Doubt with physical storage being used by postgres when storing LOBs

Hi all


I am developing an application which uses postgres 9.2 to store binaries as oid objects.

CREATE TABLE content (contentname text, contentoid oid);

I am making some tests to evaluate how much HD space I will need to allocate these objects.

To measure the space used by postgres I have used two different tools, both with the same results

1.- Checking physical HD space by making a "sudo du -sb /opt/PostgreSQL/9.2/data/base/" before and after inserting the data

2.- Asking directly postgres about the tables size estimation "select pg_size_pretty(pg_relation_size('pg_largeobject'))"

I have tested with different binaries and I am getting different results, for example when I put the content of a zipped file of 17MB size, the increment of the disk space is of 24MB. The reason for this increment seems to be an index created on the table "pg_largeobject". The index is "pg_largeobject_loid_pn_index"

In other hand when I put let's say many zeroes (same 17Mb) the increase of HD usage is much smaller.

I think it could be caused because TOAST compresses the content stored, se he can compress the zeroes but not the previously compressed zip content.

My question is: Is this increase of ~40% normal? Has someone else experienced this?

Thanks in advance for your help!

Víctor Cosqui