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AIX Versions 3.2 and 4 Performance Tuning Guide
Using Raw Disk I/O
There are three ways in which a program might access disk in raw mode:
- Block raw-disk-device special files have names of the form
/dev/hdiskn, and are used by some subsystems. These devices should
not be used by application programs.
- Character raw-disk-device special files have names of the form
/dev/rhdiskn. Use of these devices by application programs is not
recommended. If you decide to use this technique, make sure that no AIX logical
volumes occupy any part of the physical disk drive being accessed. The
performance effect of interaction between raw access and file-system access to
the same physical drive is unpredictable. Make sure you do not overwrite the
first 512 bytes of the disk since that is where the physical volume ID is
stored.
- A logical volume on which no file system has been created can be accessed in
raw mode. All writes, reads, lseeks, etc. must be in
multiples of 512 bytes. The least important consequence of violating this rule
is serious performance degradation.
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