated:
/dev/sda4 58.00GiB
root@diaspora:~# btrfs filesystem df /local Data, single: total=823.47GiB, used=493.53GiB System, single: total=32.00MiB, used=128.00KiB Metadata, single: total=2.01GiB, used=938.67MiB GlobalReserve, single: total=320.00MiB, used=0.00B
root@diaspora:~# btrfs filesystem show /local
Label: 'local' uuid: 11faaa39-5805-4e92-a891-e8ceb4afa9f7
Total devices 1 FS bytes used 494.45GiB
devid 1 size 883.51GiB used 825.51GiB path /dev/sda4What does the last line mean?
825.51GiB used = allocated by (file) data?
In this case I would have only 58 GB free disk space left?
"btrfs filesystem usage" says 494 GB used, but "btrfs filesystem show" says 825 GB used?!
The man pages do not explain the different meanings of "used" and "allocated"
And at last:
root@diaspora:~# df -HT /local Filesystem Type Size Used Avail Use% Mounted on /dev/sda4 btrfs 949G 532G 417G 57% /local
Signature · 6 lines
Ullrich Horlacher Server und Virtualisierung Rechenzentrum TIK Universitaet Stuttgart E-Mail: [email protected] Allmandring 30a Tel: ++49-711-68565868 70569 Stuttgart (Germany) WWW: http://www.tik.uni-stuttgart.de/ REF:<[email protected]>
Quoted 8 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For". As the output of the Usage command and df command clearly show, you have almost 400GiB space available.
Note that the btrfs commands are clearly and explicitly displaying values in Binary units, (Mi, and Gi prefix, respectively). If you want df command to match, use -h instead of -H (see man df)
An observation:
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option. You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55
You do *not* want to end up with all your space allocated to Data, but not actually used by data. Bad things can happen if you run out of Unallocated space for more metadata. (not catastrophic, but awkward and unexpected downtime that can be a little tricky to sort out.)
begin:vcard
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Quoted 4 lines · Remi Gauvin
Quoted 3 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
Quoted 8 lines
Quoted 5 lines
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem showIn my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
Opinions are divided. I find it almost impossible to read, and
always use btrfs fi df and btrfs fi show together.
There's short tutorials of how to read the output in both cases in
the FAQ, which is where I start out by directing people in this instance.
Hugo.
Quoted 37 lines
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For". As the output of the Usage command and df command clearly show, you have almost 400GiB space available.
Note that the btrfs commands are clearly and explicitly displaying values in Binary units, (Mi, and Gi prefix, respectively). If you want df command to match, use -h instead of -H (see man df)
An observation:
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option. You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55You do *not* want to end up with all your space allocated to Data, but not actually used by data. Bad things can happen if you run out of Unallocated space for more metadata. (not catastrophic, but awkward and unexpected downtime that can be a little tricky to sort out.)
begin:vcard
fn:Remi Gauvin
n:Gauvin;Remi
org:Georgian Infotech
adr:;;3-51 Sykes St. N.;Meaford;ON;N4L 1X3;Canada
email;internet:[email protected]
tel;work:226-256-1545
version:2.1
end:vcard
Signature · 4 lines
Hugo Mills | Great oxymorons of the world, no. 8: hugo@... carfax.org.uk | The Latest In Proven Technology http://carfax.org.uk/ | PGP: E2AB1DE4 |
Quoted 12 lines · Hugo Mills
Quoted 10 lines · Remi Gauvin
Quoted 8 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
Quoted 5 lines
Quoted 2 lines
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
Opinions are divided. I find it almost impossible to read, and
always use btrfs fi df and btrfs fi show together.
I find the tabular output via -T makes btrfs file usage much easier to read, and it's now the only command I use to look at it space usage on btrfs.
Quoted 51 lines
There's short tutorials of how to read the output in both cases in
the FAQ, which is where I start out by directing people in this instance.Hugo.
Quoted 27 lines
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For". As the output of the Usage command and df command clearly show, you have almost 400GiB space available.
Note that the btrfs commands are clearly and explicitly displaying values in Binary units, (Mi, and Gi prefix, respectively). If you want df command to match, use -h instead of -H (see man df)
An observation:
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option. You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55You do *not* want to end up with all your space allocated to Data, but not actually used by data. Bad things can happen if you run out of Unallocated space for more metadata. (not catastrophic, but awkward and unexpected downtime that can be a little tricky to sort out.)
Quoted 10 lines
begin:vcard
fn:Remi Gauvin
n:Gauvin;Remi
org:Georgian Infotech
adr:;;3-51 Sykes St. N.;Meaford;ON;N4L 1X3;Canada
email;internet:[email protected]
tel;work:226-256-1545
version:2.1
end:vcard--
Hugo Mills | Great oxymorons of the world, no. 8: hugo@... carfax.org.uk | The Latest In Proven Technology http://carfax.org.uk/ | PGP: E2AB1DE4 |
Quoted 9 lines · Remi Gauvin
Quoted 6 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
Quoted 6 lines
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For".
And what is "Device unallocated"? Not reserved?
Quoted 2 lines
As the output of the Usage command and df command clearly show, you have almost 400GiB space available.
This is the good part :-)
Quoted 5 lines
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option.
I am running kernel 4.4 (it is a Ubuntu 16.04 system) But /local is on a SSD. Should I really use nossd mount option?!
Quoted 4 lines
You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55
I run balance via cron weekly (adapted
https://software.opensuse.org/package/btrfsmaintenance)
Signature · 6 lines
Ullrich Horlacher Server und Virtualisierung Rechenzentrum TIK Universitaet Stuttgart E-Mail: [email protected] Allmandring 30a Tel: ++49-711-68565868 70569 Stuttgart (Germany) WWW: http://www.tik.uni-stuttgart.de/ REF:<[email protected]>
Quoted 27 lines · Ulli Horlacher
Quoted 9 lines · Remi Gauvin
Quoted 6 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem showQuoted 6 lines
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For".
And what is "Device unallocated"? Not reserved?
Quoted 2 lines
As the output of the Usage command and df command clearly show, you have almost 400GiB space available.
This is the good part :-)
Quoted 8 lines
Quoted 5 lines
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option.
I am running kernel 4.4 (it is a Ubuntu 16.04 system) But /local is on a SSD. Should I really use nossd mount option?!
Probably, and you may even want to use it on newer (patched) kernels.
This requires some explanation though.
SSD's are write limited media (write to them too much, and they stop working). This is generally a pretty well known fact, and while it is true, it's not anywhere near as much of an issue on modern SSD"s as people make it out to be (pretty much, if you've got an SSD made in the last 5 years, you almost certainly don't have to worry about this). The ssd code in BTRFS behaves as if this is still an issue (and does so in a way that doesn't even solve it well).
Put simply, when BTRFS goes to look for space, it treats requests for space that ask for less than a certain size as if they are that minimum size, and only tries to look for smaller spots if it can't find one at least that minimum size. This has a couple of advantages in terms of write performance, especially in the common case of a mostly empty filesystem.
For the default (nossd) case, that minimum size is 64kB. So, in most cases, the potentially wasted space actually doesn't matter much (most writes are bigger than 64k) unless you're doing certain things.
For the old (ssd) case, that minimum size is 2MB. Even with the common cases that would normally not have an issue with the 64k default, this ends up wasting a _huge_ amount of space.
For the new ssd behavior, the minimum is different for data and metadata (IIRC, metadata uses the 64k default, while data still uses the 2M size). This solves the biggest issues (which were seen with metadata), but doesn't completely remove the problem.
Expanding on this further, some unusual workloads actually benefit from the old ssd behavior, so on newer kernels ssd_spread gives that behavior. However, many workloads actually do better with the nossd behavior (especially the pathological worst case stuff like databases and VM disk images), so if you have a recent SSD, you probably want to just use that.
Quoted 11 lines
Quoted 6 lines
You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55I run balance via cron weekly (adapted
https://software.opensuse.org/package/btrfsmaintenance)
Quoted 11 lines · Ulli Horlacher
Quoted 9 lines · Remi Gauvin
Quoted 6 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
Quoted 8 lines
Quoted 6 lines
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For".
And what is "Device unallocated"? Not reserved?
That's a reasonable interpretation. Unallocated space is space that's not used for anything: no data, no metadata, and isn't reference by any block group.
It's not a relevant number day to day, I'd say it's advanced leaning toward esoteric knowledge of Btrfs internals. At this point I'd like to see a simper output by default, and have a verbose option for advanced users, and an export option that spits out a superset of all available information in a format parsable for scripts. But I know there are other project that depend on btrfs user space output, rather than having something specifically invented for them that's easily parsed, and can be kept consistent and extendible, separate from human user consumption. Oh well!
Quoted 8 lines
Quoted 5 lines
The disparity between 498GiB used and 823Gib is pretty high. This is probably the result of using an SSD with an older kernel. If your kernel is not very recent, (sorry, I forget where this was fixed, somewhere around 4.14 or 4.15), then consider mounting with the nossd option.
I am running kernel 4.4 (it is a Ubuntu 16.04 system) But /local is on a SSD. Should I really use nossd mount option?!
Yes. But it's not a file system integrity suggestion, it's an optimization.
Quoted 10 lines
Quoted 7 lines
You can improve this by running a balance.
Something like:
btrfs balance start -dusage=55I run balance via cron weekly (adapted
https://software.opensuse.org/package/btrfsmaintenance)
With a newer kernel you can probably reduce this further depending on your workload and use case. And optionally follow it up with executing fstrim, or just enable fstrim.timer (we don't recommend using discard mount option for most use cases as it too aggressively discards very recently stale Btrfs metadata and can make recovery from crashes harder).
There is a trim bug that causes FITRIM to only get applied to unallocated space on older file systems, that have been balanced such that block group logical addresses are outside the physical address space of the device which prevents the free space inside of such block groups to be passed over for FITRIM. Looks like this will be fixed in kernel 4.20/5.0
Signature · 1 line
Chris Murphy
Quoted 12 lines · Ulli Horlacher
Quoted 9 lines · Remi Gauvin
Quoted 6 lines · Ulli Horlacher
I want to know how many free space is left and have problems in interpreting the output of:
btrfs filesystem usage
btrfs filesystem df
btrfs filesystem show
Quoted 8 lines
Quoted 6 lines
In my not so humble opinion, the filesystem usage command has the easiest to understand output. It' lays out all the pertinent information.
You can clearly see 825GiB is allocated, with 494GiB used, therefore, filesystem show is actually using the "Allocated" value as "Used". Allocated can be thought of "Reserved For".
And what is "Device unallocated"? Not reserved?
The answer is in the FAQ and I am stupid not to looked at it, first :-}
https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F
Signature · 6 lines
Ullrich Horlacher Server und Virtualisierung Rechenzentrum TIK Universitaet Stuttgart E-Mail: [email protected] Allmandring 30a Tel: ++49-711-68565868 70569 Stuttgart (Germany) WWW: http://www.tik.uni-stuttgart.de/ REF:<[email protected]>