Restore to stock recovery first. You mean the Nandroid backup I just did (boot, data, system, cache) before I flashed the SuperSu zip file correct? So I restore my Nandroid backup before applying OTA update, correct?
when you do
Code:
fastboot flash recovery twrp.img
you are writing the twrp image into the tablet's recovery partition
When you run twrp, by default it will have selected backup of the system, data, and boot partitions. If you then choose to also backup the recovery partition, it will just end up backing up the twrp image you just installed in the previous fastboot command. If you end up restoring this backup, you'll get twrp in your recovery partition rather than stock recovery (which is already gone since you overwrote it with the fastboot flash) You can recover the stock recovery by grabbing it from someone else as the stock recovery is generic for everyone.
Now, if instead of using fastboot flash, you instead use
as your first command after unlocking, and then you subsequently choose to backup system, data, boot, and recovery partitions, you will have a full backup of your stock install, including recovery. This is because you never wrote/flashed the twrp recovery onto the tablet. You are only temporarily loading twrp recovery from PC into tablet memory over USB, not permanently making it the recovery installed on your tablets recovery partition.
You can still run twrp recovery whenever you want and you will be able to install OTAs when they come out (as long as you don't modify/remove ANY existing OS files) If you modify some minor OS files there's probably still a 10% chance the OTA will work (as long as it doesn't contain an update to the file you modified), but if you modify/remove something major like removing Maps.apk and replacing with a newer one, then that will almost 100% cause the OTA to fail. Installing SuperSU in and of itself won't cause an OTA to fail, because you are just adding files to the system. You aren't modifying any files at all, so the files the OTA is trying to patch will still pass the SHA integrity check. I haven't looked at the OTAs on Nexus 7, but on past phones, the OTAs often would contain updates to recovery as well. If you had twrp installed as your recovery and the OTA contained a patch-style update to the recovery partition, then having twrp installed would cause the OTA to fail the integrity check. Now if the OTA update didn't update the recovery partition then it could probably pass, as long as twrp is honoring the protocol used to pass updates from Android OS to recovery. Android OS proper (the stuff you are running daily) is responsible for checking whether you need and downloading the updates into the cache partition. Then when you select to install the update, it reboots into recovery and points to recovery to look for an update.zip in the cache directory. As long was twrp (installed on the tablet's recovery partition) is playing by those rules it should theoretically be able to install OTAs which do not modify the recovery partition.
Now if you instead load twrp temporarily from PC into memory, then your tablet's recovery partition still contains the stock recovery. You will thus still look like a stock phone from the OTAs perspective and thus any OTA will work (as long as you haven't modified/removed any existing OS files) Adding Titanium Backup or other root apps normally just adds files, so that won't prevent you from getting the OTA (though the OTA will commonly reset permissions on su, which I believe SuperSU has tried to counter, but I haven't tested) You may or may not lose root after the OTA. By "lose" root, I am referring to the "capability" of becoming root, rather than the root files itself. OTAs won't delete the root files, so you will definitely still see SuperSU in your app folder, it just might have reset the permissions, so when you try to become root, it fails. However, as I mentioned, I believe SuperSU tries to restore its permissions after an OTA update.
The disadvantage of loading twrp temporarily from PC into memory is you will need a PC connected to your tablet to initiate twrp recovery. You'll need to do
Code:
adb reboot bootloader
fastboot boot twrp.img
each time you want to run twrp.
If you install twrp onto the tablet's recovery partition, you can start twrp by doing
Code:
Power Down the tablet (ie shutoff)
Press-and-hold Power+Vol-
Select Recovery (using Vol+/- and Power)
This is convenient if you are not by your computer and want to initiate a Backup or install some update.zip or new custom ROM.
On my every day phones, I usually write custom recovery onto the recovery partition because I make backups all the time and I run customized ROMs. On this tablet, since it is from Google, I plan an getting the OTAs as soon as they are available to see the latest and greatest, so I'm leaving stock recovery on the tablet. I still want root for the convenience of what it allows me to tinker with, but don't really need access to twrp all the time.
That's just my opinion though for my usage. If you run custom ROMs all the time and have no intention of running the OTAs (instead waiting for the devs to update their ROMs), then it is better to install twrp onto the tablet's recovery partition.
Hope that clears things up.