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@xxxxxxxxxx> 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. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
