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The SAP HANA persistent devices use different I/O patterns. For more information, see SAP HANA Storage Requirements.
Access to the data volume is primarily random, with blocks ranging from 4 KB to 64 MB. The data is written asynchronously with parallel I/O operations to the datafile system. During normal operations, most of the I/Os to the datafile system are writes and data is read from the file system only during database restarts, SAP HANA backups, host autofailover, or a column store table load or reload operation.
Access to the log volume is primarily sequential, with blocks ranging from 4 KB to 1 MB. SAP HANA keeps a 1 MB buffer in memory for the redo log. When the buffer is full, it is synchronously written to the log volume. When a database transaction is committed before the log buffer is full, a smaller block is written to the file system. Because data is written synchronously to the log volume, a low latency for the I/O to the storage device is important, especially for the smaller 4 KB and 16 KB block sizes. During normal database operations, most of the I/Os to the log volume are writes and data is read from the log volume only during database restart, high availability failover, log backup, or database recovery.
For information about optimizing SAP HANA for specific storage environments, see Optimizing file I/O after the SAP HANA installation.