1 Apr
2020
1 Apr
'20
1:14 p.m.
Hi Frank,
answering the second part.
The following settings look senseless indeed:
bluestore_compression_min_blob_size_ssd 8192
bluestore_min_alloc_size_ssd 16384
Presumably this was an incomplete backport from Nautilus which has
proper numbers: 32K and 16K respectively.
Fell free to create a ticket.
Thanks,
Igor
On 4/1/2020 12:44 PM, Frank Schilder wrote:
> 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
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