Prestoserve File System Accelerator

The Prestoserve file system accelerator is a hardware option that speeds up synchronous disk writes, including NFS server access, by reducing the amount of disk I/O. Frequently-written data blocks are cached in nonvolatile memory and then written to disk asynchronously.

The software required to drive the board ships as an optional subset in Digital UNIX Version 4.0 and once it is installed can be accessed with a PAK that comes with the board.

Prestoserve uses a write cache for synchronous disk I/O. Prestoserve works in a way that is similar to the way the system buffer cache speeds up asynchronous disk I/O requests. Prestoserve is interposed between the operating system and the device drivers for the disks on a server. Mounted file systems and unmounted block devices selected by the administrator are accelerated.

When a synchronous write request is issued to a disk with accelerated file systems or block devices, it is intercepted by the Prestoserve pseudodevice driver, which stores the data in nonvolatile memory instead of on the disk. Thus, synchronous writes occur at memory speeds, not at disk speeds.

As the nonvolatile memory in the Prestoserve cache fills up, it asynchronously flushes the cached data to the disk in portions that are large enough to allow the disk drivers to optimize the order of the writes. A modified form of Least Recently Used (LRU) replacement is used to determine the order. Reads that hit (match blocks) in the Prestoserve cache also benefit.

Nonvolatile memory is required because data must not be lost if the power fails or if the system crashes. As a result, the hardware board contains a battery that protects data in case the system crashes. From the point of view of the operating system, Prestoserve appears to be a very fast disk.

Note that there is a substantial performance gain when Prestoserve is used on an NFSV2 server.

The dxpresto command allows you to monitor Prestoserve activity and to enable or disable Prestoserve on machines that allow that operation. For more information on Prestoserve see the Guide to Prestoserve and the dxpresto(8X) reference page.