linux-ext4 - Re: [PATCH 0/3] freeze feature ver 1.6

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Date:	Wed, 25 Jun 2008 15:36:40 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	<viro@...IV.linux.org.uk>, <linux-ext4@...r.kernel.org>,
	<xfs@....sgi.com>, <dm-devel@...hat.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<axboe@...nel.dk>, <mtk.manpages@...glemail.com>
Subject: Re: [PATCH 0/3] freeze feature ver 1.6

Hi,

> umm, OK, but nowhere in this patch series can I find a justification or
> reason for making these changes to Linux.  Why do we want to do this?
> What is the benefit?  What is the motivation.  What are the use-cases,
> etc?

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.

Cheers, Takashi

> On Tue, 24 Jun 2008 15:59:19 +0900
> Takashi Sato <t-sato@...jp.nec.com> wrote:
> 
>> Hi Andrew and Alexander,
>> 
>> I have implemented the ioctls for the filesystem freeze feature
>> and discussed its implementation on the ML (linux-ext4, xfs,
>> linux-fsdevel, linux-kernel) for five months.  All of the comments are
>> addressed in my patch-set.
>> Could you consider merging it into Linux?
>> 
>> The filesystem freeze feature can suspend accesses to the filesystem
>> keeping the filesystem's consistency.  We can take the consistent backup
>> with it cooperating with the backup software.
>> 
>> The patches are re-based from linux-2.6.26-rc3 to linux-2.6.26-rc7
>> There is no functional change from the previous version.
>> The patch-set consists of the following three patches.
>> 
>> [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 *timeval)
>>       fd: The file descriptor of the mountpoint
>>       FIFREEZE: request code for the freeze
>>       timeval: 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 timeval more accurately.
>>     For example, the freezer resets the timeval 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 timeval.
>>     int ioctl(int fd, int FIFREEZE_RESET_TIMEOUT, long *timeval)
>>       fd:file descriptor of mountpoint
>>       FIFREEZE_RESET_TIMEOUT: request code for reset of timeout period
>>       timeval: 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.
> 
> umm, OK, but nowhere in this patch series can I find a justification or
> reason for making these changes to Linux.  Why do we want to do this? 
> What is the benefit?  What is the motivation.  What are the use-cases,
> etc?
> 
> Thanks.
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