Subject: Re: [PATCH] Re: Reduce XFS footprint (was Re: TAKE - remove a
	function xfs added to filemap.c
From: Eric Sandeen <sandeen@sgi.com>
To: Andi Kleen <ak@suse.de>
Cc: Nathan Scott <nathans@sgi.com>, linux-xfs@oss.sgi.com
In-Reply-To: <20020129211758.A8117@wotan.suse.de>
References: <200201291751.g0THp897004750@scare.vieo.com>
	<20020129194001.A16401@wotan.suse.de> <20020129202509.A31370@wotan.suse.de>
	<1012334201.5905.25.camel@stout.americas.sgi.com> 
	<20020129211758.A8117@wotan.suse.de>
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Date: 29 Jan 2002 14:42:12 -0600
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On Tue, 2002-01-29 at 14:17, Andi Kleen wrote:
> On Tue, Jan 29, 2002 at 01:56:41PM -0600, Eric Sandeen wrote:
> > Yep, I just saw that.   And FWIW, the stated limit for the filesystem
> > size w/o XFS_BIG_FILESYSTEMS isn't quite right*.  I think these are
> > correct (these are internal XFS limitations):
> > 
> > File size:
> > o Without XFS_BIG_FILES, the max filesize is 1<<40, or 2TiB.
> 
> I misread blockno in xfs_types.h as byteno. It should be 2^31 * blocksize,
> which would be 2^43 with 4K blocks or more with bigger block size.
> I don't see any other limit.
> 
> How do you get to 2^40 ? 

in xfs_inode.h:

#if XFS_BIG_FILES
#define XFS_MAX_FILE_OFFSET     ((long long)((1ULL<<(32+PAGE_SHIFT))-1ULL))
#else
#define XFS_MAX_FILE_OFFSET     ((1LL<<40)-1LL)
#endif

and XFS_MAX_FILE_OFFSET is used to set sb->s_maxbytes
(the latter case is stock XFS, the former is something I changed, might
be off by a factor of 2?  hm....)

The comment blurb says
 * if XFS_BIG_FILES [...]
 * else 2^40 - 1 (40=31+9) (might be an int holding a block #)

Not quite sure where this comes from, actually.

> Also Linux/32bit is VM limited to 2^31 * PAGE_SIZE, so assuming PAGE_SIZE==
> blocksize XFS_BIG_FILES should not be needed at all on 32bit linux.

Does the VM actually enforce this limit?  I thought I had a case where I
could create a too-large file if XFS allowed it.  Also I thought the
limit was 2^32 * PAGE_SIZE...  /me goes back to look.
 
> On 64bit it probably makes sense to hardcode them both to 1. 
> 
> Where I am wrong ? 
> 
> > o With XFS_BIG_FILES, the max filesize is 1<<63-1.
> >   However, Linux can only handle 1<<(32+PAGE_SHIFT), or
> 
> 2^(31+PAGE_SHIFT)

Whoops...  so the page cache index is signed...?

-Eric


-- 
Eric Sandeen      XFS for Linux     http://oss.sgi.com/projects/xfs
sandeen@sgi.com   SGI, Inc.