kern/100365: snapshots on busy filesystem fail

[Stephen J. Roznowski]

>Number:         100365
>Category:       kern
>Synopsis:       snapshots on busy filesystem fail
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Jul 16 02:00:31 GMT 2006
>Closed-Date:
>Last-Modified:
>Originator:     Stephen J. Roznowski
>Release:        FreeBSD 6.1-STABLE amd64
>Organization:
>Environment:
System: FreeBSD 6.1-STABLE FreeBSD 6.1-STABLE #3: Sat Jul 8 22:34:39 EDT 2006


	
>Description:
	Taking snapshots while creating/deleting directories leads
	to corrupted snapshots.
>How-To-Repeat:

I'm running the following script as root:

	#!/bin/sh

	FS=/usr/ports

	for i in 1 2 3 4 5 6 7 8 9 10
	do
		echo $i
		mksnap_ffs $FS $FS/.snap/snapshot
		fsck_ffs $FS/.snap/snapshot
		/bin/rm -f $FS/.snap/snapshot
	done

If the filesystem is quiet, the script completes without errors.

In another window, I'm running the following:

	# mkdir /usr/ports/.a
	# cd /usr/ports/.a
	# while (1)
	> mkdir a b c d e f g h i j k l m n o p
	> rmdir a b c d e f g h i j k l m n o p
	> sleep 1
	> end

Now, when I run the previous script, occasionally I'll get snapshots
that are corrupt when the fsck is checking them. [Appears to be about
50% of the time....]

Additionally, I'm seeing the following error:

	mksnap_ffs: Cannot create /usr/ports/.snap/snapshot: Resource temporarily unavailable
	/usr/ports/.snap/snapshot is not a disk device

and dmesg shows:

  fsync: giving up on dirty
  0xffffff002fcee9b0: tag devfs, type VCHR
      usecount 1, writecount 0, refcount 635 mountedhere 0xffffff0000e7c600
      flags ()
      v_object 0xffffff002ff5a460 ref 0 pages 2556
       lock type devfs: EXCL (count 1) by thread 0xffffff002ac52260 (pid 71385)
          dev ad0s1f
  fsync: giving up on dirty
  0xffffff002fcee9b0: tag devfs, type VCHR
      usecount 1, writecount 0, refcount 634 mountedhere 0xffffff0000e7c600
      flags ()
      v_object 0xffffff002ff5a460 ref 0 pages 2552
       lock type devfs: EXCL (count 1) by thread 0xffffff002ac52260 (pid 71385)
          dev ad0s1f

I can provide more system configuration details if needed.

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: