Changes between Version 3 and Version 4 of embedded/flash

Timestamp:

Dec 10, 2007 1:17:13 PM (5 years ago)

Author:

dkg

Comment:

--

embedded/flash

@@ -1 +1 @@
 = using flash memory as a filesystem =
 
+Flash memory is different than standard magnetic non-volatile storage because the number of potential write cycles is relatively small.  On the order of 10000 writes for the life of a single bit (need sources).
+
+So in general, the guiding principle seems to be:
+
+ * avoid writes
+
+In detail, this means:
+
+ * you probably don't want to swap on flash-backed devices.
+ * you probably want the `noatime` mount option for any filesystems on flash-backed devices.
+ * you might prefer a non-journaled filesystem over a journaled filesystem.
+
 It sounds like Compact Flash has a translation layer that makes it different from writing directly to flash.  That is, the CF pseudo-IDE interface provides "wear levelling" capability directly in firmware.  It's not clear exactly how (or where?) this is implemented, or if all CF devices use the same wear-levelling scheme.
+
+If it's done in firmware like this, that makes it difficult to upgrade the system if a more sophisticated protocol comes along.
+
+== speed ==
+
+Much flash-backed storage is slow: it might have a decent burst speed, but for sustained read or write transfer, it degrades significantly.  How can we measure this?  What can we do to avoid this?  What kinds of flash-backed storage is faster than others?
 
 == links ==