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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The content on this page reflects the state of design of a Lustre feature at a particular point in time and may contain outdated information.
Purpose
Open by fid returns a file descriptor based on a FID. This is quite similar to the controversial open by inode calls found elsewhere.
Implementation Choices
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
| Description | Semantics |
|---|---|
| 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 |
