Am Donnerstag, 5. Januar 2012 schrieb Fabian Zeindl:
> On Jan 5, 2012, at 11:39 , Martin Steigerwald wrote:
> > "Allocating as many chunks as can fit across the drives" is also
> > pretty clear to me. So if BTRFS can´t allocate a new chunk on two
> > devices, its full. To me it seems obvious that BTRFS will not break
> > the RAID-1 redundancy guarentee unless a drive fails.
>
> So (assuming 1GB chunksize):
>
> if i create a raid-1, btrfs with a 3GB and a 7GB device, it will show
> me ~10GB free space, after saving a 1GB file, i will have 8GB left
> (-1GB on each device) after saving another 1GB, i will have 6GB left
> (--- " ----)
> after saving another 1GB, it's "suddenly" full?
I would say yes, but suggest that you try this out or wait for confirmation
of a BTRFS developer if you can to be sure about this. The other way of
handling this would be to break the RAID-1 redundancy guarentee and I
really hope that BTRFS is not doing this. I am not completely sure tough
as I never tested it.
The output of df -h with BTRFS and RAID is bogus anyway.
Just consider df -h with two 10GB disks. df -H will display about 20GB
free then. But when you write 100 MB it will show that 200 MB are
allocated. So an application that assumes it will be able to write 12 GB
easily will just fail doing that.
I don´t like this either, cause an application that writes something
cannot even do a rough estimate. But then an application can never now
whether a write will succeed cause another application could also write
lots of data in the same time.
But even without RAID I cannot get exaxt figures from df. Just consider:
merkaba:~> btrfs filesystem show
failed to read /dev/sr0
Label: 'debian' uuid: dd52fea8-f6c3-4a60-bd4a-7650483655e5
Total devices 1 FS bytes used 11.35GB
devid 1 size 18.62GB used 18.29GB path /dev/dm-0
Btrfs Btrfs v0.19
merkaba:~> btrfs filesystem df /
Data: total=14.01GB, used=10.55GB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=2.12GB, used=814.32MB
Metadata: total=8.00MB, used=0.00
merkaba:~> df -hT /
Dateisystem Typ Größe Benutzt Verf. Verw% Eingehängt auf
/dev/mapper/merkaba-debian btrfs 19G 13G 3,8G 77% /
merkaba:~> df -HT /
Dateisystem Typ Größe Benutzt Verf. Verw% Eingehängt auf
/dev/mapper/merkaba-debian btrfs 20G 14G 4,1G 77% /
merkaba:~>
So how much space is free?
df just seems to reflect the size of the data, system and metadata b-trees,
not their usage. Cause at least for me 10.55 GB, 4 KB and 814 KB just do
not add up to 13 / 14 GB - I am not sure whether btrfs command uses 1024
or 1000 as base.
So actually in this case I just be able to write more to the filesystem
than df -hT tells me.
I prefer this over the other way around with RAID-1 where I just can write
about half of the size that df reports.
So or so the current df output is bogus.
--
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7
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