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Go Back   Linux Archive > Redhat > Device-mapper Development

 
 
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Old 06-30-2008, 12:20 PM
Takashi Sato
 
Default freeze feature ver 1.8

Hi Andrew and Alexander,

I've addressed Andrew's comments in these patches.

Please see my previous mail for details of changes.
But only following change is different from my previous mail.
I wrote:
>>> +void del_freeze_timeout(struct block_device *bdev)
>>> +{
>>> + if (delayed_work_pending(&bdev->bd_freeze_timeout))
>>
>> Is this test needed?
>
> It's not necessary because cancel_delayed_work_sync checks it itself.
> I will remove it.

It's possible that the delayed work task (freeze_timeout()) calls
del_freeze_timeout(). If the delayed work task calls
cancel_delayed_work_sync() to delete itself, a deadlock will occur.
So we need the test of delayed_work_pending().

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

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Old 06-30-2008, 01:54 PM
Alasdair G Kergon
 
Default freeze feature ver 1.8

On Mon, Jun 30, 2008 at 09:20:05PM +0900, Takashi Sato wrote:
> 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.

Is the following a fair summary?

1. Some filesystems have a freeze/thaw feature. XFS exports this to
userspace directly through a couple of ioctls, but other filesystems
don't. For filesystems on device-mapper block devices it is exported to
userspace through the DM_DEV_SUSPEND ioctl which LVM uses.

2. There is a desire to access this feature from userspace on non-XFS
filesystems without having to use device-mapper/LVM.

Alasdair
--
agk@redhat.com

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Old 07-01-2008, 09:12 AM
"Takashi Sato"
 
Default freeze feature ver 1.8

Hi,

Alasdair G Kergon wrote:

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.


Is the following a fair summary?


Yes, you are right.
We'd like to use the freeze feature without device-mapper/LVM.


1. Some filesystems have a freeze/thaw feature. XFS exports this to
userspace directly through a couple of ioctls, but other filesystems
don't. For filesystems on device-mapper block devices it is exported to
userspace through the DM_DEV_SUSPEND ioctl which LVM uses.

2. There is a desire to access this feature from userspace on non-XFS
filesystems without having to use device-mapper/LVM.

Alasdair


Cheers, Takashi


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