linux-ext4 - Re: [RFC][PATCH] ext4: handle fast symlink properly with inline

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Date:	Fri, 30 May 2014 16:26:33 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Zheng Liu <gnehzuil.liu@...il.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Ian Nartowicz <claws@...towicz.co.uk>, Tao Ma <tm@....ma>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [RFC][PATCH] ext4: handle fast symlink properly with inline_data

On Tue, Feb 18, 2014 at 11:07:24AM -0800, Andreas Dilger wrote:
> I suspect that the stats for symlinks > 60 but < ~150 chars is only a very
> small fraction of files. If the code complexity of handling this is very
> small (i.e. it is just handled as a natural consequence of writing "data" 
> of this size) then I would be OK with it. 
> 
> Otherwise, I expect the code and maintenance overhead of supporting
> the 0.01% (?) of symlinks that are this size is probably lot worth it. 
> 
> People could check what the actual usage is via the "fsstats" tool at:
> 
> http://www.pdsi-scidac.org/fsstats/
> 
> There is also data there already that reports stats on symlink length, but
> it is mostly HPC filesystems and it might be better to redo this with a
> desktop-type workload. 

I think we should either put in this kernel patch so that we can read inline
data fast symlinks, or remove the ability to write inline data fast symlinks.
It's a bit surprising that I can do:

# mke2fs -t ext4 -O inline_data /dev/sdb         
# mount /dev/sdb /mnt/
# ln -s "Fuzzy Wuzzy was a bear. Fuzzy Wuzzy had no hair. I guess he wasn't fuzzy, was he?" /mnt/biglink
# readlink /mnt/biglink
Fuzzy Wuzzy was a bear. Fuzzy Wuzzy had no hair. I guess he wasn't fuzzy, was he?
# umount /mnt
# mount /dev/sdb /mnt/
# readlink /mnt/biglink
Fuzzy Wuzzy was a bear. Fuzzy Wuzzy had no hair. I guess he

What happened to the punchline of the limerick? ------------^^^^^^^ ???? :)

e2fsck still seems to think that you can't have inline_data fast symlinks.  I
don't see a downside to continuing to allow them.

--D

> 
> Cheers, Andreas
> 
> >> On Feb 17, 2014, at 17:52, Theodore Ts'o <tytso@....edu> wrote:
> >> 
> >> On Thu, Feb 13, 2014 at 03:07:17PM +0800, Zheng Liu wrote:
> >> 
> >> I am not sure whether or not we need to enable inline_data for a fast
> >> symlink inode.  Obviously, it brings a benefit that after enabling
> >> inline_data feature for a fast symlink we can get more space to store
> >> the path.  But it seems that the original patch doesn't want to do this
> >> Another solution for fixing this bug is to disable inline_data for a
> >> fast symlink.  Any comment?
> > 
> > Well, if we are using inline data, and we have a symlink which is
> > longer than 60 bytes, but less than extra space available for an
> > inline data, it seems like a good thing to support.
> > 
> > The downside is that it is a bit more complication to add the kernel's
> > code in both the kernel as well as e2fsprogs, but it doesn't seem that
> > bad.
> > 
> > So I don't have any objections to adding this functionality.  What do
> > other folks think?
> > 
> >                       - Ted
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