AIO virtual processors

If the platform does not support KAIO or if the I/O is to buffered-file chunks, the database server performs database I/O through the AIO class of virtual processors. All AIO virtual processors service all I/O requests equally within their class.

The database server assigns each disk chunk a queue, sometimes known as a gfd queue, which is based on the file name of the chunk. The database server orders I/O requests within a queue according to an algorithm that minimizes disk-head movement. The AIO virtual processors service queues that have pending work in round-robin fashion. All other non-chunk I/O is queued in the AIO queue.

Use the VPCLASS parameter with the aio keyword to specify the number of AIO virtual processors that the database server starts initially. You can start additional AIO virtual processors while the database server is in online mode. You cannot drop AIO virtual processors while the database server is in online mode.

You can enable the database server to add AIO virtual processors and flusher threads when the server detects that AIO VPs are not keeping up with the I/O workload. Include the autotune=1 keyword in the VPCLASS configuration parameter setting.

Manually controlling the number of AIO VPs

The goal in allocating AIO virtual processors is to allocate enough of them so that the lengths of the I/O request queues are kept short; that is, the queues have as few I/O requests in them as possible. When the gfd queues are consistently short, it indicates that I/O to the disk devices is being processed as fast as the requests occur.

The onstat-g ioq command shows the length and other statistics about I/O queues. You can use this command to monitor the length of the gfd queues for the AIO virtual processors.

One AIO virtual processor might be sufficient:
  • If the database server implements kernel asynchronous I/O (KAIO) on your platform and all of your dbspaces are composed of raw disk space
  • If your file system supports direct I/O for the page size that is used for the dbspace chunk and you use direct I/O
Allocate two AIO virtual processors per active dbspace that is composed of buffered file chunks.
  • If the database server implements KAIO, but you are using some buffered files for chunks
  • IF KAIO is not supports by the system for chunks.

If KAIO is not implemented on your platform, allocate two AIO virtual processors for each disk that the database server accesses frequently.

If you use cooked files and if you enable direct I/O using the DIRECT_IO configuration parameter, you might be able to reduce the number of AIO virtual processors.

If the database server implements KAIO and you enabled direct I/O using the DIRECT_IO configuration parameter, HCL Informix® attempts to use KAIO, so you probably do not require more than one AIO virtual processor. However, even when direct I/O is enabled, if the file system does not support either direct I/O or KAIO, you still must allocate two AIO virtual processors for every active dbspace that is composed of buffered file chunks or does not use KAIO.

Temporary dbspaces do not use direct I/O. If you have temporary dbspaces, you probably require more than one AIO virtual processors.

Allocate enough AIO virtual processors to accommodate the peak number of I/O requests. Generally, it is not detrimental to allocate too many AIO virtual processors.


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