linux-ext4 - Re: ext2/3/4: punch support?

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Message-ID: <4D6BBCA2.8030104@redhat.com>
Date:	Mon, 28 Feb 2011 09:17:54 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Dave Young <hidave.darkstar@...il.com>
CC:	Josef Bacik <josef@...hat.com>, Theodore Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: ext2/3/4: punch support?

On 2/28/11 8:29 AM, Dave Young wrote:
> On Mon, Feb 28, 2011 at 9:11 PM, Josef Bacik <josef@...hat.com> wrote:
>> On Mon, Feb 28, 2011 at 03:03:40PM +0800, Dave Young wrote:
>>> Hi, josef and ext guys
>>>
>>> Do you have any plan with ext4 hole punching support? And the
>>> fallocate for ext2/3?
>>>
>>> What's the obstacle to work on this? could someone tell the status of
>>> this issue?
>>
>> There is no obstacle, just nobody has bothered to do it.  If you want to do it
>> go for it.  Thanks,
> 
> Yes, I want to do some kvm image space discard stuff,
> I use ext4 as my filesystem, but currently only xfs support something
> like trim/discard.

As Lukas said, ext4 & ext3 support various forms of trim/discard already.

But that's different from punch...

And as for fallocate on ext3, you need somewhere to put the metadata to indicate a block is allocated but not written.  This was a lot more straightforward for ext4, since it describes a large range of blocks with a single extent structure.

-Eric

> I know general knowledge about ext filesystem, but nearly new to ext4 code,
> could you give some hint where to start?
> 
> --
> Thanks
> dave
> --
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