On Tuesday July 7, gnomeza@xxxxxxxxx wrote: > Hi > > __The Problem__ > > I'm attempting to recover a RAID5 (6 disks) with zeroed superblocks. > > mdadm -C --assume-clean -f -e 1.2 -l 5 -p ls -c 128 -n6 > > But I'm unable to get the data offsets of some of the disks in the > array to match what I had before: > Diff of mdadm --examine /dev/sdb3 (which became /dev/sde3 after reboot) > > > - Avail Dev Size : 870353369 (415.02 GiB 445.62 GB) > + Avail Dev Size : 870353233 (415.02 GiB 445.62 GB) > Array Size : 4351765760 (2075.08 GiB 2228.10 GB) > Used Dev Size : 870353152 (415.02 GiB 445.62 GB) > - Data Offset : 136 sectors > + Data Offset : 272 sectors That is unfortunate, isn't it... > > Data offset for this disk was originally 136, but on recreation, mdadm > chooses 272... It must have been created with mdadm-2.5.6 or earlier > > The array started off as RAID5 (5 disks plus a hot spare). > One disk (array slot 4) failed about a year ago and was replaced. > The array was grown to RAID5 (6 disks) shortly after. > > __The Question__ > > So, is there a way to force a particular data offset? (short of > recompiling mdadm?) No. > > __Some things I've tried__ > > It seems to be feasible to *manually* tweak data_offset with a hexeditor. > To test things I've created some sparse loopback devices of the > correct size (dd seek) with just the first 1MB or so of data in each > device. > After updating data_offset and the checksum of the first disk I > assembled the array and cryptsetup found its header. (Yay!) You would need to update the data_offset on each device. > > But does anyone know if other parts of the superblock are dependent on > the data_offset value? > Am I going to bork things horribly by fiddling with the data_offset? No, that should be safe (as long as you do it on every device). > > What data is present between the end of the superblock and the > beginning of data? That space is used for an internal bitmap. If you don't have a bitmap configured, then that space is not used. NeilBrown -- 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
