Re: linux raid wiki - backup files

[NeilBrown <neilb@suse.com>]

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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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