Okay, so I ran it with strace and the offending line is:
write(4,
"\377\377\377\377\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
1048576) = -1 EINVAL (Invalid argument)
I then looked up EINVAL errors and found the following description of cases
when EINVAL is returned:
*fd* is attached to an object which is unsuitable for writing; or the file
was opened with the *O_DIRECT* flag, and either the address specified in
*buf*, the value specified in *count*, or the current file offset is not
suitably aligned.
Then I looked at where the file was opened and found that the O_DIRECT flag
is used:
open("disk12.img", O_RDWR|O_DIRECT) = 4
So I looked up O_DIRECT and found this:
The O_DIRECT flag on its own makes an effort to transfer data
synchronously, but does not give the guarantees of the O_SYNC flag that
data and necessary metadata are transferred. To guarantee synchronous I/O,
O_SYNC must be used in addition to O_DIRECT.
So I modified mirage-block-unix to add O_SYNC in odirect_stubs.c, because I
thought that might help, but no such luck.
Luckily I then noticed that prepending "buffered:" to the filename forces
buffered IO, and that actually fixed my problem. I'm not sure what the
exact reason for the failure is, but I thought the insight that I gained
was interesting and might lead to someone fixing it. For now I will just
use the workaround though!
Rupert
On Sat, Mar 5, 2016 at 11:57 AM Thomas Leonard <[email protected]> wrote:
> On 5 March 2016 at 08:25, Rupert Horlick <[email protected]> wrote:
> > Hi all,
> >
> > I'm running into a very strange issue using mirage-block-unix and I was
> > wondering if anyone had some insight.
> >
> > The core of the issue is that connecting to the same file (disk.img or
> > whatever) and writing in two different locations is giving completely
> > different results.
> >
> > If I connect and write to offset 0 in my unix home directory (an NFS
> > directory), then everything is completely fine. If I then go to
> > /local/scratch (local scratch space on a department machine, so it's on
> the
> > directly attached disk), and connect and write in exactly the same way I
> get
> > the following error:
> >
> > (Failure "write: Invalid argument in write '' at file disk.img offset 0
> with
> > length 512")
> >
> > I've done the same thing in various other locations and never run into
> this
> > error before. I've triple checked permissions and everything should be
> > completely fine. Any ideas?
>
> Try running it with "strace". That should show you the actual error
> from the kernel.
>
>
> --
> Dr Thomas Leonard http://roscidus.com/blog/
> GPG: DA98 25AE CAD0 8975 7CDA BD8E 0713 3F96 CA74 D8BA
>
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