Documentation / filesystems / caching / fscache.rst


Based on kernel version 5.10.1. Page generated on 2020-12-14 21:14 EST.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565
.. SPDX-License-Identifier: GPL-2.0

==========================
General Filesystem Caching
==========================

Overview
========

This facility is a general purpose cache for network filesystems, though it
could be used for caching other things such as ISO9660 filesystems too.

FS-Cache mediates between cache backends (such as CacheFS) and network
filesystems::

	+---------+
	|         |                        +--------------+
	|   NFS   |--+                     |              |
	|         |  |                 +-->|   CacheFS    |
	+---------+  |   +----------+  |   |  /dev/hda5   |
	             |   |          |  |   +--------------+
	+---------+  +-->|          |  |
	|         |      |          |--+
	|   AFS   |----->| FS-Cache |
	|         |      |          |--+
	+---------+  +-->|          |  |
	             |   |          |  |   +--------------+
	+---------+  |   +----------+  |   |              |
	|         |  |                 +-->|  CacheFiles  |
	|  ISOFS  |--+                     |  /var/cache  |
	|         |                        +--------------+
	+---------+

Or to look at it another way, FS-Cache is a module that provides a caching
facility to a network filesystem such that the cache is transparent to the
user::

	+---------+
	|         |
	| Server  |
	|         |
	+---------+
	     |                  NETWORK
	~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	     |
	     |           +----------+
	     V           |          |
	+---------+      |          |
	|         |      |          |
	|   NFS   |----->| FS-Cache |
	|         |      |          |--+
	+---------+      |          |  |   +--------------+   +--------------+
	     |           |          |  |   |              |   |              |
	     V           +----------+  +-->|  CacheFiles  |-->|  Ext3        |
	+---------+                        |  /var/cache  |   |  /dev/sda6   |
	|         |                        +--------------+   +--------------+
	|   VFS   |                                ^                     ^
	|         |                                |                     |
	+---------+                                +--------------+      |
	     |                  KERNEL SPACE                      |      |
	~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
	     |                  USER SPACE                        |      |
	     V                                                    |      |
	+---------+                                           +--------------+
	|         |                                           |              |
	| Process |                                           | cachefilesd  |
	|         |                                           |              |
	+---------+                                           +--------------+


FS-Cache does not follow the idea of completely loading every netfs file
opened in its entirety into a cache before permitting it to be accessed and
then serving the pages out of that cache rather than the netfs inode because:

 (1) It must be practical to operate without a cache.

 (2) The size of any accessible file must not be limited to the size of the
     cache.

 (3) The combined size of all opened files (this includes mapped libraries)
     must not be limited to the size of the cache.

 (4) The user should not be forced to download an entire file just to do a
     one-off access of a small portion of it (such as might be done with the
     "file" program).

It instead serves the cache out in PAGE_SIZE chunks as and when requested by
the netfs('s) using it.


FS-Cache provides the following facilities:

 (1) More than one cache can be used at once.  Caches can be selected
     explicitly by use of tags.

 (2) Caches can be added / removed at any time.

 (3) The netfs is provided with an interface that allows either party to
     withdraw caching facilities from a file (required for (2)).

 (4) The interface to the netfs returns as few errors as possible, preferring
     rather to let the netfs remain oblivious.

 (5) Cookies are used to represent indices, files and other objects to the
     netfs.  The simplest cookie is just a NULL pointer - indicating nothing
     cached there.

 (6) The netfs is allowed to propose - dynamically - any index hierarchy it
     desires, though it must be aware that the index search function is
     recursive, stack space is limited, and indices can only be children of
     indices.

 (7) Data I/O is done direct to and from the netfs's pages.  The netfs
     indicates that page A is at index B of the data-file represented by cookie
     C, and that it should be read or written.  The cache backend may or may
     not start I/O on that page, but if it does, a netfs callback will be
     invoked to indicate completion.  The I/O may be either synchronous or
     asynchronous.

 (8) Cookies can be "retired" upon release.  At this point FS-Cache will mark
     them as obsolete and the index hierarchy rooted at that point will get
     recycled.

 (9) The netfs provides a "match" function for index searches.  In addition to
     saying whether a match was made or not, this can also specify that an
     entry should be updated or deleted.

(10) As much as possible is done asynchronously.


FS-Cache maintains a virtual indexing tree in which all indices, files, objects
and pages are kept.  Bits of this tree may actually reside in one or more
caches::

                                            FSDEF
                                              |
                         +------------------------------------+
                         |                                    |
                        NFS                                  AFS
                         |                                    |
            +--------------------------+                +-----------+
            |                          |                |           |
         homedir                     mirror          afs.org   redhat.com
            |                          |                            |
      +------------+           +---------------+              +----------+
      |            |           |               |              |          |
    00001        00002       00007           00125        vol00001   vol00002
      |            |           |               |                         |
  +---+---+     +-----+      +---+      +------+------+            +-----+----+
  |   |   |     |     |      |   |      |      |      |            |     |    |
 PG0 PG1 PG2   PG0  XATTR   PG0 PG1   DIRENT DIRENT DIRENT        R/W   R/O  Bak
                      |                                            |
                     PG0                                       +-------+
                                                               |       |
                                                             00001   00003
                                                               |
                                                           +---+---+
                                                           |   |   |
                                                          PG0 PG1 PG2

In the example above, you can see two netfs's being backed: NFS and AFS.  These
have different index hierarchies:

   * The NFS primary index contains per-server indices.  Each server index is
     indexed by NFS file handles to get data file objects.  Each data file
     objects can have an array of pages, but may also have further child
     objects, such as extended attributes and directory entries.  Extended
     attribute objects themselves have page-array contents.

   * The AFS primary index contains per-cell indices.  Each cell index contains
     per-logical-volume indices.  Each of volume index contains up to three
     indices for the read-write, read-only and backup mirrors of those volumes.
     Each of these contains vnode data file objects, each of which contains an
     array of pages.

The very top index is the FS-Cache master index in which individual netfs's
have entries.

Any index object may reside in more than one cache, provided it only has index
children.  Any index with non-index object children will be assumed to only
reside in one cache.


The netfs API to FS-Cache can be found in:

	Documentation/filesystems/caching/netfs-api.rst

The cache backend API to FS-Cache can be found in:

	Documentation/filesystems/caching/backend-api.rst

A description of the internal representations and object state machine can be
found in:

	Documentation/filesystems/caching/object.rst


Statistical Information
=======================

If FS-Cache is compiled with the following options enabled::

	CONFIG_FSCACHE_STATS=y
	CONFIG_FSCACHE_HISTOGRAM=y

then it will gather certain statistics and display them through a number of
proc files.

/proc/fs/fscache/stats
----------------------

     This shows counts of a number of events that can happen in FS-Cache:

+--------------+-------+-------------------------------------------------------+
|CLASS         |EVENT  |MEANING                                                |
+==============+=======+=======================================================+
|Cookies       |idx=N  |Number of index cookies allocated                      |
+              +-------+-------------------------------------------------------+
|              |dat=N  |Number of data storage cookies allocated               |
+              +-------+-------------------------------------------------------+
|              |spc=N  |Number of special cookies allocated                    |
+--------------+-------+-------------------------------------------------------+
|Objects       |alc=N  |Number of objects allocated                            |
+              +-------+-------------------------------------------------------+
|              |nal=N  |Number of object allocation failures                   |
+              +-------+-------------------------------------------------------+
|              |avl=N  |Number of objects that reached the available state     |
+              +-------+-------------------------------------------------------+
|              |ded=N  |Number of objects that reached the dead state          |
+--------------+-------+-------------------------------------------------------+
|ChkAux        |non=N  |Number of objects that didn't have a coherency check   |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of objects that passed a coherency check        |
+              +-------+-------------------------------------------------------+
|              |upd=N  |Number of objects that needed a coherency data update  |
+              +-------+-------------------------------------------------------+
|              |obs=N  |Number of objects that were declared obsolete          |
+--------------+-------+-------------------------------------------------------+
|Pages         |mrk=N  |Number of pages marked as being cached                 |
|              |unc=N  |Number of uncache page requests seen                   |
+--------------+-------+-------------------------------------------------------+
|Acquire       |n=N    |Number of acquire cookie requests seen                 |
+              +-------+-------------------------------------------------------+
|              |nul=N  |Number of acq reqs given a NULL parent                 |
+              +-------+-------------------------------------------------------+
|              |noc=N  |Number of acq reqs rejected due to no cache available  |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of acq reqs succeeded                           |
+              +-------+-------------------------------------------------------+
|              |nbf=N  |Number of acq reqs rejected due to error               |
+              +-------+-------------------------------------------------------+
|              |oom=N  |Number of acq reqs failed on ENOMEM                    |
+--------------+-------+-------------------------------------------------------+
|Lookups       |n=N    |Number of lookup calls made on cache backends          |
+              +-------+-------------------------------------------------------+
|              |neg=N  |Number of negative lookups made                        |
+              +-------+-------------------------------------------------------+
|              |pos=N  |Number of positive lookups made                        |
+              +-------+-------------------------------------------------------+
|              |crt=N  |Number of objects created by lookup                    |
+              +-------+-------------------------------------------------------+
|              |tmo=N  |Number of lookups timed out and requeued               |
+--------------+-------+-------------------------------------------------------+
|Updates       |n=N    |Number of update cookie requests seen                  |
+              +-------+-------------------------------------------------------+
|              |nul=N  |Number of upd reqs given a NULL parent                 |
+              +-------+-------------------------------------------------------+
|              |run=N  |Number of upd reqs granted CPU time                    |
+--------------+-------+-------------------------------------------------------+
|Relinqs       |n=N    |Number of relinquish cookie requests seen              |
+              +-------+-------------------------------------------------------+
|              |nul=N  |Number of rlq reqs given a NULL parent                 |
+              +-------+-------------------------------------------------------+
|              |wcr=N  |Number of rlq reqs waited on completion of creation    |
+--------------+-------+-------------------------------------------------------+
|AttrChg       |n=N    |Number of attribute changed requests seen              |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of attr changed requests queued                 |
+              +-------+-------------------------------------------------------+
|              |nbf=N  |Number of attr changed rejected -ENOBUFS               |
+              +-------+-------------------------------------------------------+
|              |oom=N  |Number of attr changed failed -ENOMEM                  |
+              +-------+-------------------------------------------------------+
|              |run=N  |Number of attr changed ops given CPU time              |
+--------------+-------+-------------------------------------------------------+
|Allocs        |n=N    |Number of allocation requests seen                     |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of successful alloc reqs                        |
+              +-------+-------------------------------------------------------+
|              |wt=N   |Number of alloc reqs that waited on lookup completion  |
+              +-------+-------------------------------------------------------+
|              |nbf=N  |Number of alloc reqs rejected -ENOBUFS                 |
+              +-------+-------------------------------------------------------+
|              |int=N  |Number of alloc reqs aborted -ERESTARTSYS              |
+              +-------+-------------------------------------------------------+
|              |ops=N  |Number of alloc reqs submitted                         |
+              +-------+-------------------------------------------------------+
|              |owt=N  |Number of alloc reqs waited for CPU time               |
+              +-------+-------------------------------------------------------+
|              |abt=N  |Number of alloc reqs aborted due to object death       |
+--------------+-------+-------------------------------------------------------+
|Retrvls       |n=N    |Number of retrieval (read) requests seen               |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of successful retr reqs                         |
+              +-------+-------------------------------------------------------+
|              |wt=N   |Number of retr reqs that waited on lookup completion   |
+              +-------+-------------------------------------------------------+
|              |nod=N  |Number of retr reqs returned -ENODATA                  |
+              +-------+-------------------------------------------------------+
|              |nbf=N  |Number of retr reqs rejected -ENOBUFS                  |
+              +-------+-------------------------------------------------------+
|              |int=N  |Number of retr reqs aborted -ERESTARTSYS               |
+              +-------+-------------------------------------------------------+
|              |oom=N  |Number of retr reqs failed -ENOMEM                     |
+              +-------+-------------------------------------------------------+
|              |ops=N  |Number of retr reqs submitted                          |
+              +-------+-------------------------------------------------------+
|              |owt=N  |Number of retr reqs waited for CPU time                |
+              +-------+-------------------------------------------------------+
|              |abt=N  |Number of retr reqs aborted due to object death        |
+--------------+-------+-------------------------------------------------------+
|Stores        |n=N    |Number of storage (write) requests seen                |
+              +-------+-------------------------------------------------------+
|              |ok=N   |Number of successful store reqs                        |
+              +-------+-------------------------------------------------------+
|              |agn=N  |Number of store reqs on a page already pending storage |
+              +-------+-------------------------------------------------------+
|              |nbf=N  |Number of store reqs rejected -ENOBUFS                 |
+              +-------+-------------------------------------------------------+
|              |oom=N  |Number of store reqs failed -ENOMEM                    |
+              +-------+-------------------------------------------------------+
|              |ops=N  |Number of store reqs submitted                         |
+              +-------+-------------------------------------------------------+
|              |run=N  |Number of store reqs granted CPU time                  |
+              +-------+-------------------------------------------------------+
|              |pgs=N  |Number of pages given store req processing time        |
+              +-------+-------------------------------------------------------+
|              |rxd=N  |Number of store reqs deleted from tracking tree        |
+              +-------+-------------------------------------------------------+
|              |olm=N  |Number of store reqs over store limit                  |
+--------------+-------+-------------------------------------------------------+
|VmScan        |nos=N  |Number of release reqs against pages with no           |
|              |       |pending store                                          |
+              +-------+-------------------------------------------------------+
|              |gon=N  |Number of release reqs against pages stored by         |
|              |       |time lock granted                                      |
+              +-------+-------------------------------------------------------+
|              |bsy=N  |Number of release reqs ignored due to in-progress store|
+              +-------+-------------------------------------------------------+
|              |can=N  |Number of page stores cancelled due to release req     |
+--------------+-------+-------------------------------------------------------+
|Ops           |pend=N |Number of times async ops added to pending queues      |
+              +-------+-------------------------------------------------------+
|              |run=N  |Number of times async ops given CPU time               |
+              +-------+-------------------------------------------------------+
|              |enq=N  |Number of times async ops queued for processing        |
+              +-------+-------------------------------------------------------+
|              |can=N  |Number of async ops cancelled                          |
+              +-------+-------------------------------------------------------+
|              |rej=N  |Number of async ops rejected due to object             |
|              |       |lookup/create failure                                  |
+              +-------+-------------------------------------------------------+
|              |ini=N  |Number of async ops initialised                        |
+              +-------+-------------------------------------------------------+
|              |dfr=N  |Number of async ops queued for deferred release        |
+              +-------+-------------------------------------------------------+
|              |rel=N  |Number of async ops released                           |
|              |       |(should equal ini=N when idle)                         |
+              +-------+-------------------------------------------------------+
|              |gc=N   |Number of deferred-release async ops garbage collected |
+--------------+-------+-------------------------------------------------------+
|CacheOp       |alo=N  |Number of in-progress alloc_object() cache ops         |
+              +-------+-------------------------------------------------------+
|              |luo=N  |Number of in-progress lookup_object() cache ops        |
+              +-------+-------------------------------------------------------+
|              |luc=N  |Number of in-progress lookup_complete() cache ops      |
+              +-------+-------------------------------------------------------+
|              |gro=N  |Number of in-progress grab_object() cache ops          |
+              +-------+-------------------------------------------------------+
|              |upo=N  |Number of in-progress update_object() cache ops        |
+              +-------+-------------------------------------------------------+
|              |dro=N  |Number of in-progress drop_object() cache ops          |
+              +-------+-------------------------------------------------------+
|              |pto=N  |Number of in-progress put_object() cache ops           |
+              +-------+-------------------------------------------------------+
|              |syn=N  |Number of in-progress sync_cache() cache ops           |
+              +-------+-------------------------------------------------------+
|              |atc=N  |Number of in-progress attr_changed() cache ops         |
+              +-------+-------------------------------------------------------+
|              |rap=N  |Number of in-progress read_or_alloc_page() cache ops   |
+              +-------+-------------------------------------------------------+
|              |ras=N  |Number of in-progress read_or_alloc_pages() cache ops  |
+              +-------+-------------------------------------------------------+
|              |alp=N  |Number of in-progress allocate_page() cache ops        |
+              +-------+-------------------------------------------------------+
|              |als=N  |Number of in-progress allocate_pages() cache ops       |
+              +-------+-------------------------------------------------------+
|              |wrp=N  |Number of in-progress write_page() cache ops           |
+              +-------+-------------------------------------------------------+
|              |ucp=N  |Number of in-progress uncache_page() cache ops         |
+              +-------+-------------------------------------------------------+
|              |dsp=N  |Number of in-progress dissociate_pages() cache ops     |
+--------------+-------+-------------------------------------------------------+
|CacheEv       |nsp=N  |Number of object lookups/creations rejected due to     |
|              |       |lack of space                                          |
+              +-------+-------------------------------------------------------+
|              |stl=N  |Number of stale objects deleted                        |
+              +-------+-------------------------------------------------------+
|              |rtr=N  |Number of objects retired when relinquished            |
+              +-------+-------------------------------------------------------+
|              |cul=N  |Number of objects culled                               |
+--------------+-------+-------------------------------------------------------+



/proc/fs/fscache/histogram
--------------------------

     ::

	cat /proc/fs/fscache/histogram
	JIFS  SECS  OBJ INST  OP RUNS   OBJ RUNS  RETRV DLY RETRIEVLS
	===== ===== ========= ========= ========= ========= =========

     This shows the breakdown of the number of times each amount of time
     between 0 jiffies and HZ-1 jiffies a variety of tasks took to run.  The
     columns are as follows:

	=========	=======================================================
	COLUMN		TIME MEASUREMENT
	=========	=======================================================
	OBJ INST	Length of time to instantiate an object
	OP RUNS		Length of time a call to process an operation took
	OBJ RUNS	Length of time a call to process an object event took
	RETRV DLY	Time between an requesting a read and lookup completing
	RETRIEVLS	Time between beginning and end of a retrieval
	=========	=======================================================

     Each row shows the number of events that took a particular range of times.
     Each step is 1 jiffy in size.  The JIFS column indicates the particular
     jiffy range covered, and the SECS field the equivalent number of seconds.



Object List
===========

If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a
list of all the objects currently allocated and allow them to be viewed
through::

	/proc/fs/fscache/objects

This will look something like::

	[root@andromeda ~]# head /proc/fs/fscache/objects
	OBJECT   PARENT   STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA       OBJECT_KEY, AUX_DATA
	======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================
	   17e4b        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a
	   1693a        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a

where the first set of columns before the '|' describe the object:

	=======	===============================================================
	COLUMN	DESCRIPTION
	=======	===============================================================
	OBJECT	Object debugging ID (appears as OBJ%x in some debug messages)
	PARENT	Debugging ID of parent object
	STAT	Object state
	CHLDN	Number of child objects of this object
	OPS	Number of outstanding operations on this object
	OOP	Number of outstanding child object management operations
	IPR
	EX	Number of outstanding exclusive operations
	READS	Number of outstanding read operations
	EM	Object's event mask
	EV	Events raised on this object
	F	Object flags
	S	Object work item busy state mask (1:pending 2:running)
	=======	===============================================================

and the second set of columns describe the object's cookie, if present:

	================ ======================================================
	COLUMN		 DESCRIPTION
	================ ======================================================
	NETFS_COOKIE_DEF Name of netfs cookie definition
	TY		 Cookie type (IX - index, DT - data, hex - special)
	FL		 Cookie flags
	NETFS_DATA	 Netfs private data stored in the cookie
	OBJECT_KEY	 Object key } 1 column, with separating comma
	AUX_DATA	 Object aux data } presence may be configured
	================ ======================================================

The data shown may be filtered by attaching the a key to an appropriate keyring
before viewing the file.  Something like::

		keyctl add user fscache:objlist <restrictions> @s

where <restrictions> are a selection of the following letters:

	==	=========================================================
	K	Show hexdump of object key (don't show if not given)
	A	Show hexdump of object aux data (don't show if not given)
	==	=========================================================

and the following paired letters:

	==	=========================================================
	C	Show objects that have a cookie
	c	Show objects that don't have a cookie
	B	Show objects that are busy
	b	Show objects that aren't busy
	W	Show objects that have pending writes
	w	Show objects that don't have pending writes
	R	Show objects that have outstanding reads
	r	Show objects that don't have outstanding reads
	S	Show objects that have work queued
	s	Show objects that don't have work queued
	==	=========================================================

If neither side of a letter pair is given, then both are implied.  For example:

	keyctl add user fscache:objlist KB @s

shows objects that are busy, and lists their object keys, but does not dump
their auxiliary data.  It also implies "CcWwRrSs", but as 'B' is given, 'b' is
not implied.

By default all objects and all fields will be shown.


Debugging
=========

If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
debugging enabled by adjusting the value in::

	/sys/module/fscache/parameters/debug

This is a bitmask of debugging streams to enable:

	=======	=======	===============================	=======================
	BIT	VALUE	STREAM				POINT
	=======	=======	===============================	=======================
	0	1	Cache management		Function entry trace
	1	2					Function exit trace
	2	4					General
	3	8	Cookie management		Function entry trace
	4	16					Function exit trace
	5	32					General
	6	64	Page handling			Function entry trace
	7	128					Function exit trace
	8	256					General
	9	512	Operation management		Function entry trace
	10	1024					Function exit trace
	11	2048					General
	=======	=======	===============================	=======================

The appropriate set of values should be OR'd together and the result written to
the control file.  For example::

	echo $((1|8|64)) >/sys/module/fscache/parameters/debug

will turn on all function entry debugging.