[Libguestfs] nbdkit build failure in Koji

[Eric Blake]

On 8/4/20 4:42 AM, Richard W.M. Jones wrote:

>>> I'll push a patch along those lines, but as you're the one doing the
>>> koji runs, there's obviously another feedback cycle to go through.
>>
>> Sure I'll give your patch a go once I see it in git, thanks!
> 
> Unfortunately no this didn't fix it.  The log is:
> 
>    https://kojipkgs.fedoraproject.org//work/tasks/9782/48609782/build.log
> 
> nozero2.img (for example) was originally:
> 
>    nozero2.img: 2048 allocated blocks of size 512 bytes, total size 1048576
> 
> but when we test it later it has grown to 4096 blocks:
> 
>    + for i in {2..6}
>    ++ stat -c %b nozero2.img
>    + test 4096 '!=' 2048
>    + echo 'nozero2.img was trimmed by mistake'
>    nozero2.img was trimmed by mistake
>    + fail=1
> 
> So it has been "trimmed" from 2048 to 4096 blocks.  Perhaps we should
> use -lt in that test to detect if the file got smaller?

Well, the point is that it wasn't trimmed - the nozero filter let 
literal zeroes be written in rather than attempting a trim.  But still, 
it is odd that writing literal zeroes in place made the file occupy more 
reserved space, even though the file size is unchanged.

I wonder if changing the file to be 2 megabytes in size, but only 
writing zeroes to the first megabyte, would be a more reliable way to 
avoid the file system thinking that because we just wrote zeroes all the 
way to the current EOF that we are likely to grow the file size, in 
order to thwart its preemptive allocations.  But you also have a point 
that merely forbidding a shrinking file is an acceptable test (the fact 
that the file grew is not nice, but the file did not become sparse, and 
that's all we really cared about).

> 
> Also I was interested in what filesystem the builders are using (since
> I still cannot reproduce any of this without Koji).  So I added
> ‘stat -f .’ in the %check section.  It printed:
> 
>      File: "."
>        ID: fc0200000000 Namelen: 255     Type: xfs
>    Block size: 4096       Fundamental block size: 4096
>    Blocks: Total: 33538048   Free: 25172957   Available: 25172957
>    Inodes: Total: 67108864   Free: 66469270
> 
> I believe the builder is running Fedora 32, at least going by the
> kernel version.  (This surprised me as I thought they were running
> RHEL.)
> 
> Rich.
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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