Why does the OSPG.dat file grows so much more than all other files?

Question

Dear Jena Community,

I'm running Jena Fuseki Version 4.4.0 as a container on an OpenShift Cluster.

OS Version Info (cat /etc/os-release):

NAME="Red Hat Enterprise Linux" 
VERSION="8.5 (Ootpa)" 
ID="rhel" 
ID_LIKE="fedora" ="8.5" 
... 

Hardware Info (from Jena Fuseki initialization log):

[2023-01-27 20:08:59] Server     INFO    Memory: 32.0 GiB 
[2023-01-27 20:08:59] Server     INFO    Java:   11.0.14.1 
[2023-01-27 20:08:59] Server     INFO    OS:     Linux 3.10.0-1160.76.1.el7.x86_64 amd64 
[2023-01-27 20:08:59] Server     INFO    PID:    1

Disk Info (df -h):

Filesystem                                                          Size  Used Avail Use% Mounted on
overlay                                                              99G   76G   18G  82% /
tmpfs                                                                64M     0   64M   0% /dev
tmpfs                                                                63G     0   63G   0% /sys/fs/cgroup
shm                                                                  64M     0   64M   0% /dev/shm
/dev/mapper/docker_data                                              99G   76G   18G  82% /config
/data                                                                1.0T  677G  348G  67% /usr/app/run
tmpfs                                                                40G   24K   40G   1% 

My dataset is built using TDB2, and currently has the following RDF Stats:

• Triples: 65KK (Approximately 65 million)
• Subjects: ~20KK (Aproximately 20 million)
• Objects: ~8KK (Aproximately 8 million)
• Graphs: ~213K (Aproximately 213 thousand)
• Predicates: 153

The files corresponding to this dataset alone on disk sum up to approximately 671GB (measured with du -h). From these, the largest files are:

• /usr/app/run/databases/my-dataset/Data-0001/OSPG.dat: 243GB
• /usr/app/run/databases/my-dataset/Data-0001/nodes.dat: 76GB
• /usr/app/run/databases/my-dataset/Data-0001/POSG.dat: 35GB
• /usr/app/run/databases/my-dataset/Data-0001/nodes.idn: 33GB
• /usr/app/run/databases/my-dataset/Data-0001/POSG.idn: 29GB
• /usr/app/run/databases/my-dataset/Data-0001/OSPG.idn: 27GB

I've looked into several documentation pages, source code, forums, ... nowhere I was able to find some explanation to why OSPG.dat is so much larger than all other files.

I've been using Jena for quite some time now and I'm well aware that its indexes grow significantly during usage, specially when triples are being added across multiple requests (transactional workloads).

Even though, the size of this particular file (OSPG.dat) surprised me, as in my prior experience the indexes would never get larger than the nodes.dat file.

Is there a reasonable explanation for this based on the content of the dataset or the way it was generated? Could this be an indexing bug within TDB2?

Thank you for your support!

For completeness, here is the assembler configuration for my dataset:

@prefix :       <http://base/#> .
@prefix fuseki: <http://jena.apache.org/fuseki#> .
@prefix ja:     <http://jena.hpl.hp.com/2005/11/Assembler#> .
@prefix rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs:   <http://www.w3.org/2000/01/rdf-schema#> .
@prefix root:   <http://dev-test-jena-fuseki/$/datasets#> .
@prefix tdb2:   <http://jena.apache.org/2016/tdb#> .

tdb2:GraphTDB  rdfs:subClassOf  ja:Model .

ja:ModelRDFS  rdfs:subClassOf  ja:Model .

ja:RDFDatasetSink  rdfs:subClassOf  ja:RDFDataset .

<http://jena.hpl.hp.com/2008/tdb#DatasetTDB>
rdfs:subClassOf  ja:RDFDataset .

tdb2:GraphTDB2  rdfs:subClassOf  ja:Model .

<http://jena.apache.org/text#TextDataset>
rdfs:subClassOf  ja:RDFDataset .

ja:RDFDatasetZero  rdfs:subClassOf  ja:RDFDataset .

:service_tdb_my-dataset
rdf:type                      fuseki:Service ;
rdfs:label                    "TDB my-dataset" ;
fuseki:dataset                :ds_my-dataset ;
fuseki:name                   "my-dataset" ;
fuseki:serviceQuery           "sparql" , "query" ;
fuseki:serviceReadGraphStore  "get" ;
fuseki:serviceReadWriteGraphStore
"data" ;
fuseki:serviceUpdate          "update" ;
fuseki:serviceUpload          "upload" .

ja:ViewGraph  rdfs:subClassOf  ja:Model .

ja:GraphRDFS  rdfs:subClassOf  ja:Model .

tdb2:DatasetTDB  rdfs:subClassOf  ja:RDFDataset .

<http://jena.hpl.hp.com/2008/tdb#GraphTDB>
rdfs:subClassOf  ja:Model .

ja:DatasetTxnMem  rdfs:subClassOf  ja:RDFDataset .

tdb2:DatasetTDB2  rdfs:subClassOf  ja:RDFDataset .

ja:RDFDatasetOne  rdfs:subClassOf  ja:RDFDataset .

ja:MemoryDataset  rdfs:subClassOf  ja:RDFDataset .

ja:DatasetRDFS  rdfs:subClassOf  ja:RDFDataset .

:ds_my-dataset  rdf:type     tdb2:DatasetTDB2 ;
tdb2:location           "run/databases/my-dataset" ;
tdb2:unionDefaultGraph  true ;
ja:context              \[ ja:cxtName   "arq:optFilterPlacement" ;
ja:cxtValue  "false"
\] .

Updates: Currently, I'm trying to recreate the dataset from an NQuads Backup (~15GB after decompression) in the hope the index sizes will decrease. Nonetheless, as this dataset will continue to grow during the system usage it would be helpful to understand what caused this astounding growth in this particular index.