I/O Performance HOWTO
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# 5. I/O Request Lock Patch

This section provides information on the I/O request lock patch, also known as the scsi concurrent queuing patch (sior1), written by Johnathan Lahr.

The I/O request lock patch improves SCSI I/O performance on Linux 2.4 multi-processor systems by providing concurrent I/O request queuing. There are significant I/O performance and CPU utilization improvements possible by enabling multi-processors to concurrently drive multiple block devices.

Before the patch is applied block I/O requests are queued one at a time holding the global spin lock, `io_request_lock`. Once the patch is applied, SCSI requests are queued while holding the lock specific to the queue associated with the request. Requests that are made to different devices are queued concurrently, and requests that are made to the same device are queued serially.

## 5.1. Locating the Patch

You can download the I/O request patch from Sourceforge at http://sourceforge.net/projects/lse/io. The latest version is sior1-v1.2416. Patches that enable concurrent queuing for specific drivers are also available at SourceForge. The patch for the Emulex SCSI/FC is lpfc_sior1-v0.249 and the patch for Adaptec SCSI is aic_sior1-v0.249.

## 5.2. Modifying Your Driver for the I/O Request Lock Patch

The I/O request lock patch installs concurrent queuing capability into the SCSI midlayer. Concurrent queuing is activated for each SCSI adapter device driver. To activate the driver, the `concurrent_queue` field in the `Scsi_Host_Template` structure must be set when the driver is registered.

| Drivers that activate concurrent queuing must ensure that any access of the `request_queue` by the driver is protected by the `request_queue.queue_lock`. Additional information is available on rebuilding device drivers at http://www.xml.com/ldd/chapter/book/index.html.

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