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Re: [parted-devel] [parted 1.8.8] msdos label and file system corruption

From: H. Peter Anvin
Subject: Re: [parted-devel] [parted 1.8.8] msdos label and file system corruption issues with > 2TB disk
Date: Thu, 27 Dec 2007 11:32:45 -0800
User-agent: Thunderbird (X11/20071115)

Michael Reed wrote:

Via analysis of the MSDOS partition table, I can state that it can address
a maximum of 4TB of space.  Each partition entry consists of a starting block
and a length.  Both of these fields are 32 bits in length.  No individual
partition can begin beyond 2TB.  No individual partition can be longer than
2TB.  In order to address 4TB, at least two partition table entries must be
used, for example:

That's a very optimistic view that both depends on optimal layout and no 32-bit bugs anywhere.

I'm considering adding GPT support to extlinux (syslinux), although it is complicated by the fact that there is no standard protocol to support booting from GPT partitions on BIOS systems. I'm thinking of doing something like the following:

DS:SI, which normally points to the partition table entry of the booted partition, points to a synthetic MBR partition entry with type 0xEE, and if < 2 TB has valid start and length fields; otherwise -1. The GPT partition table entry follows the 16 bytes of the synthetic MBR PTE.

extlinux itself doesn't need any support to handle GPT per se, although it won't support /boot extending past the 2 TB boundary any time soon.


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