Commitlog O_DSYNC is intended to make Raft and schema writes durable
in the face of power loss. To make O_DSYNC performant, we preallocate
the commitlog segments, so that the commitlog writes only change file
data and not file metadata (which would require the filesystem to commit
its own log).
However, in tests, this causes each ScyllaDB instance to write 384MB
of commitlog segments. This overloads the disks and slows everything
down.
Fix this by disabling O_DSYNC (and therefore preallocation) during
the tests. They can't survive power loss, and run with
--unsafe-bypass-fsync anyway.
Closes#12542
9029b8dead