Problem with mkfs.xfs on a regular file

[Dave Chinner]

On Tue, Nov 26, 2013 at 06:31:19PM -0800, Phil White wrote:
> Gents:
> 
> I was making an image for a VM using everyone's favorite fs with a line
> that looked something like this:
> -------------
> dd if=/dev/zero of=~/image bs=1024 count=1048576 && ./mkfs/mkfs.xfs && mount -o loop ~/image /mnt/loop
> -------------
> 
> 
> mkfs.xfs gave me this output:
> -------------
> meta-data=/root/image            isize=256    agcount=4, agsize=65536 blks
>          =                       sectsz=512   attr=2, projid32bit=0
> data     =                       bsize=4096   blocks=262144, imaxpct=25
>          =                       sunit=0      swidth=0 blks
> naming   =version 2              bsize=4096   ascii-ci=0
> log      =internal log           bsize=4096   blocks=2560, version=2
>          =                       sectsz=512   sunit=0 blks, lazy-count=1
> realtime =none                   extsz=4096   blocks=0, rtextents=0
> existing superblock read failed: Invalid argument
> mkfs.xfs: pwrite64 failed: Invalid argument
> mkfs.xfs: read failed: Invalid argument
> -------------
.....
> 
> While it occurred to me that the problem might just be line 806 of some files
> in xfsprogs, I threw it under a debugger and took a closer look.  The file
> descriptor value in xi->dfd pointed at ~/image.  errno was set to 22.  I
> thought that might indicate a problem with lseek(), so I rewrote the pwrite64()
> and pread() as lseek()s and read()/write()
> 
> As you may have guessed, this did me no good at all.
> 
> It's trying to read/write 512 bytes at the beginning of the file which seems
> reasonably innocuous.  I double checked the man page which says that under
> 2.6, O_DIRECT writes can be aligned to 512 bytes without a problem.

That doesn't mean it is correct, because the man page also says:

" In Linux alignment restrictions vary by filesystem and kernel
version and might be absent entirely."

So, I bet that your underlying filesystem (i.e. the host filesystem)
has a sector size of 4k, and that's why direct Io on 512 byte
alignment is failing. In that case, run "mkfs.xfs -s size=4k ..."
and mkfs should just work fine...

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com