Note 1645824 - Unexpected data increase during log recovery

Summary

Symptom

After a log recovery, the liveCache fill level is higher than before.

In a system that is configured as a 'shadow database' or a 'hot standby database', the data volumes are filled to 100% after some time due to the permanent log redo even though the original system requires significantly less space.

Other terms

Log recovery, shadow system, data growth, hot standby database, OmsVarObject

Reason and Prerequisites

Prerequisite:

You use SAP liveCache Version 7.7.01.05 to 7.7.07.39.
You use SAP liveCache Version 7.9.00.00 to 7.9.08.01.

Reason:

During the log recovery, the system creates too many pages (data pages) for data of the type OmsVarObject.
This does not affect data recovery; it affects only the log recovery.

The data itself is identical and consistent; this means that nothing is lost and also that nothing is added but the page occupancy is so poor due to the error that the system requires several pages for the same dataset.

The more objects of the type OmsVarObject that are redone via a log, the larger the difference in memory occupation when compared to the original system.
In a normal recovery (data backup + log backups), the effect is generally not critical; however, you can expect a large increase in data in shadow systems and hot standby systems.

Solution

A correction is available as of the following SAP liveCache versions:

7.7.07.40 (PTS 1239906)
7.9.08.02 (PTS 1239908)

Workaround:
A HotFix is provided on request.


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Example of expert analysis (optional):

You can use the following select to display the page size and the page occupancy for objects with the type 'omsVarObject'. In the following example, you can see that the number of entries remains the same but that more pages are required and, therefore, that page occupancy is decreasing.

select
R.Class_ID, R.root, F.type, R.deleted,
F.entrycount, C.pagecount, C."OCCUPANCY (%)"
from
classcontainer_roots R, classcontainers C,
files F
where
R.CLASS_ID = C.CLASS_ID and
F.root = R.root and
F.TYPE = 'VARIABLE OBJECT'
order by
pagecount desc


Recovery via data backup:

TYPE ENTRYC PAGEC OCCUP(%)
---------------------------------------
...VARIABLE OBJECT 952 107 18
...VARIABLE OBJECT 2095 78 55
...VARIABLE OBJECT 540 69 16

Recovery using an old data backup and redoing approximately 500 logs from the same system:

TYPE ENTRYC PAGEC OCCUP(%)
---------------------------------------
...VARIABLE OBJECT 952 943 2
...VARIABLE OBJECT 2095 445 9
...VARIABLE OBJECT 540 406 2

Header Data

Release Status:	Released for Customer
Released on:	26.10.2011 08:02:33
Master Language:	German
Priority:	Correction with high priority
Category:	Program error
Primary Component:	BC-DB-LVC liveCache
Secondary Components:	BC-DB-LCA liveCache Applications