Btrfs devel - Mar 2013 - Creating zero-filled file aborts after 20GB in a 4GB volume with compress=lzo

Hi,

When running ...
> dd if=/dev/zero of=testfile bs=1M
on a compressed btrfs volume of 4GB mounted with compress=lzo, dd
aborts after about 20GB written.

As I don''t think this can be attributed to metadata consumed ... Any
ideas why lzo achieves such a poor ratio for even highly compressible
data?

Thank you in advance, Clemens
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On Mon, Mar 11, 2013 at 04:16:34PM +0100, Clemens Eisserer
wrote:
> When running ...
> 
> > dd if=/dev/zero of=testfile bs=1M
> 
> on a compressed btrfs volume of 4GB mounted with compress=lzo, dd
> aborts after about 20GB written.
# mkfs 4g
# dd if=/dev/zero of=testfile bs=1M
dd: writing `testfile'': No space left on device
58623787008 bytes (59 GB) copied, 154.548 s, 379 MB/s
# btrfs fi df .
Data: total=2.04GB, used=1.71GB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=204.75MB, used=77.56MB
Metadata: total=8.00MB, used=0.00
# pretty $(filesize testfile)
58,623,787,008 bytes (54.6 GiB)

I''m not sure why the enospc came so early (never seen that before
*cough*),
maybe the other tests running in parallel, so a usuall

# cat /dev/zero > zerofill
cat: write error: No space left on device
# pretty $(filesize zerofill)
47648243712 bytes (44.4 GiB)

# btrfs fi df .
Data: total=3.32GB, used=3.10GB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=332.75MB, used=144.19MB
Metadata: total=8.00MB, used=0.00

Now that looks like a full fs, with ~100GB worth of compressed zeros.
> As I don''t think this can be attributed to metadata consumed ...
Any
> ideas why lzo achieves such a poor ratio for even highly compressible
> data?
What does your ''fi df'' report before and after the test?

david
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