On 09/28/2012 05:17 AM, Roman Mamedov wrote:

On Thu, 27 Sep 2012 23:02:35 +0200
Goffredo Baroncelli[off-list ref]  wrote:

quoted

Sorry for the space error:
Below a more correct example

$ btrfs filesystem disk-free /
Summary:
     Total:             	        135.00GB
     Allocated:             	 10.51GB
     Unallocated:            	124.49GB
     Free_(Estimated)              86.56GB
     Average_disk_efficiency:         62 %

How do you estimate "Free" here? Sorry I didn't check the source code in git,
but from the "Details" below nothing leads me to believe that this FS is
doomed to only be able to usefully utilize only ~86GB of the partition, and not
more.

The estimation is made on the basis of the real allocated space on the 
disk and the available space.

In the example we know that BTRFS allocate:
- 4GB	in Single mode (4GB available, 2.16GB used)
- 16MB  in DUP mode (so  16/2=8MB available, 4kb used)
- 4MB   in Single mode (4MB available)
- 6GB   in DUP mode (6/2=3GB available, 429MB used)
- 8MB   in Single mode (8MB available)


So BTRFS allocated on disk 4GB+16MB+4MB+6GB+8MB = ~10GB, but the space 
availabled (regarding these allocated chunks) is 4GB+8MB+4MB+3GB+8MB = ~7GB.

This means that the ration of space physically allocated on the disk and 
the space available is 7GB/10GB = 0.7 . So on 135GB of disk, only 94GB 
are available.

Yes my previous 0.62 was wrong. The real ratio is 0.7.

Are you ready to answer the flood of questions from people why their disk is
only 62% efficient, and how to tune it to 100%? :-)

I don't understand your question: by default BTRFS store all metadata 
DUP-ed, this means that on the disk the space allocated are 2 times the 
space required. Because on BTRFS the metadata are a lot, this means that 
BTRFS is not so efficiency as other file-systems. This is a well know fact.

If you want to use all the space with the maximum efficiency, you could 
format the filesystem with the options "-m single".

Why use underscores instead of spaces?

Simplify the parsing in scripts

quoted

Details:
     Chunk-type    Mode       Allocated        Used        Free
     ----------    ----       ---------    --------   ---------
     Data          Single        4.01GB      2.16GB      1.87GB
     System        DUP          16.00MB      4.00KB      7.99MB
     System        Single        4.00MB        0.00      4.00MB
     Metadata      DUP           6.00GB    429.16MB      2.57GB
     Metadata      Single        8.00MB        0.00      8.00MB