The filesystem level only displays a certain greyscale value for entire block groups. By specifying a block group address, we can also get a view on the distribution of data and metadata inside a single block group. This shows for example how fragmented the free space inside the block group is.
| 1GiB DATA block group | 512MiB DUP METADATA block group |
|---|---|
 |
 |
Metadata tree blocks get colored depending on the tree they belong to:
| Color | Tree | Color | Tree |
|---|---|---|---|
 |
ROOT (1) |  |
CSUM (7) |
 |
EXTENT (2) |  |
QUOTA (8) |
 |
CHUNK (3) |  |
UUID (9) |
 |
DEV (4) |  |
FREE SPACE (10) |
 |
FS (5, 256+) |  |
DATA RELOC (-9) |
The btrfs-heatmap program can take a --blockgroup argument, which needs a
vaddr of a block group as argument.
In order to list all block groups, we can use the btrfs-search-metadata
program that is included with python-btrfs since v12. Example part of the
output:
-# btrfs-search-metadata block_groups /
[...]
block group vaddr 722187845632 transid 1871184 length 536870912 flags METADATA|DUP used 409714688 used_pct 76
block group vaddr 783391129600 transid 1851697 length 1073741824 flags DATA used 573911040 used_pct 53
[...]
Then I created the images using the following commands:
-# btrfs-heatmap --blockgroup 722187845632 --size 8 /
max_id 1 num_devices 1 fsid 64ac42f5-4ff7-4be0-b94a-90def45e6c1e nodesize 16384 sectorsize 4096 clone_alignment 4096
scope block_group 722187845632
grid order 8 size 8 height 256 width 256 total_bytes 536870912 bytes_per_pixel 8192.0
pngfile fsid_64ac42f5-4ff7-4be0-b94a-90def45e6c1e_blockgroup_722187845632_at_1484322493.png
-# btrfs-heatmap --blockgroup 783391129600 --size 8 /
max_id 1 num_devices 1 fsid 64ac42f5-4ff7-4be0-b94a-90def45e6c1e nodesize 16384 sectorsize 4096 clone_alignment 4096
scope block_group 783391129600
grid order 8 size 8 height 256 width 256 total_bytes 1073741824 bytes_per_pixel 16384.0
pngfile fsid_64ac42f5-4ff7-4be0-b94a-90def45e6c1e_blockgroup_783391129600_at_1484322668.png
Also note:
- These are pictures from the virtual address space. The DUP in the header of the metdata picture doesn't mean much.
- By default, size of pictures is 10 (2^10=1024 width/height). I used 8 to make them smaller for the documentation page.
Next: Scripting btrfs-heatmap