[Nexus 6P Q&A, Help & Troubleshooting] vacaloca:
With the issue of TWRP 3.0.2-1 causing bootloops from people going back to MM from N because that version had a bug in the EFS restore functionality, I mistakenly formatted the /persist partition of my Nexus 6P attempting to find a fix last weekend.

I noticed no ill effects at first, and then I realized that some sensor readings (for example the tilt reading on GPS Status & Toolbox app) were inconsistent. After looking at the /persist partition on different Nexus 6P, I was able to identify most of the partition contents with a bit of searching, and what is affected.

/persist/audio/speaker_calibdata.bin - text file, holds values for calibrating speaker (e.g. L:6.778258 R:6.556932 ), not sure of the scale of values
/persist/data/st_offset and /persist/data/st_xtalk - laser autofocus calibration data (according to Nexus 6P Android [URL="https://android.googlesource.com/device/huawei/angler/+/android-6.0.1_r62/init.angler.rc"]source code[/URL])
/persist/data/sfs - some sort of DRM files
/persist/data/tz - related to a qualcomm trustzone increment only counter
/persist/usf - related to digital pen calibration (except for /persist/usf/mixer)
/persist/usf/mixer - related to sound device specific mixer settings
/persist/.bt_nv.bin - contains what looks to be a MAC address if opened in a hex editor, however it is not the address of the BT MAC of the phone
/persist/sensorcal.json - contains data for Nexus 6P sensors (in this case acceleration, barometer, gyro, and proximity)

A lot of these files don't seem like they are even used, except for /persist/sensorcal.json

Using the post here, I was able to figure out the right syntax to regenerate this file:

(Note, the following requires SELinux to be set as permissive, as well as root access)

sensortool.angler -c all:1013

where 1013 in the above command is the true barometer reading in mb (mmHg) from the known calibrated Nexus 6P. Also to note, if the calibration data stored on the device significantly deviates from the current regenerated values, the calibration of some sensors might fail. In this case, deleting the /persist/sensorcal.json file if it exists, rebooting, and re-issuing the command should fix that issue, and the sensors should all calibrate successfully. Sample output:

root@angler:/vendor/bin # sensortool.angler -c all:1013
resetting target.
sensorhub said: 'Hello, world!'
sensorhub said: 'Found Bosch BMP280 Pressure/Temperature Sensor'
sensorhub said: 'Found AMS TMD27723 Proximity/ALS Sensor'
sensorhub said: 'Found Bosch BMI160 Accel/Gyro Sensor'

sensorhub said: 'init: 0 0 0, 0 0 0, 0.00, 0'

sensorhub said: 'batch 18 flags:0, sampling_rate_Hz:1.00, max_report_latency_us:0'

sensorhub said: 'activate 18 enable:1'
onMagAccuracyUpdate 0
onMagBiasUpdate 0.00, 0.00, 0.00
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'calibrate 4'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'calibrate 1'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'calibrate 2'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'calibrate 6'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'sensor 4 calibration SUCCESS: -1.96'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'Found Bosch BMM150 Mag Sensor'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'sensor 1 calibration SUCCESS: 16 / -1 / -11'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'sensor 2 calibration SUCCESS: 7 / -2 / -1'
18: 283042619 us, 0.00 0.00 0.00

sensorhub said: 'sensor 6 calibration SUCCESS: 14'
18: 283042619 us, 0.00 0.00 0.00

writing json: {
"accel": [
16,
-1,
-11
],
"barometer": -1.960000,
"gyro": [
7,
-2,
-1
],
"proximity": 14
}

Figured I'd update this in case anyone else comes across this issue. Lastly, since there are no personally identifiable contents in this partition, I have perfomed a backup that is flashable from TWRP on a Nexus 6P. After copying persist.img to /tmp on the phone's filesystem, the following command will restore it:

dd if=/tmp/persist.img of=/dev/block/platform/soc.0/f9824900.sdhci/by-name/persist

Backup of Nexus 6P /persist partition (ext4 image) is here.