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 in size from 4 KB to 64 MB. The data is written asynchronously with parallel I/O to the data file system. During normal operations, most I/O to the data file system consists of writes. Data is read from the file system only during database restarts, SAP HANA backups, host auto-failover, or a column store table load or reload operation.
Access to the log volume is primarily sequential, with blocks ranging in size 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 I/O to the log volume consists of writes, and data is read from the log volume only during database restart, HA failover, log backup, or database recovery.
SAP HANA I/O can be optimized for specific storage environments. For more information, see Optimizing file I/O after the SAP HANA installation.