On 6 Feb 2016 04:18 +0100, from sven@xxxxxxxxxxxxxxxxxxxxx (Sven Eschenberg): > (A secondary header implies that > all changes on both headers need to be atomic and in sync. While > this is doable, LVM clearly shows, that it is not trivial, otherwise > it would certainly be available as feature by now). I'm not so sure it does imply that. It does certainly imply the need to know that a, and which one out of the lot, header is most up to date, but that does not necessarily require writes to both to be done atomically and in sync. (In fact, truly atomic, in-sync writes to multiple distinct locations seems a physical impossibility at least in the case of a single spinning disk, since the write head can only be in one location at any one time.) This is where the "update counter" and a checksum that I mentioned earlier comes in. An example of how to actually do this might be to first discard (or perhaps rather, remove from consideration) any header which doesn't match its checksum (for integrity purposes), then use the one with the highest update counter value (taking care to allow for wraparound) as a starting point for the operation at hand, then rewrite any previously discarded headers (ideally writing the checksum last, such that the header remains considered invalid until it has been fully rewritten). Or maybe even better, rewrite a previously considered invalid header (if any) first; that should ensure that as long as the storage itself works properly, if any header is ever valid at the beginning of an operation, there exists at all times at least one header which is valid. By placing the headers far apart from each other, this forces at least spinning disks to seek, which naturally introduces a sequence point into the write process; even if the two write requests were to be put onto the I/O bus at the same instant, one write must complete before the other can physically begin. (Finally, a good use for the seek delay in rotational storage!) This should work equally well for any number of header copies. -- Michael Kjörling • https://michael.kjorling.se • michael@xxxxxxxxxxx “People who think they know everything really annoy those of us who know we don’t.” (Bjarne Stroustrup) _______________________________________________ dm-crypt mailing list dm-crypt@xxxxxxxx http://www.saout.de/mailman/listinfo/dm-crypt
