[BC] Digital Storage

[RichardBJohnson at comcast.net]

From: "Powell E. Way III" <w4opw at yahoo.com>

>One thing I forgot, is that the "solid state" type USB drives have a very limited read / write over capability compared to hard drives with platters. Using it for mission crititcal operations on a 24/7 basis is a disaster waiting to happen. BE VERY CAREFUL.

Maybe it is time the users learned how these things work! It is true that the "permanent" storage medium, "Flash RAM," can only take a few million recording cycles. However, in actual use as a file-system, this should not be a problem.

Because Flash RAM bits can only be reset to zero, the flash RAM is written in the following manner. First, a page to be written is erased. This takes a long time, perhaps ten milliseconds or so. This makes all bits high. Second, the data to be written is written one byte at a time after a special charge-pump cycle consisting of a number of alternate bits (0xaa, 0x55, etc.,) a key byte value, and an offset into the page to be written.

In the case where even a single byte on a page is to be changed, the page's contents are read into SRAM (static RAM); the byte is changed in SRAM and then written back in the complicated procedure described earlier.

Obviously, such a device is not suitable for random access. It could take a minute to update a directory entry! What is done is the random access is accomplished with SRAM. Enough charge remains in an ultra capacitor to complete a write cycle if the device is unplugged.

Every time a new page is accessed, any previous "dirty" pages are written to the device. This allows transparent read and write to the device, even though there may be an occasional write to a particular page.

The result is that even if you are writing a 10-megabyte file, the actual writes to the device are 10 MB / PageSize. The page size depends upon the device and may vary from 512 bytes to 2048 or even greater.

For all practical purposes, you will lose your USB Pen Drive long before you wear it out!

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/