On Thu, Jul 14, 2011 at 12:50 PM, John Stoffel <j...@stoffel.org> wrote: >>>>>> "Alasdair" == Alasdair G Kergon <a...@redhat.com> writes: > > Alasdair> On Thu, Jul 14, 2011 at 04:38:36PM +1000, Neil Brown wrote: >>> It might make sense for a device to be able to report what the maximum >>> 'N' supported is... that might make stacked raid easier to manage... > > Alasdair> I'll just say that any solution ought to be stackable. > > I've been mulling this over too and wondering how you'd handle this, > because upper layers really can't peak down into lower layers easily. > As far as I understand things. > > So if you have btrfs -> luks -> raid1 -> raid6 -> nbd -> remote disks > > How does btrfs handle errors (or does it even see them!) from the > raid6 level when a single nbd device goes away? Or taking the > original example, when btrfs notices a checksum isn't correct, how > would it push down multiple levels to try and find the correct data? > > Alasdair> This means understanding both that the number of data access > Alasdair> routes may vary as you move through the stack, and that this > Alasdair> number may depend on the offset within the device. > > It almost seems to me that the retry needs to be done at each level on > it's own, without pushing down or up the stack. But this doesn't > handle the wrong file checksum issue. > > Hmm... maybe instead of just one number, we need another to count the > levels down you go (or just split 16bit integer in half, bottom half > being count of tries, the upper half being levels down to try that > read?) > > It seems to defeat the purpose of layers if you can go down and find > out how many layers there are underneath you.... > > John
A random thought: What if we allow the number to wrap at each level, and, each time it wraps, increment the number passed to the next lower level. A zero would propagate down, letting each level do what it wants: luks: 0 raid1: 0 raid6: 0 nbd: 0 And higher numbers would indicate the method at each level: For a 1: luks: 1 raid1: 1 raid6: 1 nbd: 1 For a 3: luks: 1 (only one possibility, passes three down) raid1: 1 (two possibilities, so we wrap back to one and pass two down, since we wrapped once) raid6: 2 (not wrapped) nbd: 1 When the bottom-most level gets an N that it can't handle, it would return EINVAL, which would be propagated up the stack. This would allow the same algorithm of incrementing N until we receive good data or EINVAL, and would exhaust all ways of reading the data at all levels. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
