oli writes: > take the liberty...
So I think there are a couple of interesting questions about how well you can clear flash storage by simple overwriting of free space. Remember that you have several layers in between your write operation and the actual flash blocks. Wei et al. say from experiments that overwriting free space is _not_ very effective. https://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf One issue I wonder about is whether a regular user program can succeed in filling the whole flash device. On Linux filesystems in the ext2 series, there is a notion of "blocks reserved for the superuser". E.g., from tune2fs(8): Set the percentage of the filesystem which may only be allocated by privileged processes. Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesys‐ tem fragmentation, and to allow system daemons, such as sys‐ logd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. Nor‐ mally, the default percentage of reserved blocks is 5%. Some Android systems may use these filesystems on their flash storage; I don't know if the same concept exists in other filesystems. (Due to wear leveling, I guess you would have a different 5% of the underlying blocks that you fail to overwrite each time.) The other is whether the controller actually keeps some blocks in reserve relative to those that it reports the existence of to software. My understanding is that for magnetic storage, there are more blocks on the physical disk than are reported to the ATA layer, and the controller uses the extra blocks for transparent remapping in case of physical errors, and maybe for other purposes. If I understand Wei et al. correctly, they found this issue was even _more_ pronounced on flash devices and is a major reason that overwriting free space is not so effective. They say that "[t]he SSDs we tested contain between 6 and 25% more physical flash storage than they advertise as their logical capacity". -- Seth Schoen <[email protected]> Senior Staff Technologist https://www.eff.org/ Electronic Frontier Foundation https://www.eff.org/join 454 Shotwell Street, San Francisco, CA 94110 +1 415 436 9333 x107 _______________________________________________ liberationtech mailing list [email protected] Should you need to change your subscription options, please go to: https://mailman.stanford.edu/mailman/listinfo/liberationtech If you would like to receive a daily digest, click "yes" (once you click above) next to "would you like to receive list mail batched in a daily digest?" You will need the user name and password you receive from the list moderator in monthly reminders. You may ask for a reminder here: https://mailman.stanford.edu/mailman/listinfo/liberationtech Should you need immediate assistance, please contact the list moderator. Please don't forget to follow us on http://twitter.com/#!/Liberationtech
