linux-kernel - Re: [PATCH v3 0/2] ext4: increase mbcache scalability

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Date:	Tue, 10 Sep 2013 17:10:13 +0000
From:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
To:	Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>
CC:	T Makphaibulchoke <tmac@...com>, Al Viro <viro@...iv.linux.org.uk>,
	"linux-ext4@...r.kernel.org List" <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org Devel" <linux-fsdevel@...r.kernel.org>,
	aswin@...com, Linus Torvalds <torvalds@...ux-foundation.org>,
	aswin_proj@...ts.hp.com
Subject: Re: [PATCH v3 0/2] ext4: increase mbcache scalability

On 09/10/2013 09:02 PM, Theodore Ts'o wrote:
> On Tue, Sep 10, 2013 at 02:47:33PM -0600, Andreas Dilger wrote:
>> I agree that SELinux is enabled on enterprise distributions by default,
>> but I'm also interested to know how much overhead this imposes.  I would
>> expect that writing large external xattrs for each file would have quite
>> a significant performance overhead that should not be ignored.  Reducing
>> the mbcache overhead is good, but eliminating it entirely is better.
> 
> I was under the impression that using a 256 byte inode (which gives a
> bit over 100 bytes worth of xattr space) was plenty for SELinux.  If
> it turns out that SELinux's use of xattrs have gotten especially
> piggy, then we may need to revisit the recommended inode size for
> those systems who insist on using SELinux...  even if we eliminate the
> overhead associated with mbcache, the fact that files are requiring a
> separate xattr is going to seriously degrade performance.
> 
> 	       	  	   	     - Ted
> 

Thank you Andreas and Ted for the explanations and comments.  Yes, I see both of your points now.  Though we may reduce the mbcache overhead, due to the overhead of additional xattr I/O it would be better to provide some data to help users or distros to determine whether they will be better off completely disabling SELinux or increasing the inode size.  I will go ahead and run the suggested experiments and get back with the results.

Thanks,
Mak.

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