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Architecture - Open by fid
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Note: The content on this page reflects the state of design of a Lustre feature at a particular point in time and may contain outdated information.
Open by fid returns a file descriptor based on a FID. This is quite similar to the controversial open by inode calls found elsewhere.
There several considerations for implementation:
- use an ioctl on the root directory of the lustre file system - advantage: simple implementation
- open the file /proc/fs/lustre/.../open_by_fid/FID - major advantage: this is a standard open call, so it can be used by unmodified utilities
- an easy to use, in kernel API is also required
Decomposition of the design
The architecture of open by fid decomposes into several pieces:
- a local mechanism to generate a file descriptor from a ficticious pathname in /proc and/or through an ioctl
- the rpc mechanisms with the MDS to get that file descriptor (based on existing re-open by fid code, used during recovery)
- a mechanism to set flags and default flags on fid file descriptors
- verification and implementation of the recovery of the file descriptor during an MDS failure
- verification and implementation of correct behavior when file descriptor calls are made
- IO using the file descriptor: this can be dependent on flags, e.g. IO may or may not trigger cache miss handling
- fstat, fchown, ftruncate etc.
Use cases to cover
|deployment||open by fid is done on Lustre client nodes|
|open a fid residing the mds||file handle for file io|
|open a fid residing on oss||file handle for object io|
|open /proc/fs/lustre/something/FID||returns synthetic file descriptor for the object|
|open /proc/fs/lustre/something/FID_FLAGS||returns synthetic file descriptor for the object and sets flags on the file descriptor, for example to avoid cache misses|
|open_by_fid()||ioctl with the same functionality as opening the /proc file.|
|authorization||servers allow open_by_fid() only from certain nodes|
|mount options||clients do not enable open by fid unless a mount option is passed, clients can set the default flags to be used when open by fid file descriptors are generated.|
|Failure modes||the open calls should be subject to replay / resend, including the restoration of flags on the server|