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