do we support using a file as logdev?

[Wengang Wang]

On 10-04-20 12:01, Eric Sandeen wrote:
> On 04/20/2010 09:36 AM, Wengang Wang wrote:
> > Hi experts,
> > 
> > Do we support using a file as log device?
> > If no, it's strange that we support running xfs on a file but don't support log
> > file.
> > If yes, it fails at mount.
> > 
> > [root at desk test-xfsprogs]# mkfs.xfs
> > -llogdev=/root/test-xfsprogs/test-xfsprogs-tmp/imagefile,size=32768b
> > /dev/sda10 -f
> > meta-data=/dev/sda10             isize=256    agcount=4, agsize=1250558
> > blks
> >          =                       sectsz=512   attr=2
> > data     =                       bsize=4096   blocks=5002231, imaxpct=25
> >          =                       sunit=0      swidth=0 blks
> > naming   =version 2              bsize=4096   ascii-ci=0
> > log      =/root/test-xfsprogs/test-xfsprogs-tmp/imagefile bsize=4096
> > blocks=32768, version=2
> >          =                       sectsz=512   sunit=0 blks, lazy-count=0
> > realtime =none                   extsz=4096   blocks=0, rtextents=0
> > [root at desk test-xfsprogs]# mount
> > -ologdev=/root/test-xfsprogs/test-xfsprogs-tmp/imagefile /dev/sda10 /xfs
> > mount: the kernel does not recognize /dev/sda10 as a block device
> >        (maybe `insmod driver'?)
> 
> Well, -o logdev=$BLAH sets up m_logname with that value in the mount
> point, and xfs_open_devices() then tries:
> 
>         if (mp->m_logname) {
>                 error = xfs_blkdev_get(mp, mp->m_logname, &logdev);
> 
> which eventually does
> 
> 	open_bdev_exclusive(name, FMODE_READ|FMODE_WRITE, mp);
> /**
>  * open_bdev_exclusive  -  open a block device by name and set it up for use
>  *
>  * @path:       special file representing the block device
>  * @mode:       FMODE_... combination to pass be used
>  * @holder:     owner for exclusion
>  *
>  * Open the blockdevice described by the special file at @path, claim it
>  * for the @holder.
>  */
> 
> ... this is all in kernelspace, which has no notion of what
> /root/test-xfsprogs/test-xfsprogs-tmp/imagefile might be.  You'll need
> to use a devicename that open_bdev_exclusive() can understand (like
> /dev/loop0, as suggested)

I see.
Thanks Eric for your detailed explanation!

regards,
wengang.