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diff --git a/doc/cephfs/cephfs-io-path.rst b/doc/cephfs/cephfs-io-path.rst new file mode 100644 index 000000000..8c7810ba0 --- /dev/null +++ b/doc/cephfs/cephfs-io-path.rst @@ -0,0 +1,50 @@ +========================= + Ceph File System IO Path +========================= + +All file data in CephFS is stored as RADOS objects. CephFS clients can directly +access RADOS to operate on file data. MDS only handles metadata operations. + +To read/write a CephFS file, client needs to have 'file read/write' capabilities +for corresponding inode. If client does not have required capabilities, it sends +a 'cap message' to MDS, telling MDS what it wants. MDS will issue capabilities +to client when it is possible. Once client has 'file read/write' capabilities, +it can directly access RADOS to read/write file data. File data are stored as +RADOS objects in the form of <inode number>.<object index>. See 'Data Striping' +section of `Architecture`_ for more information. If the file is only opened by +one client, MDS also issues 'file cache/buffer' capabilities to the only client. +The 'file cache' capability means that file read can be satisfied by client +cache. The 'file buffer' capability means that file write can be buffered in +client cache. + + +.. ditaa:: + + +---------------------+ + | Application | + +---------------------+ + | + V + +---------------------+ Data IOs +--------------------+ + | CephFS Library | ---------> | LibRados | + +---------------------+ +--------------------+ + | | + | Metadata Operations | Objects Read/Write + V V + +---------------------+ +--------------------+ + | MDSs | -=-------> | OSDs | + +---------------------+ +--------------------+ + + + +----------------------+ +---------------------+ + | CephFS kernel client | Data IOs | Ceph kernel library | + | (ceph.ko) | --------> | (libceph.ko) | + +----------------------+ +---------------------+ + | | + | Metadata Operations | Objects Read/Write + v v + +---------------------+ +--------------------+ + | MDSs | -=-------> | OSDs | + +---------------------+ +--------------------+ + +.. _Architecture: ../architecture |