Cache Advisories

Direct I/O

Direct I/O is an unbuffered form of I/O. If the VX_DIRECT advisory is set, the user is requesting direct data transfer between the disk and the user-supplied buffer for reads andwrites. This bypasses the kernel buffering of data, and reduces the CPU overhead associated with I/O by eliminating the data copy between the kernel buffer and the user's buffer. This also avoids taking up space in the buffer cache that might be better used for something else. The direct I/O feature can provide significant performance gains for some applications.

For an I/O operation to be performed as direct I/O, it must meet certain alignment criteria. The alignment constraints are usually determined by the disk driver, the disk controller, and the system memory management hardware and software. The file offset must be aligned on a 4-byte boundary.

If a request fails to meet the alignment constraints for direct I/O, the request is performed as data synchronous I/O. If the file is currently being accessed by using memory mapped I/O, any direct I/O accesses are done as data synchronous I/O.

Since direct I/O maintains the same data integrity as synchronous I/O, it can be used in many applications that currently use synchronous I/O. If a direct I/O request does not allocate storage or extend the file, the inode is not immediately written.

The CPU cost of direct I/O is about the same as a raw disk transfer. For sequential I/O to very large files, using direct I/O with large transfer sizes can provide the same speed as buffered I/O with much less CPU overhead.

If the file is being extended or storage is being allocated, direct I/O must write the inode change before returning to the application. This eliminates some of the performance advantages of direct I/O.

The direct I/O and VX_DIRECT advisories are maintained on a per-file-descriptor basis.

Unbuffered I/O

If the VX_UNBUFFERED advisory is set, I/O behavior is the same as direct I/O with the VX_DIRECT advisory set, so the alignment constraints that apply to direct I/O also apply to unbuffered. For I/O with unbuffered I/O, however, if the file is being extended, or storage is being allocated to the file, inode changes are not updated synchronously before the write returns to the user. The VX_UNBUFFERED advisory is maintained on a per-file-descriptor basis.

Discovered Direct I/O

Discovered Direct I/O is not a cache advisory that the user can set using the VX_SETCACHE ioctl. When the file system gets an I/O request larger than the default size of 128K, it tries to use direct I/O on the request. For large I/O sizes, Discovered Direct I/O can perform much better than buffered I/O.

Discovered Direct I/O behavior is similar to direct I/O and has the same alignment constraints, except writes that allocate storage or extend the file size do not require writing the inode changes before returning to the application.

Data Synchronous I/O

If the VX_DSYNC advisory is set, the user is requesting data synchronous I/O. In synchronous I/O, the data is written, and the inode is written with updated times and (if necessary) an increased file size. In data synchronous I/O, the data is transferred to disk synchronously before the write returns to the user. If the file is not extended by the write, the times are updated in memory, and the call returns to the user. If the file is extended by the operation, the inode is written before the write returns.

Like direct I/O, the data synchronous I/O feature can provide significant application performance gains. Since data synchronous I/O maintains the same data integrity as synchronous I/O, it can be used in many applications that currently use synchronous I/O. If the data synchronous I/O does not allocate storage or extend the file, the inode is not immediately written. The data synchronous I/O does not have any alignment constraints, so applications that find it difficult to meet the alignment constraints of direct I/O should use data synchronous I/O.

If the file is being extended or storage is allocated, data synchronous I/O must write the inode change before returning to the application. This case eliminates the performance advantage of data synchronous I/O.

The direct I/O and VX_DSYNC advisories are maintained on a per-file-descriptor basis.

Other Advisories

The VX_SEQ advisory indicates that the file is being accessed sequentially. When the file is being read, the maximum read-ahead is always performed. When the file is written, instead of trying to determine whether the I/O is sequential or random by examining the write offset, sequential I/O is assumed. The pages for the write are not immediately flushed. Instead, pages are flushed some distance behind the current write point.

The VX_RANDOM advisory indicates that the file is being accessed randomly. For reads, this disables read-ahead. For writes, this disables the flush-behind. The data is flushed by the pager, at a rate based on memory contention.

The VX_NOREUSE advisory is used as a modifier. If both VX_RANDOM and VX_NOREUSE are set, pages are immediately freed and put on the quick reuse free list as soon as the data has been used. If VX_NOREUSE is set when doing sequential I/O, pages are also put on the quick reuse free list when they are flushed. The VX_NOREUSE may slow down access to the file, but it can reduce the cached data held by the system. This can allow more data to be cached for other files and may speed up those accesses.