cache的基本原理（cachefilesd缓存项目介绍）

1. More than one cache can be used at once. Caches can be selected explicitly by use of tags. 一次可以使用多个Cache，不同的Cache使用不同的tag区分 2. Caches can be added / removed at any time. Cache可以在任何时间被移除或者添加 3. The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (2)). 网络文件系统提供接口，能够允许其他方删除文件cache相关的能力 4. The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious. 网络文件系统尽量不要返回错误 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表示文件系统的目录、文件、其他对象 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(也可以限制文件数量). dir <path> 存放缓存的根目录 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> 用于开启日志 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: -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. -s Send messages to stderr instead of syslog. -n Don’t daemonise and go into background. -f <configfile> Use an alternative configuration file rather than the default one.

brun, frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off.当缓存中空闲的block和file 都高于该值时，不进行缓存剔除 bcull, fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started.当缓存中可以使用的block或者files 有一个低于该值时，进行缓存剔除 bstop, fstop If 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.当缓存中可以使用的block或者files有一个低于该值时，除非缓存剔除机制进行了缓存剔除，否则不会再分配磁盘空间，

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

/INDEX /INDEX /INDEX /DATA FILES /=========/==========/=================================/================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@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 ENCODED Index “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).