OK, here is a summary of some information about flash which hopefully someone else will find useful.
Wear leveling:
Wear leveling means attempting to wear out a flash chip at the same rate. Each write degrades part of the flash, but if the writes are distributed the flash chip will last longer overall.
The flash inside the internet tablet is not set up the same way as the SD card. At least some SD cards have a built-in computer which does wear-leveling and handles erasing flash. I haven't figured out if all the SD cards do wear leveling.
The flash inside the tablet itself does not have this helper computer so the tablet must handle wear-leveling with jffs2.
Types of flash:
There 2 types of flash (NOR and NAND) which have different capabilities for density and speed, but most if not all consumer devices use NAND. There is also a factor which is either SLC (single level cell) or MLC (multi-level cell.) SLC tends to be faster than MLC, but MLC is higher density. Also, each block in a card using SLC can be erased more times and still be reliable - 100,000 times vs. 10,000 for MLC.
Ext3 (not flash but related for us)
Ext3 has three different modes of operation. All three modes
will write information about a file, such as the file size, to a disk twice. The first time the information about the file is written to a journal, which the file system writes to keep track of what it intends to do. The second time is the "real" write. This is not much data overall.
Only one of the ext3 modes writes the contents of the file twice.
Expected life time:
A site gave a formula for calculating the lifetime of an SD card which does optimal wear leveling. It is
Maximum number of erase Cycles * (Size of Card - Size of Static Files - size of 1 block)/((bigger of file size or block size)*write frequency)
This says that how fast your card wears out depends not only on how often you write, but how much free space you have compared to how much data you are writing.
So if I have a 4 GB card with 2 GB of MP3s, and I rewrite maybe 100MB total per day, and it is a MLC card, it will last about 561 years. If I rewrite 1GB per day that is about 54 years.
I am guessing that most memory cards don't do optimal wear leveling but still this is not bad.
Wear leveling:
Wear leveling means attempting to wear out a flash chip at the same rate. Each write degrades part of the flash, but if the writes are distributed the flash chip will last longer overall.
The flash inside the internet tablet is not set up the same way as the SD card. At least some SD cards have a built-in computer which does wear-leveling and handles erasing flash. I haven't figured out if all the SD cards do wear leveling.
The flash inside the tablet itself does not have this helper computer so the tablet must handle wear-leveling with jffs2.
Types of flash:
There 2 types of flash (NOR and NAND) which have different capabilities for density and speed, but most if not all consumer devices use NAND. There is also a factor which is either SLC (single level cell) or MLC (multi-level cell.) SLC tends to be faster than MLC, but MLC is higher density. Also, each block in a card using SLC can be erased more times and still be reliable - 100,000 times vs. 10,000 for MLC.
Ext3 (not flash but related for us)
Ext3 has three different modes of operation. All three modes
will write information about a file, such as the file size, to a disk twice. The first time the information about the file is written to a journal, which the file system writes to keep track of what it intends to do. The second time is the "real" write. This is not much data overall.
Only one of the ext3 modes writes the contents of the file twice.
Expected life time:
A site gave a formula for calculating the lifetime of an SD card which does optimal wear leveling. It is
Maximum number of erase Cycles * (Size of Card - Size of Static Files - size of 1 block)/((bigger of file size or block size)*write frequency)
This says that how fast your card wears out depends not only on how often you write, but how much free space you have compared to how much data you are writing.
So if I have a 4 GB card with 2 GB of MP3s, and I rewrite maybe 100MB total per day, and it is a MLC card, it will last about 561 years. If I rewrite 1GB per day that is about 54 years.
I am guessing that most memory cards don't do optimal wear leveling but still this is not bad.
References: http://www.windowsfordevices.com/fil...whitepaper.pdf
http://www.edn.com/index.asp?layout=...leid=CA6319917
http://www.gentoo.org/doc/en/articles/l-afig-p8.xml
http://www.stec-inc.com/downloads/AN...CALCULATOR.pdf