linux-ext4 - Re: [PATCH 14/38] fallthru: ext2 fallthru support

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Message-Id: <E1OlrzM-0001lZ-0z@be1.7eggert.dyndns.org>
Date:	Thu, 19 Aug 2010 01:24:07 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Miklos Szeredi <miklos@...redi.hu>, miklos@...redi.hu,
	jack@...e.cz, agruen@...e.de, viro@...iv.linux.org.uk,
	jblunck@...e.de, hch@...radead.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu,
	linux-ext4@...r.kernel.org, Valerie Aurora <vaurora@...hat.com>
Subject: Re: [PATCH 14/38] fallthru: ext2 fallthru support

Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, 17 Aug 2010, Valerie Aurora wrote:

>> >  - hard links to make sure a separate inode is not necessary for each
>> >    whiteout/fallthrough entry
>> 
>> The problem with hard links is that you run into hard link limits.  I
>> don't think we can do hard links for whiteouts and fallthrus.  Each
>> whiteout or fallthru will cost an inode if we implement them as
>> extended attributes.  This cost has to be balanced against the cost of
>> implementing them as dentries, which is mainly code complexity in
>> individual file systems.

Not knowing the details, I'd suggest to implement a generic function to
create an attributed inode and let the fs override it to create an
unlinked-file-dentry instead.

Benefit: All fs supporting extended attributes will be able to support
whiteout. If the fs has other means of supporting whiteout, they may fake
the attribute.

Possible problems:
- Having two ways of reporting a whiteout? Or can it be reported using a
  (static) fake inode?
- How do you un-whiteout while (not) having an overlaying fs?

> get_unlinked_inode() is a great idea.  But I feel that individual
> inodes for each fallthrough is excessive.  It'll make the first
> readdir() really really expensive and wastes a lot of disk and memory
> for no good reason.
> 
> Not sure how to fix the hard link limits problem though...

Do a hardlink if you can create a hard link, otherwise use a fresh inode
and use that for the next hardlink(s).


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