linux-ext4 - Re: [LSF/FS TOPIC] Ext4 snapshots status update

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Message-ID: <AANLkTikPzr-RTLcnxWm+qqRNTmBP6DkYRV=KJtkY7zG-@mail.gmail.com>
Date:	Wed, 30 Mar 2011 12:46:24 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Joel Becker <jlbec@...lplan.org>
Cc:	Tao Ma <tm@....ma>, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Theodore Tso <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Josef Bacik <josef@...hat.com>
Subject: Re: [LSF/FS TOPIC] Ext4 snapshots status update

On Wed, Mar 30, 2011 at 12:33 PM, Joel Becker <jlbec@...lplan.org> wrote:
> On Wed, Mar 30, 2011 at 08:05:38AM +0200, Amir Goldstein wrote:
>> Just wanted to clarify there are 2 differences I notice between mmap
>> write to a hole
>> and mmap write to COWed file with ENOSPC:
>>
>> 1. A "good" application can avoid mmap write to a hole.
>>
>> 2. when initiating a hole, the mkwrite callback is in used (in ext4) to
>> reserve disk space for delayed allocation when a page becomes writable.
>> with COW a page may already be writable when the flush encounters COW
>> with ENOSPC. that flush can even happen after the application has exited,
>> so the data will be dropped on the floor silently (like in ext3).
>
>        ocfs2 doesn't have delayed allocation yet, so we try and fail
> the allocation in write_begin() right under mkwrite().
>

And what if the page is already writable?
Do you go over all inode pages and make them RO after fastcopy?
For volume level snapshot this isn't a sensible option.

Amir.
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