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

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Message-Id: <651BBBE5-D7E1-43C5-BC21-7E56CF12848C@dilger.ca>
Date:	Mon, 27 Aug 2012 11:51:41 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	raphel johnson <iraphel78@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: why online shrink is not supported with EXT4_IOC_RESIZE_FS?

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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