UBC(9)			 BSD Kernel Developer's Manual			UBC(9)

NAME
     ubc — unified buffer cache

SYNOPSIS
     #include <uvm/uvm.h>

     void *
     ubc_alloc(struct uvm_object *uobj, voff_t offset, vsize_t *lenp,
	 int advice, int flags);

     void
     ubc_release(void *va, int flags);

     int
     ubc_uiomove(struct uvm_object *uobj, struct uio *uio, vsize_t todo,
	 int advice, int flags);

     void
     ubc_purge(struct uvm_object *uobj);

DESCRIPTION
     ubc_alloc() creates a kernel mapping of uobj starting at offset offset.
     The desired length of the mapping is pointed to by lenp, but the actual
     mapping may be smaller than this.	lenp is updated to contain the actual
     length mapped.  advice is the access pattern hint, which must be one of

	   UVM_ADV_NORMAL      No hint
	   UVM_ADV_RANDOM      Random access hint
	   UVM_ADV_SEQUENTIAL  Sequential access hint (from lower offset to
			       higher offset)

     The possible flags are

	   UBC_READ	       Mapping will be accessed for read.
	   UBC_WRITE	       Mapping will be accessed for write.
	   UBC_FAULTBUSY       Fault in window's pages already during mapping
			       operation.  Makes sense only for write.

     Once the mapping is created, it must be accessed only by methods that can
     handle faults, such as uiomove() or kcopy().  Page faults on the mapping
     will result in the object's pager method being called to resolve the
     fault.

     ubc_release() frees the mapping at va for reuse.  The mapping may be
     cached to speed future accesses to the same region of the object.	The
     flags can be any of

	   UBC_UNMAP	       Do not cache mapping.

     ubc_uiomove() allocates an UBC memory window, performs I/O on it and
     unmaps the window.	 The advice parameter takes the same values as the
     respective parameter in ubc_alloc() and the flags parameter takes the
     same arguments as ubc_alloc() and ubc_release().  Additionally, the flag
     UBC_PARTIALOK can be provided to indicate that it is acceptable to return
     if an error occurs mid-transfer.

     ubc_purge() disassociates all UBC structures from an empty UVM object,
     specified by uobj.

CODE REFERENCES
     The ubc subsystem is implemented within the file sys/uvm/uvm_bio.c.

SEE ALSO
     pmap(9), uiomove(9), uvm(9), vnode(9), vnodeops(9)

     Chuck Silvers, "UBC: An Efficient Unified I/O and Memory Caching
     Subsystem for NetBSD", Proceedings of the FREENIX Track: 2000 USENIX
     Annual Technical Conference, USENIX Association,
     http://www.usenix.org/event/usenix2000/freenix/full_papers/silvers/silvers.pdf,
     285-290, June 18-23, 2000.

HISTORY
     UBC first appeared in NetBSD 1.6.

AUTHORS
     Chuck Silvers ⟨chuq@chuq.com⟩ designed and implemented the UBC part of
     UVM, which uses UVM pages to cache vnode data rather than the traditional
     buffer cache buffers.

BSD				 June 14, 2011				   BSD

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