1. More than one cache can be used at once. Caches can be selected explicitly by use of tags. 一次可以使用多個Cache，不同的Cache使用不同的tag區分2. Caches can be added / removed at any time. Cache可以在任何時間被移除或者新增3. The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (2)). 網路檔案系統提供介面，能夠允許其他方刪除檔案cache相關的能力4. The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious. 網路檔案系統儘量不要返回錯誤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. 使用cookie表示檔案系統的目錄、檔案、其他物件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.

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.

brun <N>%, bcull <N>%, bstop <N>%, frun <N>%, fcull <N>%, fstop <N>%Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.The commands beginning with a ‘b’ are file space (block) limits, those beginning with an ‘f’ are file count limits(也可以限制檔案數量).dir <path> 存放快取的根目錄Specify the directory containing the root of the cache. Mandatory.tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is “CacheFiles”.debug <mask>  用於開啟日誌Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR’d into the mask to collect various information:1	Turn on trace of function entry (_enter() macros)2	Turn on trace of function exit (_leave() macros)4	Turn on trace of internal debug points (_debug())This mask can also be set through sysfs, eg:echo 5 >/sys/modules/cachefiles/parameters/debug

The daemon is run as follows:/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]The flags are:-dIncrease the debugging level. This can be specified multiple times and is cumulative with itself.-sSend messages to stderr instead of syslog.-nDon’t daemonise and go into background.-f <configfile>Use an alternative configuration file rather than the default one.

brun, frunIf the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off.當快取中空閒的block和file 都高於該值時，不進行快取剔除bcull, fcullIf the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started.當快取中可以使用的block或者files 有一個低於該值時，進行快取剔除bstop, fstopIf the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again.當快取中可以使用的block或者files有一個低於該值時，除非快取剔除機制進行了快取剔除，否則不會再分配磁碟空間，

0 <= bstop < bcull < brun < 1000 <= fstop < fcull < frun < 100

 /INDEX    /INDEX     /INDEX                            /DATA FILES/=========/==========/=================================/================cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1rycache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22rycache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry

J1223/@23/+xy...z/+kl...m/Epqr

Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like ‘/’ and NUL characters, and so they may not be suitable for turning directly into a filename.To handle this, CacheFiles will use a suitably printable filename directly and “base-64” encode ones that aren’t directly suitable. The two versions of object filenames indicate the encoding:OBJECT TYPE	PRINTABLE	ENCODEDIndex	“I…”	“J…”Data	“D…”	“E…”Special	“S…”	“T…”Intermediate directories are always “@” or “+” as appropriate.Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them.Note that CacheFiles will erase from the cache any file it doesn’t recognise or any file of an incorrect type (such as a FIFO file or a device file).