[Libguestfs] mkfs.ext2 succeeds despite nbd write errors?

[Richard W.M. Jones]

On Sat, Nov 07, 2015 at 05:09:52PM -0600, Jason Pepas wrote:
> On Sat, Nov 7, 2015 at 3:02 PM, Richard W.M. Jones <rjones at redhat.com> wrote:
> >> I'm not sure where to start with hunting down why mkfs's pwrite()
> >> calls aren't failing.  I'd look to the kernel source for that?
> >
> > It looks like it's really an e2fsprogs problem, not a kernel problem.
> > That's pretty surprising - I wasn't expecting it.
> 
> I agree the fsync() issue is an e2fsprogs problem, but as for
> specifically the pwrite() calls not getting a -1 return value, that's
> the kernel's fault, right?

I'm definitely not an expert here, but I do recall being told that
writes and reads are allowed to return an "OK" indication, but a later
close(2) or fsync(2) might fail.  That is particularly a problem with NFS.

I'll leave the rest to the true experts on the ext4 mailing list.

> I've been rolling this around in my mind and I think I can see why the
> kernel would correctly make fsync() fail but not pwrite() fail.  Let
> me run this by you:
> 
> When a pwrite() happens, that doesn't immediately cause nbd to send a
> network packet out, and doesn't wait on a network reply before
> returning, right?  It just ends up in some dirty block device queue,
> I'm guessing?  And then something triggers a bunch of dirty blocks to
> get flushed out to "disk"?  If that's the case, then its impossible
> for the kernel to give an accurate return code to pwrite(), because it
> doesn't know those blocks will eventually fail to be written to "disk"
> (nbd).
> 
> But as for fsync(), the kernel is probably waiting until every last
> dirty sector gets written before it decides what the return code is,
> which is why we see that pwrite() isn't failing, but fsync() is
> failing.
> 
> Does that make sense?
> 
> I wonder if the block device were opened with O_DIRECT by e2fsprogs if
> that would cause the pwrite() calls to fail correctly?

Rich.

-- 
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