On Sun, Jun 28, 2009 at 10:34:17AM +1000, Neil Brown wrote: > On Tuesday May 26, albertito@xxxxxxxxxxxxxx wrote: > > On Tue, May 26, 2009 at 12:33:01PM +0200, Goswin von Brederlow wrote: > > > > This scheme assumes writes to a single sector are atomic in the presence of > > > > normal crashes, which I'm not sure if it's something sane to assume in > > > > practise. If it's not, then the scheme can be modified to cope with that. > > > > > > What happens if you have multiple writes to the same sector? (assuming > > > you ment "before" above) > > > > > > - user writes to sector > > > - queue up write for M1 and data1 > > > - M1 writes > > > - user writes to sector > > > - queue up writes for M2 and data2 > > > - data1 is thrown away as data2 overwrites it > > > - M2 writes > > > - system crashes > > > > > > Now both M1 and M2 have a different checksum than the old data left on > > > disk. > > > > > > Can this happen? > > > > No, parallel writes that affect the same metadata sectors will not be allowed. > > At the moment there is a rough lock which does not allow simultaneous updates > > at all, I plan to make that more fine-grained in the future. > > Can I suggest a variation on the above which, I think, can cause a > problem. > > - user writes data-A' to sector-A (which currently contains data-A) > - queue up write for M1 and data-A' > - M1 is written correctly. > - power fails (before data-A' is written) > reboot > - read sector-A, find data-A which matches checksum on M2, so > success. > > So everything is working perfectly so far... > > - write sector-B (in same 62-sector range as sector-A). > - queue up write for M2 and data-B > - those writes complete > - read sector-A. find data-A, which doesn't match M1 (that has > data-A') and doesn't match M2 (which is mostly a copy of M1), > so the read fails. The thing is that M2 is not a copy of M1. When updating M2 for data-B, the procedure is not "copy M1, update sector-B's checksum, write" but "read M2, update sector-B's checksum, write". So as long as there are no writes to sector-A, M1 will have the incorrect checksum and M2 will have the correct one, regardless of writes to the other sectors. However, a troubling scenario based on yours could be: - M2 has the right checksum but is older, M1 has the wrong checksum but is newer. - user writes data-A'' to sector'A - queue up write for M2 (chosen because it is older) - M2 is written correctly - power fails before data-A'' is written At that point, data-A is written at sector-A, but both M1 and M2 have incorrect checksums for it. I'll try to come up with a better scheme that copes with this kind of scenarios and post an updated patch. Thanks a lot, Alberto -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
