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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. |