s390/dasd: Add full_track_bias to control fulltrack write mode

Add a single per-device 'full_track_bias' sysfs attribute (0..100) that
gates the full-track write path. 0 disables it, 100 routes every aligned,
full-track write through dasd_eckd_build_cp_tpm_writefulltrack(). Values in
between are reserved for the adaptive heuristic added in the next patch.
For now any non-zero value simply enables full-track writes. Internally the
value is kept in the per-device 'ft_bias' field.

This will control the default IO path only.
In case we get an unformatted track error it will always be used to format
and write the track in one go.

The WRITE_FULL_TRACK command has an advantage on sparse formatted ESE
devices but it has an overall penalty for maximum throughput compared to
usual track based IO.

The attribute lives at /sys/bus/ccw/devices/<devid>/full_track_bias and
accepts 0..100. The default is DASD_FT_BIAS_DEFAULT; together with the
adaptive heuristic added in the next patch it uses full-track writes only
where they pay off, avoiding the ESE format penalty out of the box while
keeping the throughput cost off already-formatted volumes.

A 'full_track_bias' module parameter sets the initial value applied to
every device at online time; individual volumes can still be re-tuned
through their sysfs attribute afterwards.

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://patch.msgid.link/20260805111612.1285190-14-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions