[Libguestfs] [PATCH] [v2] WIP: ddrescue mapfile filter

[Richard W.M. Jones]

On Fri, May 01, 2020 at 02:41:59PM -0500, Eric Blake wrote:
> Oh cool, this sounds very similar to the existing extentlist plugin,
> but with a difference that extentlist uses the input to decide which
> portions of the file are advertised as sparse, while this one uses
> which portions of the file cause EIO errors during read.  Is it
> worth trying to combine the two concepts into one filter, or do we
> want to keep it orthogonal as two separate filters?  Can we teach
> both filters to reuse common code for parsing extent lists in
> different formats (the extentlist format is the same format
> documented by the nbdkit-sh-plugin .extents; yours is the ddrescue
> format, I am also interested in the 'qemu-img map --output=json'
> format)?  In fact, if we introduce a common library for extent list
> parsing, teaching nbdkit-sh-plugin to accept all formats might be
> useful.

I think common code for parsing sounds useful (eventually), but the
filters are quite different in their effects so fully combining them
would be difficult and seems unlikely to achieve much.  However ...

> Or even make it configurable (via another .config knob) whether bad
> blocks read as zero or cause EIO.

... this is definitely an interesting idea.  I guess some clients will
stop at an EIO but would continue if the data read as zeroes (although
whether such clients would do the right thing is questionable).

Rich.

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