f734301d22
Implement semi-compatible functionality for mode=0 (preallocation) and mode=FALLOC_FL_KEEP_SIZE (preallocation beyond EOF) for ZPL. Since ZFS does COW and snapshots, preallocating blocks for a file cannot guarantee that writes to the file will not run out of space. Even if the first overwrite was guaranteed, it would not handle any later overwrite of blocks due to COW, so strict compliance is futile. Instead, make a best-effort check that at least enough free space is currently available in the pool (with a bit of margin), then create a sparse file of the requested size and continue on with life. This does not handle all cases (e.g. several fallocate() calls before writing into the files when the filesystem is nearly full), which would require a more complex mechanism to be implemented, probably based on a modified version of dmu_prealloc(), but is usable as-is. A new module option zfs_fallocate_reserve_percent is used to control the reserve margin for any single fallocate call. By default, this is 110% of the requested preallocation size, so an additional 10% of available space is reserved for overhead to allow the application a good chance of finishing the write when the fallocate() succeeds. If the heuristics of this basic fallocate implementation are not desirable, the old non-functional behavior of returning EOPNOTSUPP for calls can be restored by setting zfs_fallocate_reserve_percent=0. The parameter of zfs_statvfs() is changed to take an inode instead of a dentry, since no dentry is available in zfs_fallocate_common(). A few tests from @behlendorf cover basic fallocate functionality. Reviewed-by: Richard Laager <rlaager@wiktel.com> Reviewed-by: Arshad Hussain <arshad.super@gmail.com> Reviewed-by: Matthew Ahrens <mahrens@delphix.com> Co-authored-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Andreas Dilger <adilger@dilger.ca> Issue #326 Closes #10408 |
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