Newbie question about xfs_db with the response message "unexpected XFS SB magic number 0x2e524d46"

OpenSUSE10.2. and 2.6.22-rc7-default XFS
1)
I'd like to run "xfs_db -f -c "sb 0" -c "p" ./myfile | egrep 'agcount|inopblog|agblklog'" to see the three values. I mounted my xfs_disk.
Then I run :

               xfs_db -f -c "sb 0" -c "p" ./loop | egrep

'agcount|inopblog|agblklog'
It said:
xfs_db: unexpected XFS SB magic number 0x2e524d46 Floating point exception
I don't know what the problem is.
But if I umount my xfs_disk,
Then I run

              xfs_db {my_xfs_device}
              xfs_db>sb
              xfs_db>p

It's OK.But I can not see things about ./myfile now, since my xfs_disk is umounted.
2)
I have to umount my xfs_disk before I use xfs_db, correct?
3)
If I want to read the inode content of a file, say, myfile, I should "ls -li myfile" to get the inode number of myfile
Then
"xfs_db> inode <inode number>" to read the content of inode Is what I said correct?
Thanks in advance.

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if "myfile" is a file on your xfs filesystem, then that's not right. xfs_db needs to be pointed at a block device (or filesystem image file) containing an xfs filesystem. You can look at it while it's mounted, though (IIRC) the information may not be perfectly in sync with the fs's data.

agcount, inopblog, etc are parameters of an xfs filesystem, not a file on that filesystem.

What are you trying to learn (about your file or filesystem?)

-Eric

Thanks.
But what should I do to " look at it while it's mounted,though (IIRC)"? Only thing I know is that I have to umount xfs_disk to use xfs_db. But at this time, I can not see files in my xfs_disk.

Actually, I'd like to know where exactly myfile is located in xfs_disk.

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xfs_bmap -v myfile

-Eric