Bluestore compression parameters in ceph.conf not used in mimic 13.2.8? - ceph-users

1 Apr 2020

Dear all,

I have two observations regarding bluestore compression config:

1) ceph.conf settings seem to be ignored.
2) The SSD default values seem not to save space using compression.

To 1) We are running a mimic 13.2.8 cluster with OSDs deployed under mimic 13.2.2. Back
then the interpretation of compression parameters was messed up, which has been fixed
along the way from 13.2.2 to 13.2.8. To get it to work properly under 13.2.2 I needed to
include these settings in ceph.conf:

[osd]
bluestore compression mode = aggressive
bluestore compression min blob size hdd = 262144

and then also enable compression on all pools that should use compression. These settings
are still present in ceph.conf, but they seem to be ignored when populating the config
data base on mon startup or querying config parameters:

# ceph config get osd.16 bluestore_compression_min_blob_size_hdd
131072

However:

# ceph tell osd.16 config get bluestore_compression_min_blob_size_hdd
262144

and:

# ceph config show osd.16
NAME                                    VALUE                         SOURCE   OVERRIDES  
IGNORES 
bluestore_compression_min_blob_size_hdd 262144                        file                

This is really confusing. Is this intended? Which values will be used when deploying new
OSDs?

In general, it would really be helpful if one could query daemon/parameter groups as in
" ceph config get osd bluestore_compression_min_blob_size_hdd" to get a list
right away.

To 2) In a long-long-ago discussion about how compression works, I was told that a blob of
bluestore_compression_min_blob_size will be compressed and then distributed over a number
of allocations of bluestore_min_alloc_size. The defaults for HDD and SSD are:

bluestore_compression_min_blob_size_hdd 131072
bluestore_min_alloc_size_hdd 65536

bluestore_compression_min_blob_size_ssd 8192
bluestore_min_alloc_size_ssd 16384

If this explanation of the compression method is correct, these defaults allow up to 50%
savings for HDD, but, erm, 0% for SSD as the uncompressed blob will use the same amount of
space as the compressed one as both will require the same allocation size.

Did something change here? Are compressed blobs now co-located in allocations?

Thanks for your help,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14