On Monday January 19, davidsen@xxxxxxx wrote: > Neil Brown wrote: > > > > The difficulty is "How do you handle a crash?" > > If you crash and restart in the middle of a reshape, you need to know > > where all the data is. > > To follow your scheme, you would need to update the metadata for > > every block that is moved. > > > > > That would seem to be an issue doing it in a large task as well, if I > understand what you want to do. After the data have been written to the > spare space I believe you are going to write it back. If you update the > metadata before the write and something happens you have old metadata > but data in another format. Conversely, if you update the metadata > first, and something happens, you have the metadata describing a change > which didn't happen. So You would have to have a metadata reflecting the > data not being in the array, but actually somewhere else, or you would > have to keep the spare space inside the array, where room might not be > available. I "freeze" a section of the array so that any writes block, I copy that data to somewhere safe, and mark that copy as being a valid copy of the moved data. Then I copy it back in the new layout and update the array metadata to show that the change-over point between old-layout and new-layout has changed. Then I invalidate the copy. Then repeat. When restarting an array, mdadm checks the copy (which could be in a file on a different device) and if it is valid, it is copied on to the array and the metadata is updated. 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
