On 04/05/17 23:57, NeilBrown wrote: > On Thu, May 04 2017, David Brown wrote: > >> >> I have another couple of questions that might be relevant, but I am >> really not sure about the correct answers. >> >> First, if you have a stripe that you know is unused - it has not been >> written to since the array was created - could the raid layer safely >> return all zeros if an attempt was made to read the stripe? > > "know is unused" and "it has not been written to since the array was > created" are not necessarily the same thing. > > If I have some devices which used to have a RAID5 array but for which > the metadata got destroyed, I might carefully "create" a RAID5 over the > devices and then have access to my data. This has been done more than > once - it is not just theoretical. That is true, of course - anything like this would have to be optional (command line switches in mdadm, for example). There is also the opposite situation - when you /have/ had something written to the array, but now you know it is unused (due to a trim). Knowing the stripe is unused might make a later partial write a little faster, and it would certainly speed up a scrub or other consistency check since unused stripes can be skipped. > > But if you really "know" it is unused, then returning zeros should be fine. > >> >> Second, when syncing an unused stripe (such as during creation), rather >> than reading the old data and copying it or generating parities, could >> we simply write all zeros to all the blocks in the stripes? For many >> SSDs, this is very efficient. > > If you were happy to destroy whatever was there before (see above > recovery example for when you wouldn't), then it might be possible to > make this work. As above, this would have to be option-controlled. (I have had occasion to pull disks from one dead server to recover them on another machine - it's nerve-racking enough at the best of times, without fearing that you will zero out your remaining good disks!) > You would need to be careful not to write zeros over a region that the > filesystem has already used. Yes, but that should not be a difficult problem - the array is created before the filesystem. > That means you either disable all writes until the initialization > completes (waste of time), or you add complexity to track which strips > have been written and which haven't, and only initialise strips that have > not been written. This complexity would only be used once in the entire > life of the RAID. That might not be best use of resources. > I am not sure I see how this would be a problem. But it is something that would need to be considered carefully when looking at details of implementing these ideas (if anyone thinks they would be worth implementing). mvh., David -- 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
