linux-ext4 - Re: Ext4 slow on links

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Message-ID: <20120620033831.GA2395@gamma.logic.tuwien.ac.at>
Date:	Wed, 20 Jun 2012 12:38:31 +0900
From:	Norbert Preining <preining@...ic.at>
To:	Ted Ts'o <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Ext4 slow on links

Hi Ted, hi Eric,

thanks for the answers, here some remarks.

On Di, 19 Jun 2012, Ted Ts'o wrote:
> The inode has room for 60 characters; after that, the symlink target
> gets stored in an external block.  The seek to read in the symlink
> target could be one of the causes of the delay.  The other is

Ok.

> Nothing has changed here between ext2/ext3 and ext4 here, so ext2/ext3
> will behave exactly the same.  There are changes in the block and
> inode allocation algorithms which might make a minor difference, but
> the same is potentially true of a very fragmented file system.

Ok.

Thinking about that, even if I dereference the files, I still am a 
bit surprised. For each file we have the following times:
1- read the inode and determine if it is a link
2- check if link target fits in the the 60chars
3- read the additional block for long link target
4- read the target inode

I assume that the items 1,3, and 4 are the time consuming ones and
about the same time.

Now what I don't understand, why doing a 
	time ls -l >/dev/null
on the directory with the original files takes 1.2s,
but reading the links with ls -l >/dev/null takes 1m13s, both
after reboot on cold cache.

I assume that some data is hashed in the directory inode, so doing
ls -l on the real files only reads the directory inode and not 
each file invividually, while reading all the links read all the
individual files.

Is this the explanation? If not, I cannot imagine any way that reading
a list of links and dereferencing them plus reading the ttargets 
takes 60times as long.

On Di, 19 Jun 2012, Eric Sandeen wrote:
> As Ted said, the targets might be far-flung.  If you do /bin/ls -l instead
> of maybe an aliased ls which stats everything to make pretty colors,
> is that faster?

Might be the problem, but I saw the same with a program doing
opendir readdir etc, so no allias or external program involved.

Best wishes

Norbert
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JAIST, Japan                                 TeX Live & Debian Developer
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