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Re: Directories > 2GB

To: Andreas Dilger <adilger@xxxxxxxxxxxxx>
Subject: Re: Directories > 2GB
From: Dave Kleikamp <shaggy@xxxxxxxxxxxxxx>
Date: Wed, 04 Oct 2006 12:51:20 -0500
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20061004165655.GD22010@xxxxxxxxxxxxxxxxxxxx>
References: <20061004165655.GD22010@xxxxxxxxxxxxxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
On Wed, 2006-10-04 at 10:56 -0600, Andreas Dilger wrote:
> For ext4 we are exploring the possibility of directories being larger
> than 2GB in size.  For ext3/ext4 the 2GB limit is about 50M files, and
> the 2-level htree limit is about 25M files (this is a kernel code and not
> disk format limit).
> Amusingly (or not) some users of very large filesystems hit this limit
> with their HPC batch jobs because they have 10,000 or 128,000 processes
> creating files in a directory on an hourly basis (job restart files,
> data dumps for visualization, etc) and it is not always easy to change
> the apps.
> My question (esp. for XFS folks) is if anyone has looked at this problem
> before, and what kind of problems they might have hit in userspace and in
> the kernel due to "large" directory sizes (i.e. > 2GB).  It appears at
> first glance that 64-bit systems will do OK because off_t is a long
> (for telldir output), but that 32-bit systems would need to use O_LARGEFILE
> when opening the file in order to be able to read the full directory
> contents.  It might also be possible to return -EFBIG only in the case
> that telldir is used beyond 2GB (the LFS spec doesn't really talk about
> large directories at all).

ext3 directory entries are always multiples of 4 bytes in length.  So
the lowest 2 bits of the offset are always zero.  Right?  Why not shift
the returned offset and f_pos 2 bits right?

JFS uses an index into an array for the position (which isn't even in
the directory traversal order) so it can handle about 2G files in a
directory (although deleted entries aren't reused).
David Kleikamp
IBM Linux Technology Center

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