Scenario Description
Savepoint A savepoint ensures that all changed persistent data since the last savepoint gets written to
disk. The SAP HANA database triggers savepoints in 5 minutes intervals by default. Data is
automatically saved from memory to the data volume located on disk. Depending on the
type of data the block sizes vary between 4 KB and 16 MB. Savepoints run asynchronously
to SAP HANA update operations. Database update transactions only wait at the critical
phase of the savepoint, which is usually taking some microseconds.
Snapshot
The SAP HANA database snapshots are used by certain operations like backup and system
copy. They are created by triggering a system wide consistent savepoint. The system keeps
the blocks belonging to the snapshot at least until the drop of the snapshot. Detailed infor
mation about snapshots can be found in the
SAP HANA Administration Guide.
Delta Merge
The delta merge itself takes place in memory. Updates on column store tables are stored in
the delta storage. During the delta merge these changes are applied to the main storage,
where they are stored read optimized and compressed. Right after the delta merge, the new
main storage is persisted in the data volume, that is, written to disk. The delta merge does
not block parallel read and update transactions.
Write Transactions
All changes to persistent data are captured in the redo log. SAP HANA asynchronously
writes the redo log with I/O orders of 4 KB to 1 MB size into log segments. Transactions writ
ing a commit into the redo log wait until the buer containing the commit has been written
to the log volume.
Database restart
At database startup the services load their persistence including catalog and row store ta
bles into memory, that is, the persistence is read from the storage. Additionally the redo log
entries written after the last savepoint have to be read from the log volume and replayed in
the data area in memory. When this is nished the database is accessible. The bigger the
row store is, the longer it takes until the system is available for operations again.
Failover (Host Auto-Fail
over)
On the standby host the services are running in idle mode. Upon failover, the data and log
volumes of the failed host are automatically assigned to the standby host, which then has
read and write access to the les of the failed active host. Row as well as column store tables
(the latter on demand) must be loaded into memory. The log entries have to be replayed.
Takeover (System Replica
tion)
The secondary system is already running, that is the services are active but cannot accept
SQL and thus are not usable by the application. Just like in the database restart (see above)
the row store tables need to be loaded into memory from persistent storage. If table preload
is used, then most of the column store tables are already in memory. During takeover the
replicated redo logs that were shipped since the last data transport from primary to secon
dary have to be replayed.
Data Backup
For a data backup the current payload of the data volumes is read and copied to the backup
storage. For writing a data backup it is essential that on the I/O connection there are no col
lisions with other transactional operations running against the database.
Log Backup Log backups store the content of a closed log segment. They are automatically and asyn
chronously created by reading the payload from the log segments and writing them to the
backup area.
Database Recovery The restore of a data backup reads the backup content from the backup device and writes it
to the SAP HANA data volumes. The I/O write orders of the data recovery have a size of 64
MB. Also the redo log can be replayed during a database recovery, that is the log backups
are read from the backup device and the log entries get replayed.
56 P U B L I C
SAP HANA Troubleshooting and Performance Analysis Guide
Root Causes and Solutions