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

# NAME

`ubc` —
unified buffer cache

# SYNOPSIS

`#include <uvm/uvm.h>`

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

void
`ubc_zerorange`(struct uvm_bject *uobj, off_t off, size_t len, int flags);

void
`ubc_purge`(struct uvm_object *uobj);

# DESCRIPTION

`ubc_uiomove`() allocates an UBC memory window, performs I/O on it and unmaps the window. The advice parameter 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)

and the flags parameter is

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.
UBC_UNMAP
  Do not cache mapping.
UBC_PARTIALOK
  Indicate that it is acceptable to return if an error occurs mid-transfer.

UBC memory window is a kernel mapping of uobj starting at offset offset.

Once the mapping is created, it must be accessed only by methods that can handle faults, such as uiomove(9) or kcopy(9). Page faults on the mapping will result in the object's pager method being called to resolve the fault.

Size of individual UBC memory window is limited to ubc_winsize. `ubc_uiomove`() sequentially creates the UBC memory windows to eventually process the whole range according to offset and len parameters.

The mappings may be cached to speed future accesses to the same region of the object, unless `UBC_UNMAP` was specified in flags parameter.

`ubc_zerorange`() sets a range of bytes in a UVM object to zero. The flags parameter takes the same arguments as `ubc_uiomove`().

`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

kcopy(9), 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.

May 12, 2018 | NetBSD 9.2