From: Takashi Sato <t-sato@yk.jp.nec.com>
Subject: [PATCH 0/3] freeze feature ver 1.9
Date: Tue, 22 Jul 2008 18:36:59 +0900
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Hi,

When multiple freeze requests arrive simultaneously, only the last
unfreeze process should unfreeze the frozen filesystem actually
(as Dave Chinner, Eric Sandeen and Alasdair G Kergon commented).
So I've added the reference counter to the freeze feature.
It counts up in freeze_bdev() and counts down in thaw_bdev().
When it becomes "0", thaw_bdev() will unfreeze actually.

The following regular cases have worked correctly.

A)
1. dmsetup suspend
2. FIFREEZE
3. FITHAW
4. dmsetup resume
B)
1. FIFREEZE
2. dmsetup suspend
3. dmsetup resume
4. FITHAW

But in the following case, the last FITHAW has been frozen
for writing the super block because device-mapper layer is still frozen.
It's a irregular case (app's bug) and the next "dmsetup resume"
can solve it.  So I don't think it is a problem.
C)
1. dmsetup suspend
2. FIFREEZE
3. FITHAW
4. FITHAW<- The thaw process was frozen.

In my previous mail, I have mentioned considering removing the timeout
feature.  But I leave it in my patch-set because we need it for the case
someone dirties so much data that the freeze process is swapped out
(as some people said).

Currently, ext3 in mainline Linux doesn't have the freeze feature which
suspends write requests.  So, we cannot take a backup which keeps
the filesystem's consistency with the storage device's features
(snapshot and replication) while it is mounted.
In many case, a commercial filesystem (e.g. VxFS) has
the freeze feature and it would be used to get the consistent backup.
If Linux's standard filesytem ext3 has the freeze feature, we can do it
without a commercial filesystem.

So I have implemented the ioctls of the freeze feature.
I think we can take the consistent backup with the following steps.
1. Freeze the filesystem with the freeze ioctl.
2. Separate the replication volume or create the snapshot
   with the storage device's feature.
3. Unfreeze the filesystem with the unfreeze ioctl.
4. Take the backup from the separated replication volume
   or the snapshot.

[PATCH 1/3] Implement generic freeze feature
  I have modified to set the suitable error number (EOPNOTSUPP)
  in case the filesystem doesn't support the freeze feature.

  The ioctls for the generic freeze feature are below.
  o Freeze the filesystem
    int ioctl(int fd, int FIFREEZE, arg)
      fd: The file descriptor of the mountpoint
      FIFREEZE: request code for the freeze
      arg: Ignored
      Return value: 0 if the operation succeeds. Otherwise, -1

  o Unfreeze the filesystem
    int ioctl(int fd, int FITHAW, arg)
      fd: The file descriptor of the mountpoint
      FITHAW: request code for unfreeze
      arg: Ignored
      Return value: 0 if the operation succeeds. Otherwise, -1

[PATCH 2/3] Remove XFS specific ioctl interfaces for freeze feature
  It removes XFS specific ioctl interfaces and request codes
  for freeze feature.
  This patch has been supplied by David Chinner.

[PATCH 3/3] Add timeout feature
  The timeout feature is added to freeze ioctl.  And new ioctl
  to reset the timeout period is added.
  o Freeze the filesystem
    int ioctl(int fd, int FIFREEZE, long *timeout_sec)
      fd: The file descriptor of the mountpoint
      FIFREEZE: request code for the freeze
      timeout_sec: the timeout period in seconds
               If it's 0 or 1, the timeout isn't set.
               This special case of "1" is implemented to keep
               the compatibility with XFS applications.
      Return value: 0 if the operation succeeds. Otherwise, -1

  o Reset the timeout period
    This is useful for the application to set the timeout_sec more accurately.
    For example, the freezer resets the timeout_sec to 10 seconds every 5
    seconds.  In this approach, even if the freezer causes a deadlock
    by accessing the frozen filesystem, it will be solved by the timeout
    in 10 seconds and the freezer can recognize that at the next reset
    of timeout_sec.
    int ioctl(int fd, int FIFREEZE_RESET_TIMEOUT, long *timeout_sec)
      fd:file descriptor of mountpoint
      FIFREEZE_RESET_TIMEOUT: request code for reset of timeout period
      timeout_sec: new timeout period in seconds
      Return value: 0 if the operation succeeds. Otherwise, -1
      Error number: If the filesystem has already been unfrozen,
                    errno is set to EINVAL.

Any comments are very welcome.

Cheers, Takashi