fs_mgr: overlayfs readme caveats
Itemized list of additional overlayfs caveats
Test: none
Bug: 109821005
Change-Id: I3550ddf6544749cdc68c803f0577002346890518
diff --git a/fs_mgr/README.overlayfs.md b/fs_mgr/README.overlayfs.md
index f89e598..d204bfd 100644
--- a/fs_mgr/README.overlayfs.md
+++ b/fs_mgr/README.overlayfs.md
@@ -117,3 +117,25 @@
be used to clear scratch storage to permit the flash.
Then reinstate the overrides and continue.
- File bugs or submit fixes for review.
+- There are other subtle caveats requiring complex logic to solve.
+ Have evaluated them as too complex or not worth the trouble, please
+ File a bug if a use case needs to be covered.
+ - The backing storage is treated fragile, if anything else has
+ issue with the space taken, the backing storage will be cleared
+ out and we reserve the right to not inform, if the layering
+ does not prevent any messaging.
+ - Space remaining threshold is hard coded. If 1% or more space
+ still remains, overlayfs will not be used, yet that amount of
+ space remaining is problematic.
+ - Flashing a partition via bootloader fastboot, as opposed to user
+ space fastbootd, is not detected, thus a partition may have
+ override content remaining. adb enable-verity to wipe.
+ - Space is limited, there is near unlimited space on userdata,
+ we have made an architectural decision to not utilize
+ /data/overlay/ at this time. Acquiring space to use for
+ backing remains an ongoing battle.
+ - First stage init, or ramdisk, can not be overriden.
+ - Backing storage will be discarded or ignored on errors, leading
+ to confusion. When debugging using **adb remount** it is
+ currently advised to confirm update is present after a reboot
+ to develop confidence.