Hi,
I have several subvolumes mounted with compress-force=lzo and autodefrag.
Since I use lots of snapshots (snapper keeps around 24 hourly snapshots, 7
daily snapshots and 4 weekly snapshots) I had to create a systemd timer to
perform a full balance and deduplication each night. In fact data needs to
be already deduplicated when snapshots are created, otherwise I have no
other way to deduplicate snapshots.
This is how I performe balance: btrfs balance start --full-balance rootfs
This is how I perform deduplication (duperemove is from git master):
duperemove -drh --dedupe-options=noblock --hashfile=../rootfs.hash
<all_subvols_except_snapshots_ones>
Looking at the logs I noticed something weird: available free space
actually decreases after balance or deduplication.
This is just before the timer starts:
Overall:
Device size: 128.00GiB
Device allocated: 49.03GiB
Device unallocated: 78.97GiB
Device missing: 0.00B
Used: 43.78GiB
Free (estimated): 82.97GiB (min: 82.97GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 0.00B)
Data,single: Size:44.00GiB, Used:40.00GiB
/dev/sda5 44.00GiB
Metadata,single: Size:5.00GiB, Used:3.78GiB
/dev/sda5 5.00GiB
System,single: Size:32.00MiB, Used:16.00KiB
/dev/sda5 32.00MiB
Unallocated:
/dev/sda5 78.97GiB
I also manually performed a full balance just before the timer starts:
Overall:
Device size: 128.00GiB
Device allocated: 46.03GiB
Device unallocated: 81.97GiB
Device missing: 0.00B
Used: 43.78GiB
Free (estimated): 82.96GiB (min: 82.96GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 0.00B)
Data,single: Size:41.00GiB, Used:40.01GiB
/dev/sda5 41.00GiB
Metadata,single: Size:5.00GiB, Used:3.77GiB
/dev/sda5 5.00GiB
System,single: Size:32.00MiB, Used:16.00KiB
/dev/sda5 32.00MiB
Unallocated:
/dev/sda5 81.97GiB
As you can see even doing a full balance was enough to reduce the available
free space!
Then the timer started and it performed the deduplication:
Overall:
Device size: 128.00GiB
Device allocated: 46.03GiB
Device unallocated: 81.97GiB
Device missing: 0.00B
Used: 43.87GiB
Free (estimated): 82.94GiB (min: 82.94GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 176.00KiB)
Data,single: Size:41.00GiB, Used:40.03GiB
/dev/sda5 41.00GiB
Metadata,single: Size:5.00GiB, Used:3.84GiB
/dev/sda5 5.00GiB
System,single: Size:32.00MiB, Used:16.00KiB
/dev/sda5 32.00MiB
Unallocated:
/dev/sda5 81.97GiB
Once again it reduced the available free space!
Then, after the deduplication, the timer also performed a full balance:
Overall:
Device size: 128.00GiB
Device allocated: 46.03GiB
Device unallocated: 81.97GiB
Device missing: 0.00B
Used: 44.00GiB
Free (estimated): 82.93GiB (min: 82.93GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 512.00MiB (used: 0.00B)
Data,single: Size:41.00GiB, Used:40.04GiB
/dev/sda5 41.00GiB
Metadata,single: Size:5.00GiB, Used:3.97GiB
/dev/sda5 5.00GiB
System,single: Size:32.00MiB, Used:16.00KiB
/dev/sda5 32.00MiB
Unallocated:
/dev/sda5 81.97GiB
It further reduced the available free space! Balance and deduplication
actually reduced my available free space of 400MB!
400MB each night!
How is it possible? Should I avoid doing balances and deduplications at
all?
Thanks,
Niccolò
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