Delta Merge Log

[Former Member]

Hi Team,

I was trying to understand the delta merge topic ,
From cost function Calculation I noted a parameter called

DLS  - Delta log size [MB]

What does this space contains .

After that i noted this term in many other part of this documentation , can someone help to understand this .

Thanks,

Razal

[lbreddemann · Accepted solution]

Hi Razal,

the delta log is a storage area used for column store tables.

It's size in MB is a possible decision variable for the automatic triggering of the delta merge.

There's plenty of information about the delta merge available here in SCN and on SAPHANA.COM so please use the existing resources for more details.

- Lars

[Former Member · Accepted solution]

Hi Lars ,

This is what written in the Admin Guide :

A further result of the delta merge operation is truncation of the delta log. The delta storage structure itself exists only in memory and is not persisted to disk. The column store creates its logical redo log entries for all operations executed on the delta storage. This log is called the delta log. In the event of a system restart, the delta log entries are replayed to rebuild the in-memory delta storage After the changes in the delta storage have been merged into the main storage, the delta log file is truncated by removing those entries that were written before the merge operation.

Does this mean that delta log persisted in the Disk  ?

Thanks,

Razal

[lbreddemann · Accepted solution]

Yes, the information for the delta log are persisted on disk.

However, the data structure in memory and on disk are very different from each other.

- Lars

[Former Member · Accepted solution]

Hi Lars ,

That made the concept clear for me .

Thank you Lars  for your great support as always

Thanks,

Razal

[Former Member · Accepted solution]

Hey Lars are there any resources that explain the differences between in memory and on disk storage?

[lbreddemann · Accepted solution]

Nope, nothing I am aware of.

The main difference is that the delta store is a table/partition wide in-memory data structure.

It is made up from the joined per-column persisted delta data structures.

That way reading changed values can be done column wise, without the need to access the other columns.

Only when we want to change data, we need the whole delta store to be in memory.