Add WALSegmentSize as an option, and the corresponding flag "storage.tsdb.wal-segment-size" to tune the max size of wal segment files.
The addressed base problem is to reduce the disk space used by wal segment files : on a raspberry pi, for instance, we often want to reduce write load of the sd card, then, the wal directory is mounted on a memory (space limited) partition.
the default value of the segment max file size, pushed the size of directory to 128 MB for each segment , which is too much ram consumption on a rasp.
the initial discussion is at https://github.com/prometheus/tsdb/pull/450
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