Re: RFD: question/probe regarding adding dump-levels A-Z after 0-9 for xfs{dump,restore}

[Dave Chinner <david@fromorbit.com>]

On Fri, Dec 07, 2018 at 05:09:17AM -0800, L A Walsh wrote:
> How difficult would it be to allow 26 more dump levels
> in xfs dump/restore?  Theoretically anyone using 0-9
> would see no change.  Would there be some "problem" or
> "gotcha" in adding in what would _seem_ to be a simple
> change?

Everything _seems_ simple with xfsdump. e.g. the inventory
session headers supports up to 255 levels in it's on-disk format,
but the xfsdump command is limited to just 10 levels by a separate
"max level" define. 

IOWs, on the surface it _seems_ simple to change it, but I don't
know all the places that the dump level is stored, nor what would
happen if an older xfsdump/xfsrestore binary tripped over an
inventory with a dump level larger than they were compiled to
handle.

I also don't know if there are scalability problems with resolving
the contents of incremental dump levels. Certainly restoring from 30
incremental dumps is more onerous than 6-7, but that's just time.
What I don't know is how the invenetory scales to more than 10
incremental and whether there's some exponential algorithm in there
that falls apart. So there's a lot of testing that would be needed
to validate what looks like a 1 line change to the code.

So, yes, technically we could increase the dump level, but there's a
*lot* of verification work after doing so and it's not a risk-free
modification. 

FWIW, the idea behind 10 dump levels is that it is enough for a
full dump every week w/ an incremental every night of the week until
the next full dump is done on the weekend. What's the backup plan
you want to use/need that requires a larger number of incrementals?
Knowing what you want more dump levels for helps us understand how
it would be used and what really needs to be testedi first....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com