Currently, I have an 4GB sdcard on which I have an journaling FS partition (EXT3 and EXT4). I am testing the journaling recovery aspect of these filesystems to fix any corruption on an sd.
I have an SDCARD on a piece of hardware that simply boots linux then runs a copy.sh I wrote. I run a script that powers the machine for 150 seconds then hard shutdowns the machine for 30. This process is repeated for an extended period of time. I am running a script that copies a directory recursively back and forth on the journaling FS, deleting the directory from which it read from after it finishes. I keep track of how many times the directory was copied per boot.
I noticed something interesting in my results. At first, the directory may be copied successfully 20 times back and forth, but after hours of running, it only copies once or twice.
I was wondering why that was?
This trend is consistent with both EXT3 and EXT4. I've researched online for answers, but haven't found an answer for why the number of writes would decrease over time.
Does this explanation of how sdcards work help? http://www.anandtech.com/show/2738/8 Read that page and the couple following. This explains how deletes and overwrites are handled within the sd memory chips themselves, and implications for systems that dont implement the TRIM command.