Hi All,
I’ve been investigating compression and (long-story-short,) found that I can
never get better than 50% compression ratio.
My setup:
Mimic 13.2.2
OSDs: Bluestore, Sparse files looped to /dev/loop0, lvm to create logical
volumes. bluestore_compression_mode: passive
Pool: 3-replica, compression_mode: force, compression_algorithm: zstd
Data: dd if=/dev/zero of=dd_test bs=2M count=1
Before addition to rados there are zero objects in the pool and, perf dump of
one of the osds contains:
"bluestore_allocated": 165281792,
"bluestore_stored": 54092500,
"bluestore_compressed": 0,
"bluestore_compressed_allocated": 0,
"bluestore_compressed_original": 0,
And after `sudo rados -p rep3_testing put dd_test dd_test`, `rados ls` shows
only the one object, and the same osd as above contains:
"bluestore_allocated": 166330368,
"bluestore_stored": 56189652,
"bluestore_compressed": 592,
"bluestore_compressed_allocated": 1048576,
"bluestore_compressed_original": 2097152,
… which means that the bluestore_stored went up by 2097152 bytes (exactly the
file size of dd_test) and the bluestore_stored went up by 1048576 - which is
exactly half as much. (Note that these numbers are the same as the
bluestore_compressed_* values.) When I repeated this test using zlib or
snappy, I got exactly 50% compression with both as well. If instead I use the
zstd command line program, this file ends up being just 207 bytes - which is
what you’d expect for a file of just zeros.
Please can explain why I'm not getting better compression, or ideally tell me
I’ve forgotten a setting and which setting I need to change?
Thank you,
-TJ Ragan
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