On Sun, 28 Feb 2010 09:09:49 +0100 Luca Berra <bluca@xxxxxxxxxx> wrote: > On Thu, Feb 25, 2010 at 08:39:36AM +1100, Neil Brown wrote: > >On Wed, 24 Feb 2010 11:12:09 -0500 > >"Martin K. Petersen" <martin.petersen@xxxxxxxxxx> wrote: > > > >> So realistically both disk blocks are wrong and there's a window until > >> the new, correct block is written. That window will only cause problems > >> if there is a crash and we'll need to recover. My main concern here is > >> how big the discrepancy between the disks can get, and whether we'll end > >> up corrupting the filesystem during recovery because we could > >> potentially be matching metadata from one disk with journal entries from > >> another. > > > >After a crash, md will only read from one of the devices (the first) until a > >resync has completed. So there should be no room for more confusion than you > >would expect on a single device. > > After thinking more about this i could come up with another concern > about write ordering. > > example > app writes block A, B, C > md writes A on both disks > md writes B on disk1 > app writes B again (B') > md writes B' on disk2 > now md would write B' again on both disks, but the system crashes > (note, C is never written due to crash) > > Disk 1 contains A and B in the correct order, it is missing C and B' but we > dont care, app should be able to recover from a crash > > Disk 2 contains A and B', but they are wrongly ordered because C is > missing > > If in the above case A and C are data blocks and B contains a journal > related to A and C, booting from disk 2 could result in inconsistent > data. > > can the above really happen? > would using barriers remove the above concern? > am i missing something else? These is no inconsistency here that a filesystem would not equally expect from a single device. After the crash-while-writing B', it should expect to see either B or B', and it does, depending on which device is primary. Nothing to see here. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
