linux-ext4 - Re: Ext4 Size Restrictions

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Message-ID: <5395F8D6.6010409@redhat.com>
Date:	Mon, 09 Jun 2014 13:11:34 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Mahesh Khanwalkar <maheshkhanwalkar@...il.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: Ext4 Size Restrictions

On 6/9/14, 1:03 PM, Mahesh Khanwalkar wrote:
> Hello,
> Ext4 supports max file size of 16TiB and max volume of 1EiB. What
> exactly in the implementation causes this restriction? Also is there
> any plans (as of now) to make it max sizes larger?

The ext4 extent format on disk is:

struct ext4_extent {
        __le32  ee_block;       /* first logical block extent covers */
        __le16  ee_len;         /* number of blocks covered by extent */
        __le16  ee_start_hi;    /* high 16 bits of physical block */
        __le32  ee_start_lo;    /* low 32 bits of physical block */
};

The file size is limited by a 32-bit number for the starting logical block
of an extent, ee_block.

2^32 * 4k blocks gives 16TiB

The 48 bits for physical start high/low gives you the 1 EiB (2^48*4096).

I don't know of any plans to make either of these larger.  Surely 1EiB
suffices for now.  16TiB, perhaps not so much.

-Eric

> Thanks,
> Mahesh Khanwalkar

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