write operations in device controllers

Posted Jun 30, 2019 16:25 UTC (Sun) by Jonno (subscriber, #49613)
In reply to: write operations in device controllers by epa
Parent article: The io.weight I/O-bandwidth controller

> If you just mean that you leave some of the capacity unused, wouldn’t a large file full of zero bytes do that as effectively?

The idea is to keep a large consecutive range of LBAs to which you have _never_* written _anything_. Writing zeroes to the range you want to reserve is counterproductive, as it forces the firmware to store those zeroes, tying up a large amount of flash it could otherwise have used as an additional spare space (unless the drive firmware inspects every write to check if the whole write is just zeroes, and treat it as a TRIM if it is, but I doubt any consumer grade SSD has gone to the trouble to optimize for that case).

* On modern SSDs that support TRIM, substitute "since last consecutive TRIM covering the whole range"...