ktune parameter npbuf

[John R. MacMillan]

|In the User's Manual for the 3B1 Unix-PC, vers 3.5, there is a command called
|ktune(7), which allows one to tune certain Unix parameters in the kernel.
|One of the parameters is called npbuf:
|
|npbuf	Number of buffer headers available in the raw I/O pool of headers
|	Range: 4 up to system capacity.  (Default: 16)
|
|Can someone give me more information on this parameter.  When is one of
|these used?  Are they consumed by uucico?  rn?  One per open fd (raw mode)?
|I thought I knew Unix, but this one is new to me.

Someone can correct me if I'm wrong (like I have to mention that :-) ),
but I believe this is a small pool of buffer headers used on the
occassions when the kernel wants to do device I/O using a buffer that
isn't in the buffer cache, so doesn't have a header.  I think this
happens swapping pages, and also when a device is reading or writing a
buffer directly out of user space (``raw'' I/O).

I would think 16 would be plenty on a typical 3B1.