On 2022-05-06 12:00, David Sterba wrote: >> The current approach for npo2 devices is to place the superblock mirror >> zones near 512GB and 4TB that is **aligned to the zone size**. > > I don't like that, the offsets have been chosen so the values are fixed > and also future proof in case the zone size increases significantly. The > natural alignment of the pow2 zones makes it fairly trivial. > > If I understand correctly what you suggest, it would mean that if zone > is eg. 5G and starts at 510G then the superblock should start at 510G, > right? And with another device that has 7G zone size the nearest > multiple is 511G. And so on. > > That makes it all less predictable, depending on the physical device > constraints that are affecting the logical data structures of the > filesystem. We tried to avoid that with pow2, the only thing that > depends on the device is that the range from the super block offsets is > always 2 zones. > > I really want to keep the offsets for all zoned devices the same and > adapt the code that's handling the writes. This is possible with the > non-pow2 too, the first write is set to the expected offset, leaving the > beginning of the zone unused. > I agree. Having a known place for superblocks is important for recovery tools. We were thinking along the lines of what you have suggested. I will add this support in the next revision. >> This >> is of no issue for normal operation as we keep track where the superblock >> mirror are placed but this can cause an issue with recovery tools for >> zoned devices as they expect mirror superblock to be in 512GB and 4TB. > > Yeah the tools need to be updated, btrfs-progs and suite of blk* in > util-linux. > >> Note that ATM, recovery tools such as `btrfs check` does not work for >> image dumps for zoned devices even for po2 zone sizes. > > I thought this worked, but if you find something that does not please > report that to Johannes or Naohiro. Ok. Thanks.
