Okay, after all that I've ended up not mentioning backup files at all. I
get the feeling that in pretty much all modern scenarios they are not
required. If we have v1.1 or v1.2 there should be enough spare metadata
space, and a modern system shouldn't be using v1.0. It's only of
interest if you are doing a bios boot off a mirror with no initramfs.
Not a likely option for a system being set up today. And from the man
page, mdadm leaves 128K at the end of each device - would a v1.0 array
have that? Would that be used for backup?
I probably ought to work out how I can get it to fit neatly in my mdadm
page, especially if it's only needed for older setups.
Can you take a look at the page and make sure it's not got anything
obviously wrong ...
https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm
And then I'll try and sort out a page on finding lost arrays - we've had
a few on the list recently ...
Cheers,
Wol
On 21/11/16 22:45, NeilBrown wrote:
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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