maxphys

Description

Maximum size of physical I/O requests. If a driver sees a request larger than this size, the driver breaks the request into maxphys size chunks. File systems can and do impose their own limit.

Data Type

Signed integer

Default

126,976 (sun4m), 131,072 (sun4u), 57,344 (Intel). The sd driver uses the value of 1,048,576 if the drive supports wide transfers. The ssd driver uses 1,048,576 by default.

Range

Machine-specific page size to MAXINT

Units

Bytes

Dynamic?

Yes, but many file systems load this value into a per-mount point data structure when the file system is mounted. A number of drivers load the value at the time a device is attached into a driver-specific data structure.

Validation

None

When to Change

When doing I/O to and from raw devices in large chunks. Note that a DBMS doing OLTP operations issues large numbers of small I/Os. Changing maxphys does not result in any performance improvement in that case.

When doing I/O to and from a UFS file system where large amounts of data (greater than 64 Kbytes) are being read or written at any one time. Note that the file system should be optimized to increase contiguity (for example, increase the size of the cylinder groups and decrease the number of inodes per cylinder group). UFS imposes an internal limit of 1 Mbyte on the maximum I/O size it transfers.

Commitment Level

Unstable