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

On Sat, Nov 7, 2015 at 5:03 AM, Richard W.M. Jones <rjones(a)redhat.com&gt; wrote: > How about 'strace mkfs.ext2 ..' and see if any system calls are > returning errors. That would show you whether nbd-client is throwing > errors away, or whether mkfs is getting the errors and ignoring them > (seems pretty unlikely, but you never know). > > After that, it'd be down to tracing where the errors end up in the > kernel. Thanks for the tip! The results are interesting. It looks like all of mkfs's pwrite() calls succeed, but its final fsync() calls do actually fail: root@debian:~# strace mkfs.ext2 /dev/nbd0 2>&1 | tee strace.out root@debian:~# cat strace.out | grep pwrite pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 32768, 8187379712) = 32768 pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 32768, 8187412480) = 32768 pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 32768, 8187445248) = 32768 ... root@debian:~# cat strace.out | grep fsync fsync(3) = -1 EIO (Input/output error) fsync(3) = -1 EIO (Input/output error) The fsync() calls happen just before mkfs exists success: root@debian:~# cat strace.out | tail pwrite(3, "\1\2\0\0\2\2\0\0\3\2\0\0\367{\365\37\2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096, 6576672768) = 4096 fsync(3) = -1 EIO (Input/output error) pwrite(3, "\0\0\10\0\0\0 \0\231\231\1\0qm\37\0\365\377\7\0\0\0\0\0\2\0\0\0\2\0\0\0"..., 1024, 1024) = 1024 fsync(3) = -1 EIO (Input/output error) close(3) = 0 write(1, "done\n\n", 6done ) = 6 exit_group(0) = ? +++ exited with 0 +++ root@debian:~# I did manage to find two calls to fsync in the e2fsprogs source which are not return-value-checked: https://github.com/tytso/e2fsprogs/blob/956b0f18a5ddb6815a9dff4f10a1e3125... https://github.com/tytso/e2fsprogs/blob/956b0f18a5ddb6815a9dff4f10a1e3125...

I'll see about submitting a patch there. 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?