On Tue, 31 Aug 2010 13:00:45 +0200 Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > On Tue, 31 Aug 2010, Neil Brown wrote: > > So: is this a problem? It may seem a bit confusing to someone who doesn't > > understand what is happening, but we define that as not being a problem (to > > avoid confusion: don't change U or L). > > The important questions are: Can it cause corruption, and can it cause a > > deadlock? > > No, I don't think this design will do that. So it might be enough > just to document that online modification of upper or lower > filesystems results in undefined behavior. > > But to prevent accidental damage, it's prudent (at least by default) > to enforce the no-modification policy. > > Why do you think this feature of allowing modification is important? > Lets take some typical use cases: > > - live cd: lower layer is hard r/o, upper layer makes no sense to > modify online > > - thin client: lower layer is static except upgrades, which need > special tools to support and is done offline, upper layer makes no > sense to modify online > > Do you have some cases in mind where it makes at least a little sense > to allow online modification of the underlying filesystems? No, I don't have a particular use case in mind that would take advantage of the layers being directly modifiable. But I know that sys-admins can be very ingenious and may well come up with something clever. My point is more that I don't think that is it *possible* to prevent changes to the underlying filesystem (NFS being the prime example) so if there are easy steps we can take to make the behaviour of overlayfs more predictable in those cases, we should. Further I think that insisting that the underlying filesystems remain unchangeable is overly restrictive. If I were not allowed to perform an overlay mount on a read/write lower filesystem, that would make it significantly harder to explore the possibilities of overlayfs and experiment with it. I certainly don't think we should put a lot of work or a lot of code into making it work "perfectly" in any sense at all. But if there are *easy* things to do that allow us to avoid some weird behaviours, then I think it is worth that effort to do it. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
