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50 lines
2.8 KiB
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50 lines
2.8 KiB
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Differences from POSIX
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CephFS aims to adhere to POSIX semantics wherever possible. For
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example, in contrast to many other common network file systems like
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NFS, CephFS maintains strong cache coherency across clients. The goal
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is for processes communicating via the file system to behave the same
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when they are on different hosts as when they are on the same host.
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However, there are a few places where CephFS diverges from strict
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POSIX semantics for various reasons:
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- If a client is writing to a file and fails, its writes are not
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necessarily atomic. That is, the client may call write(2) on a file
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opened with O_SYNC with an 8 MB buffer and then crash and the write
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may be only partially applied. (Almost all file systems, even local
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file systems, have this behavior.)
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- In shared simultaneous writer situations, a write that crosses
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object boundaries is not necessarily atomic. This means that you
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could have writer A write "aa|aa" and writer B write "bb|bb"
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simultaneously (where | is the object boundary), and end up with
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"aa|bb" rather than the proper "aa|aa" or "bb|bb".
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- POSIX includes the telldir(2) and seekdir(2) system calls that allow
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you to obtain the current directory offset and seek back to it.
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Because CephFS may refragment directories at any time, it is
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difficult to return a stable integer offset for a directory. As
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such, a seekdir to a non-zero offset may often work but is not
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guaranteed to do so. A seekdir to offset 0 will always work (and is
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equivalent to rewinddir(2)).
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- Sparse files propagate incorrectly to the stat(2) st_blocks field.
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Because CephFS does not explicitly track which parts of a file are
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allocated/written, the st_blocks field is always populated by the
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file size divided by the block size. This will cause tools like
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du(1) to overestimate consumed space. (The recursive size field,
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maintained by CephFS, also includes file "holes" in its count.)
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- When a file is mapped into memory via mmap(2) on multiple hosts,
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writes are not coherently propagated to other clients' caches. That
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is, if a page is cached on host A, and then updated on host B, host
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A's page is not coherently invalidated. (Shared writable mmap
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appears to be quite rare--we have yet to here any complaints about this
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behavior, and implementing cache coherency properly is complex.)
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- CephFS clients present a hidden ``.snap`` directory that is used to
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access, create, delete, and rename snapshots. Although the virtual
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directory is excluded from readdir(2), any process that tries to
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create a file or directory with the same name will get an error
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code. The name of this hidden directory can be changed at mount
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time with ``-o snapdirname=.somethingelse`` (Linux) or the config
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option ``client_snapdir`` (libcephfs, ceph-fuse).
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