Quantum 6-01376-05 manual StorNext File System Tuning Metadata Controller System

StorNext File System Tuning

The Metadata Controller System

The buffer cache I/O size is adjusted using the cachebufsize setting. The default setting is usually optimal; however, sometimes performance can be improved by increasing this setting to match the RAID5 stripe size.

Unfortunately, this is often not possible on Linux due to kernel memory fragmentation. In this case performance may degrade severely because the full amount of buffer cache cannot be allocated. Using a large cachebufsize setting also decreases random I/O performance when the amount of data being read is smaller than the cache buffer size.

Buffer cache read-ahead can be adjusted with the buffercache_readahead setting. When the system detects that a file is being read in its entirety, several buffer cache I/O daemons pre-fetch data from the file in the background for improved performance. The default setting is optimal in most scenarios.

The buffer flusher can be tuned with the buffercache_iods setting. A single flusher daemon is responsible for flushing dirty buffers. Instead of synchronously writing each buffer, the flusher places the buffers in an I/ O queue that is processed by multiple daemons. By default there are eight I/O daemons that simultaneously perform disk I/O. Provided that the system supports SCSI Command Tagged Queuing, this concurrency dramatically improves throughput.

RAID systems such as the EMC CX series and Engenio that provide excellent concurrent small I/O performance can usually benefit from increasing the buffercache_iods setting.

The auto_dma_read_length and auto_dma_write_length settings determine the minimum transfer size where direct DMA I/O is performed instead of using the buffer cache for well-formed I/O. These settings can be useful when performance degradation is observed for small DMA I/O sizes compared to buffer cache.

For example, if buffer cache I/O throughput is 200 MB/sec but 512K DMA I/O size observes only 100MB/sec, it would be useful to determine which DMA I/O size matches the buffer cache performance and adjust auto_dma_read_length and auto_dma_write_length accordingly. The lmdd utility is handy here.

The dircachesize option sets the size of the directory information cache on the client. This cache can dramatically improve the speed of readdir operations by reducing metadata network message traffic between the SNFS client and FSM. Increasing this value improves performance in scenarios where very large directories are not observing the benefit of the client directory cache.

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