lightnvm: pblk: export write amplification counters to sysfs
In a SSD, write amplification, WA, is defined as the average number of page writes per user page write. Write amplification negatively affects write performance and decreases the lifetime of the disk, so it's a useful metric to add to sysfs. In plkb's case, the number of writes per user sector is the sum of: (1) number of user writes (2) number of sectors written by the garbage collector (3) number of sectors padded (i.e. due to syncs) This patch adds persistent counters for 1-3 and two sysfs attributes to export these along with WA calculated with five decimals: write_amp_mileage: the accumulated write amplification stats for the lifetime of the pblk instance write_amp_trip: resetable stats to facilitate delta measurements, values reset at creation and if 0 is written to the attribute. 64-bit counters are used as a 32 bit counter would wrap around already after about 17 TB worth of user data. It will take a long long time before the 64 bit sector counters wrap around. The counters are stored after the bad block bitmap in the first emeta sector of each written line. There is plenty of space in the first emeta sector, so we don't need to bump the major version of the line data format. Signed-off-by: NHans Holmberg <hans.holmberg@cnexlabs.com> Signed-off-by: NJavier González <javier@cnexlabs.com> Signed-off-by: NMatias Bjørling <mb@lightnvm.io> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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