Criteria for HANA administrator to decide on proceeding with DATA Volume reclaim activity

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Introduction: 

This Blog will  guide HANA administrators to collect requirements and take decision proceeding with De-fragmentation which called “DATA Volume reclaim”.

Back Ground : 

Fragmentation in the context of persistence is defined as difference between used and allocated space in the data area on disk. It doesn't necessarily mean that a lot of small space fragments exist. Fragmentation in general shows up when the used space on disk is significantly smaller than before, because the allocated space won't automatically reduce. Typical scenarios are:

• Reduction of business data, e.g. due to archiving and deletion operations (SAP Note 2388483)


• Cleanup after blocked garbage collection (SAP Note 2169283)


• Table optimization (i.e. merge, compression) of a large table (SAP Note 2057046), requiring temporarily double space


• Snapshots / save points preserved for a longer time (SAP Note 2100009), resulting in an increased amount of shadow pages

You can use SQL: "HANA_Disks_Overview" (SAP Note  1969700) to check the current and historic data fragmentation. The output column FRAGMENTATION_PCT indicates the percentage of fragmentation. Values below 30 % are usually acceptable, in case of larger values you can consider a defragmentation. If you know that the free space will be used in the future or the fragmentation will return in a similar way, also values of more than 30 % can be fine.

                 Guidance to take Decision 

Volume Size vs. Disk Usage 

In general, two types of storage space utilization have to be distinguished: Disk (file system) usage and space consumed (internally) in the volumes attached to the HANA database.

Both aspects are summarized in the overview tab in HANA studio:



Total Disk Usage represents the file system point of view and is identical to what OS commands like 'df' are displaying:

Volume Size and Volume Utilization

 If there is a huge difference between “Volume Size” and “Disk Usage”, this would indicate that a lot of space on files system level is consumed by files not related to data volumes. If the percentage of unused space is high, shrinking the volume can be considered, below step will guide you to make a decision.

At Volumes Tab in HANA studio

from "Volumes" Tab if we expand ‘service/Volume’ for ‘indexserver’ and click on Data we will get Details for Data Volumes of service: 'indexserver' in the bottom pan.



There we can see column  "Used/Total Size (%)"

if that Used/TotalSize is low (< 75%) we can consider for Data Volume Reclaim.

in the above picture it is 32% and HANA administrator can go/recommend  for Data Volume Reclaim