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s390/dasd: Derive adaptive ESE fulltrack heuristic from ft_bias

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Turn the middle of the ft_bias range (1..99) into an adaptive heuristic
that switches between fulltrack write (ft1) and plain write ft0 depending
on how sparse the device still is.

A sparse device benefits from fulltrack writes (it avoids the format/retry
cycle); once enough tracks are formatted the per-write overhead of
ft1 outweighs that. An state machine  measures the NRF rate in short ft0
probe windows and flips back to ft1 when it is high
(FT1_ACTIVE -> PROBING -> FT0_STABLE, with a backing-off reprobe interval).

The four parameters are derived from ft_bias by linear interpolation,
anchored so ft_bias == 50 derives the following values:

ese_heu_start_interval - 2000 - IOs in ft1, before first ft0-Probe starts
ese_heu_probe_window - 100 - IOs in probe window
ese_heu_nrf_high - 10 ‰ (= 1 %) - TRACK_FORMAT rate that leads to ft1
ese_heu_max_interval - 500000 - Backoff-Cap: max. IOs between two probes

Higher is more eager to use ft1, and 0/100 skips the heuristic.
The NRF counter is bumped in dasd_eckd_ese_format() for both the classic
NRF sense and the HPF INV_TRACK_FORMAT equivalent.
The state machine resets to ft1 on check_characteristics, full format,
and release-space.

A read-only ese_heuristic_state sysfs attribute exposes the current mode.

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://patch.msgid.link/20260805111612.1285190-15-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Stefan Haberland 2026-08-05 13:16:07 +02:00 • committed by Jens Axboe
commit 4e304b2e56
3 changed files with 223 additions and 6 deletions
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