On 21/11/16 21:02, Phil Turmel wrote:
On 11/21/2016 03:22 PM, Anthony Youngman wrote:
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?
(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 ... ?)
I suppose it would be possible, but defeats the purpose of having data
offset == 0: making non-parity array contents directly readable outside
the array. Commonly used to raid boot partitions but still have grub
able to read them directly. Less of an issue today with grub2, I
suppose (but I don't use bootloaders anymore, so I'm not a good resource
for that).
Only works for mirrors ...
So basically, there's no point using 1.0 for any form of parity raid. In
which case, if you convert a mirror to parity, it would be nice to be
able to create said offset and move the superblock.
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?
V1.1 and v1.2 are identical except for the superblock offset (one 4k
block difference). v1.1 reshapes just like v1.2.
Any reason for that difference? How big is the superblock? Did v1.1 fill
most of the first 4K and just leave not much room with a default 4K data
offset initially?
Phil
Cheers,
Wol
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