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Re: 4K drives, sectsz=512, bsize=4096

To: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Subject: Re: 4K drives, sectsz=512, bsize=4096
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 06:13:36 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <201009010938.51894@xxxxxx>
References: <881036.47658.qm@xxxxxxxxxxxxxxxxxxxxxxxxxxx> <201009010938.51894@xxxxxx>
User-agent: Mutt/1.5.20 (2009-08-17)
On Wed, Sep 01, 2010 at 09:38:46AM +0200, Michael Monnerie wrote:
> On Mittwoch, 1. September 2010 Gim Leong Chin wrote:
> > Should we leave it at the default 256 bytes, or set it to the maximum
> >  of 2 kB?
>  
> An interesting question. Why are inodes sizes configurable at all? To 
> store ACLs? How would one know when bigger Inodes should be used? And 
> what is the implication when they would be needed but aren't used?

The XFS inode consists of three parts:

 - the fixed format dinode
 - the data fork
 - the attribute fork

the fixed format inode is fixed size, so any change in the inode size
only applies to the data and attribute forks.  For regular files we
generally don't use much space in the data fork as it just contains
the extent list, and most files have rather few of them.  But we can
also store short smbolic links directly inside it, as well as the
content of directories.  The attribute fork is used to store extent
attributes and if it's large enough we can store them inline instead
of using external blocks.

You want large inodes mostly if you store lots of extentded attributes,
either for ACLs, Selinux or posisbly DMAPI.  It will also help if you
have enough directories that are just too big for the inline directory
format with smaller inode sizes.


> 
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