HP StoreOnce 4430 HP StoreOnce Backup System Concepts and Configuration Guidel - Page 74

Disk space pre-allocation Some backup applications allow the user to choose whether to "pre-allocate" the disk space for each file at creation time, i.e. as soon as a backup file is created an empty file is created of the maximum size that the backup file can reach. This is done to ensure that there is enough disk space available to write the entire backup file. This setting has no value for StoreOnce devices because it will not result in any physical disk space actually being allocated due to the deduplication system. It is advised that this setting is NOT used because it can result in unrealistically high deduplication ratios being presented when pre-allocated files are not completely filled with backup data or, in extreme cases, it will cause a backup failure due to a timeout if the application tries to write a small amount of data at the end of a large empty file. This results in the entire file having to be padded-out with zeros at creation time, which is a very time consuming operation. Block/transfer size Some backup applications provide a setting for block or transfer size for backup data in the same way as for tape type devices. Larger block sizes are beneficial in the same way for NAS devices as they are for virtual tape devices because they allow for more efficient use of the network interface by reducing the amount of metadata required for each data transfer. In general, set block or transfer size to the largest value allowed by the backup application. Concurrent operations For best StoreOnce performance it is important to either perform multiple concurrent backup jobs or use multiple streams for each backup (whilst staying within the limit of concurrently open files per NAS share). Backup applications provide an option to set the maximum number of concurrent backup streams per file device; this parameter is generally referred to as the number of writers. Setting this to the maximum values shown in the table below ensures that multiple backups or streams can run concurrently whilst remaining within the concurrent file limits for each StoreOnce share. The table in Key parameters (page 115) shows the recommended maximum number of backup streams or jobs per share to ensure that backups will not fail due to exceeding the maximum number of concurrently open files. Note however that optimal performance may be achieved at a lower number of concurrent backup streams. These values are based on standard "file" backup using most major backup applications. If backing up using application agents (e.g. Exchange, SQL, Oracle) it is recommended that only one backup per share is run concurrently because these application agents frequently open more concurrent files than standard file type backups. Buffering If the backup application provides a setting to enable buffering for Read and/or Write this will generally improve performance by ensuring that the application does not wait for write or read operations to report completion before sending the next write or read command. However, this setting could result in the backup application inadvertently causing the StoreOnce appliance to have more concurrently open files than the specified limits (because files may not have had time to close before a new open request is sent). If backup failures occur, disabling buffered writes and reads may fix the problem, in which case, reducing the number of concurrent backup streams then re-enabling buffering will provide best performance. Overwrite versus append This setting allows the backup application to either always start a new backup file for each backup job (overwrite) or continue to fill any backup file that has not reached its size limit before starting new ones (append). 74 NAS shares