[LUDOC-487] Document llite.*.fallocate_zero_blocks tunable

Related — Thu, 30 Nov 2023 21:58:38 +0000

In 2.14.0 the fallocate interface was added to allow efficient preallocation of space for OST objects. The preallocated space should return zeroes when read. A new tunable parameter "llite.*.fallocate_zero_blocks" was added to allow tuning the fallocate() behavior:

=0 (default) uses unwritten extents, like ext4.
=1 zeroes out allocated blocks immediately on the OSS
=-1 disables fallocate (return -EOPNOTSUPP)

The default mode=0 is the standard "allocate unwritten extents" behavior used by ext4. This is by far the fastest for space allocation, but requires the unwritten extents to be split and/or zeroed when they are overwritten.

The OST fallocate mode=1 can also be set to use "zeroed extents", which may be handled by "WRITE SAME", "TRIM zeroes data", or other low-level functionality in the underlying block device. This is somewhat slower at fallocate() time (especially for very large allocations), but still avoids sending any data over the network, avoids runtime overhead from managing the extents. There is not yet an FALLOC_FL_* flag to request this behavior from the client on a per-file basis.

If fallocate() is not working properly or is undesirable to have, it can also be disabled with mode=-1 at runtime on one or more clients, or persistently on all clients.

Whamcloud Community JIRA

[LUDOC-487] Document llite.*.fallocate_zero_blocks tunable