Just imagine that no any servers at all
, forget about them, the task is fully offline here.
Yes, fast measurement, and full logging. And later the user will touch the device and download the whole latest storage to view on PC as charts.
No care about the oldest records that were deleted when storage is full, the important just recent, but all and fastly measured.
It seems to me this is a normal situation for, say, video surveillance: all must be saved (i mean all video frames, or all measured sensor records...), but when it's needed - we can see any moment among the latest X hours, days... detailed. Earlier were deleted already, it's normal.
It looks like the file system idea is OK: store 100...1000 records and save them into "file0001.csv" file......
The free space is
controllable so the oldest file "file0001.csv" can be deleted when free space is near some AlmostZERO_value.