Hi

I'm testing In-Memory and my question is why the figures in v$im_segments differ from the used_bytes in v$inmemory_area?

I have read a lot of great posts (for example https://blogs.oracle.com/in-memory/what-is-an-in-memory-compression-unit-imcu ) and it is always an exact match between those columns, but that isn't the case for me.

This is what I have done in my environment:
I have enabled In-Memory for 291 tables and materialized views of vairous size (but all big enough to be enabled)

ALTER TABLE/MATERIALIZED VIEW [table_name/materialized_view_name] INMEMORY MEMCOMPRESS FOR QUERY LOW PRIORITY CRITICAL

select count(*) from dba_tables where inmemory = 'ENABLED'
count(*)
291

I wait some time to let all the data be populated into In-Memory.

This is the result:

select count(*), sum(bytes), sum(inmemory_size) from v$im_segments
count(*) sum(bytes)    sum(inmemory_size) 
233      6351224832    2660237312

Only 233 with a total size of approximately 2550 MB.
Let us take a look at v$inmemory_area:

select pool, alloc_bytes, used_bytes, populate_status from v$inmemory_area;
pool         alloc_bytes   used_bytes    populate_status
1MB POOL     3421503488    3421503488    OUT OF MEMORY
64KB POOL    855638016     32505856      DONE

I of course realize after the v$inmemory_area query that I don't have enough memory allocated, but if only taking a look at v$im_segments it isn't obvious.

So what is the reason for the difference between inmemory_size and used_bytes?
Can I only from v$im_segments calculate how much amount of memory currently used in the 1MB pool?

Thanks in advance

Out of memory status is not shown in view V$IM_SEGMENTS.
 
Rediscovery Notes
 View V$IM_SEGMENTS shows status STARTED for a table which does not fit in memory. 
 The populate status may incorrectly remain STARTED after a failure. 
 Additionally, there is no out-of-memory status. It should show an 
 indication that it is not possible to load the table completely in memory. 
 
The fix of this bug adds an out-of-memory status for populate enabling V$IM_SEGMENTS 
view to display the out-of-memory status.
The populate status is tracked by taskid, ensuring no false STARTED status.
 
Workaround
 The undocumented view V$INMEMORY_AREA shows correctly the status OUT OF MEMORY 
 for one memory pool. This view can be used instead of V$IM_SEGMENTS.