How multiphysical buffers are used

The multiphysical buffer pool is an area of memory that can be allocated to various tasks associated with moving data between memory and physical devices:

• 16KB scatter-gather buffers are used to transfer contiguous blocks of data on disk to and from the buffer cache. This mechanism is normally only used if the disk controller does not support scatter-gather in hardware.

  If no buffers are available, a process will normally sleep.

• 1KB copy request buffers (or copy buffers) are used to move data to and from buffers in the buffer cache that lie in memory above 16MB. Copy buffers are necessary for DMA and peripheral controllers that cannot address memory above 16MB as shown in ``How the system uses multiphysical buffers to overcome 24-bit addressing limitations''. If a copy buffer is not available, a process sleeps until one becomes available.

• 4KB transfer buffers are used for moving data between memory and peripheral devices on behalf of applications whose data may lie in memory above the first 16MB. These buffers are only necessary for DMA and peripheral controllers that cannot address memory above 16MB. If no transfer buffers are available, the process is put to sleep.

The system dynamically allocates memory to the following data structures which are used in performing certain I/O operations:

• Scatter-gather buffer headers are used to control scatter-gather requests if the disk hardware supports scatter-gather. If no headers are available, the requests are sent to the disk controller one at a time.

• Control blocks are used to send raw disk I/O (AIO) requests including access to swap space. If no control blocks are available, the process is put to sleep.

How the system uses multiphysical buffers to overcome 24-bit addressing limitations