linux-ext4 - Re: why online shrink is not supported with EXT4_IOC_RESIZE

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Date:	Thu, 30 Aug 2012 11:24:31 +0530
From:	raphel johnson <iraphel78@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: why online shrink is not supported with EXT4_IOC_RESIZE_FS?

Hi Andreas,

Thanks a lot for your reply.

Regards,
Raphel


On Mon, Aug 27, 2012 at 11:21 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> On 2012-08-27, at 7:30 AM, raphel johnson wrote:
>> I was referring EXT4_IOC_RESIZE_FS ioctl implementation in linux
>> kernel ver 3.3.8.
>>
>> It is given in the code that online shrinking not supported.
>>
>> if (n_blocks_count < o_blocks_count) {
>> /* On-line shrinking not supported */
>> ext4_warning(sb, "can't shrink FS - resize aborted");
>> return -EINVAL;
>> }
>>
>> I am new to ext4 file system. Could you please tell online shrinking
>> issues in ext4.
>>
>> Thanks in adavance.
>
> There are several reasons for this:
> - if blocks are in use beyond the new end of the filesystem, potentially
>   all inodes in the filesystem need to be scanned to find the owner
> - this also causes consistency issues with the page cache, since moving
>   in-use disk blocks is tricky, and it isn't always possible to remove
>   them from memory
> - if in-use inodes are beyond the end of the new filesystem, potentially
>   all of the directories in the filesystem need to be scanned to find the
>   parent directory
> - moving inodes in ext4 will also cause the inode number to change, which
>   would break NFS and potentially other programs
>
> In the end, rarely do users need to shrink a filesystem while mounted, so
> the effort and complexity of doing this isn't worthwhile.
>
> Cheers, Andreas
>
>
>
>
>
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