I too started running into the out of storage space problem a few weeks ago and arrived at the large mbcache.db and mbcache.db-wal files. I SCP'd them off of the firetv to look at offline, and they are SQLite database files that seem to cache all of the app/media metadata information that is pulled down from amazon (name/description/links to images and background images/etc).
Some sample snippets from the database:
Code:
$strings -a mbcache.db | head -n1
SQLite format 3
$strings -a mbcache.db | egrep ^text | head -n10
textOFields of Battle
textMBest of Movies
textPRecently Added TV
textJTrending TV
textZOriginal & Exclusive Movies
textSRecommended Drama TV
textaRecommended Science Fiction Movies
textNElection Movies
textWTop Movies & TV on Prime
textWAmazon Original Comedies
Note: XDA wouldn't let me post links, but I wanted to demonstrate above that the database also calls images/background images via http for items in the launcher.
I don't think removing the permissions that make these files writable is necessarily the correct fix though, or at least if you still plan on using the stock Amazon launcher, which many of us do. When this file is not writable (chmod 0), several of the menu items (home/prime/tv/movies/etc) will not populate with anything and get stuck on a loading screen and remain empty. The device needs to be able to pull down metadata and write to the mbcache files for items to populate in these menus.
After I removed the files and rebooted the firetv, the new mbcache file created was about 5mb. I'll need to do some monitoring to see how often this file is written to and how quickly it grows, but I suspect what is happening is this file is append only and continues to grow as new items are added to the amazon library online and are cached locally on the firetv. If that is the case, it makes me wonder if it was an oversight in the firmware coding to not set a max file size and clean/purge old cached items. If it's a slow increase of ~5mb per day or so, that might explain why it took so long to finally exhaust all available free space on the firetv and only recently started becoming an apparently issue for us.
I'm currently running 51.1.6.3. I'm going to monitor/test for a few days on this firmware, and then I'm going to flash the latest pre-rooted stock 5.2.1.1 and see if there's any difference. If there is nothing in the firmware to limit the size that this file can grow to or purge old cached items out, then it might need to be a file that needs to be manually removed by users at some regular interval (1-12 months) or something.
For anyone not using the amazon launcher (firedtvlauncher or similar), then the chmod 0 solution will probably be fine. My comments are really more aimed at retaining stock amazon launcher functionality while addressing the file size issue.
This is just after some preliminary troubleshooting and will need to do some further monitoring/testing, but hope this info helps for anyone else running into the problem.
