On Tue, Nov 22 2016, Anthony Youngman wrote: > On 21/11/16 14:07, Phil Turmel wrote: >> On 11/20/2016 09:48 AM, Wols Lists wrote: >>> On 20/11/16 00:27, Phil Turmel wrote: >>>> Yes. But the new stripes lay on top of the old stripes, unless you move >>>> the data offset. Which is why a backup file holds the old stripe just >>>> in case. If you can move the offset, you use the lower offset for the >>>> lower addresses in the array, and the higher offset for the higher >>>> addresses, on either side of the reshape position. >>> >>> Okay, understood. So v0.9 and v1.0 always need a backup for a reshape. > > Having looked at the man page, this now seems obvious - the superblock > is at the end, so the data offset is 0. But for a 1.0 array, could we > create a data offset? Yes. But usually the purpose for using 1.0 is to have data_offset == 0, so you might not want to. > > (So, if we created a data offset, we could then move the superblock and > convert a 1.0 to 1.1 or 1.2? Okay, it can't do it now, but it looks to > me like it shouldn't be that hard ... ?) It would be quite easy to extend "--update=metadata" to change the version once the data_offset had been changed. >>> >>> But if we have a data offset with v1.2, a reshape will use that space if >>> it can rather than needing a backup file? >> > I'm guessing that 1.0 and 1.1 defaulted to no data offset to speak of? > And if we (can) create a decent data offset, we can then use that in > exactly the same way as with v1.2? 1.0 defaults to no data_offset. 1.1 uses the safe choice function as 1.2. NeilBrown
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