On Wed, Jul 15, 2015 at 12:47:10AM -0400, Samuel Greenfeld wrote: > Would periodically erasing SD cards and eMMC storage help to extend > their useful life?
No, I don't think so. Nor would it hurt their life. It is just a few writes to metadata mapping tables kept in a different physical area of the device. (The tables relate virtual block numbers used by the host to physical pages used by the device.) > Or is this potentially dangerous for chips known to be used which > implement these instructions incorrectly? Doesn't seem likely, but good point; there may exist a set of devices that implement the command incorrectly. Although the same could be said for other corner cases, like writing a block in the last position of a page after the page has triggered the read disturb rewrite handler. Hopefully the test suites will have caught both scenarios. > A few years ago, XO firmware would erase the SD card prior to > running fs-update. But if I recall correctly this was disabled > because it caused certain SD cards to hang. If I recall correctly it was because certain SD cards would refuse the command; they didn't really hang, they just gave an error. While investigating we found that the erasure was only covering a tiny part of the device block numbers (our mistake), so it wasn't being effective anyway. We didn't want to increase the time taken to fs-update, so we removed it. We also switched from linear to sparse input files. > It might also be possible to enable the eMMC equivalent of TRIM in > XO software builds provided XO eMMC's don't accidentally discard the > wrong block. Yes. TRIM is like ERASE except the device is not obligated to return a block of zeros. Both become a write to the mapping table. Acting on the wrong block seems unlikely though. A secure ERASE is different; in addition to a mapping table write they also COPY the page less any unerased blocks and then do an erase page. I guess writing zero to all blocks might not erase any page; it might instead just write to the mapping table to map each block to a zero page. -- James Cameron http://quozl.linux.org.au/ _______________________________________________ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
