Hi,
As I've promised earlier I took TDB2 for a little test drive, using the
3.5.0rc1 builds.
I tested two scenarios: A server running Fuseki, and command line tools
operating directly on a database directory.
1. Server running Fuseki
First the server (running as a VM). Currently I've been using Fuseki
with HDT support, from the hdt-java repository. I'm serving a dataset of
about 39M triples, which occasionally changes (eventually this will be
updated once per month, or perhaps more frequently, even once per day).
With HDT, I can simply rebuild the HDT file (less than 10 minutes) and
then restart Fuseki. Downtime for the endpoint is only a few seconds.
But I'm worried about the state of the hdt-java project, it is not being
actively maintained and it's still based on Fuseki1.
So I switched (for now) to Fuseki2 with TDB2. It was rather smooth
thanks to the documentation that Andy provided. I usually create Fuseki2
datasets via the API (using curl), but I noticed that, like the UI, the
API only supports "mem" and "tdb". So I created a "tdb" dataset first,
then edited the configuration file so it uses tdb2 instead.
Loading the data took about 17 minutes. I used wget for this, per Andy's
example. This is a bit slower than regenerating the HDT, but acceptable
since I'm only doing it occasionally. I also tested executing queries
while reloading the data. This seemed to work OK even though performance
obviously did suffer. But at least the endpoint remained up.
The TDB2 directory ended up at 4.6GB. In contrast, the HDT file + index
for the same data is 560MB.
I reloaded the same data, and the TDB2 directory grew to 8.5GB, almost
twice its original size. I understand that the TDB2 needs to be
compacted regularly, otherwise it will keep growing. I'm OK with the
large disk space usage if it's constant, not growing over time like TDB1.
2. Command line tools
For this I used an older version of the same dataset with 30M triples,
the same one I used for my HDT vs TDB comparison that I posted on the
users mailing list:
http://mail-archives.apache.org/mod_mbox/jena-users/201704.mbox/%3C90c0130b-244d-f0a7-03d3-83b47564c990%40iki.fi%3E
This was on my i3-2330M laptop with 8GB RAM and SSD.
Loading the data using tdb2.tdbloader took about 18 minutes (about 28k
triples per second). The TDB2 directory is 3.7GB. In contrast, using
tdbloader2, loading took 11 minutes and the TDB directory was 2.7GB. So
TDB2 is slower to load and takes more disk space than TDB.
I ran the same example query I used before on the TDB2. The first time
was slow (33 seconds), but subsequent queries took 16.1-18.0 seconds.
I also re-ran the same query on TDB using tdbquery on Jena 3.5.0rc1. The
query took 13.7-14.0 seconds after the first run (24 seconds).
I also reloaded the same data to the TDB2 to see the effect. Reloading
took 11 minutes and the database grew to 5.7GB. Then I compacted it
using tdb2.tdbcompact. Compacting took 18 minutes and the disk usage
just grew further, to 9.7GB. The database directory then contained both
Data-0001 and Data-0002 directories. I removed Data-0001 and disk usage
fell to 4.0GB. Not quite the same as the original 3.7GB, but close.
My impressions so far: It works, but it's slower than TDB and needs more
disk space. Compaction seems to work, but initially it will just
increase disk usage. The stale data has to be manually removed to
actually reclaim any space. I didn't test subsequent load/compact
cycles, but I assume there may still be some disk space growth (e.g. due
to blank nodes, of which there are some in my dataset) even if the data
is regularly compacted.
For me, not growing over time like TDB is really the crucial feature
that TDB2 seems to promise. Right now it's not clear whether it entirely
fulfills this promise, since compaction needs to be done manually and
doesn't actually reclaim disk space by itself.
Questions/suggestions:
1. Is is possible to trigger a TDB2 compaction from within Fuseki? I'd
prefer not taking the endpoint down for compaction.
2. Should the stale data be deleted after compaction, at least as an option?
3. Should there be a JIRA issue about UI and API support for creating
TDB2 datasets?
4. Should there be a JIRA issue about the bad Content-Length values
reported by Fuseki?
-Osma
--
Osma Suominen
D.Sc. (Tech), Information Systems Specialist
National Library of Finland
P.O. Box 26 (Kaikukatu 4)
00014 HELSINGIN YLIOPISTO
Tel. +358 50 3199529
osma.suomi...@helsinki.fi
http://www.nationallibrary.fi
