[TriLUG] how big and where is the GPT

[MrB]

For flash drives .. mostly ...

Brent R Brian on Samsung GS III Android Phone
On Jul 28, 2014 7:44 AM, "William Sutton" <william at trilug.org> wrote:

> People are still using ext2 in 2014?  the horror :-)
>
> William Sutton
>
> On Mon, 28 Jul 2014, Ron Kelley wrote:
>
>  Someone mentioned earlier the inability to script gparted (“but I can
>> script writing bits”).  You can easily script parted using the “—script”
>> command-line argument.  In fact, you can script an entire disk partition
>> (including GPT or MSDOS disk labels).  Here is a quick way to script out
>> parted on a 40G disk:
>>
>> ------------------------------------------------------
>> parted -s /dev/sda mklabel msdos
>> parted -s /dev/sda mkpart primary ext2 1 5000
>> parted -s /dev/sda mkpart primary linux-swap 5001 7000
>> parted -s /dev/sda mkpart primary ext2 7001 100%
>> ------------------------------------------------------
>>
>> I use this in one of my CentOS kickstart scripts.
>>
>> If you just wanted to convert an msdos disk to a gpt disk, simply use
>> “parted -s /dev/sda mklabel gpt”. Replace gpt with msdos to go the other
>> way...
>>
>>
>> Hope this helps.
>>
>> -Ron
>>
>>
>>
>> On Jul 28, 2014, at 12:26 AM, Aaron Joyner <aaron at joyner.ws> wrote:
>>
>> I haven't played with GPT much, so I was eager to see where this thread
>> went, and read up some if time allowed.  Gregory's assertions do seem to
>> align with the Wikipedia description of GPT:
>> http://en.wikipedia.org/wiki/GUID_Partition_Table
>>
>> The image on that page is a most excellent and concise summary of the
>> information you want, but I'd suggest the relevant text commentary from
>> the
>> page would be:
>> ----- 8< snip 8<-----
>> The header contains the disk globally unique identifier (GUID). It records
>> its own size and location (always LBA 1!) and the size and location of the
>> secondary GPT header and table (always the last sectors on the disk).
>> ----- 8< snip 8<-----
>>
>> More reading of that page is required to understand the intricacies of why
>> blocksize might be 512 or 4k, but suffice to say that the blocksize
>> presented by the disk determines how large the headers are, so you should
>> look to see what it is.  You can easily check with lsblk or hdparm:
>> $ lsblk -o NAME,PHY-SeC
>> $ sudo hdparm -I /dev/sda | grep -i physical
>>
>> To rephrase the answer into my own words, if you want to smash the GPT
>> from
>> a disk, you need to wipe out the headers which are used to determine
>> there's a GPT on the disk.  These are stored as the first 2 blocks and the
>> last block on the disk.  That's pretty straightforward with dd, if you
>> know
>> the LBA blocksize and the size of the disk expressed in those terms:
>> $ dd if=/dev/zero of=/dev/your_disk bs=<blocksize, see above> count=2
>> $ dd if=/dev/zero of=/dev/your_disk bs=<blocksize, see above> count=1
>> oseek=<((size of disk in bytes / <blocksize>) - 1)>
>>
>> Please use caution determining the value of oseek correctly, or you'll be
>> writing a small patch of zeros to some random part of the disk with
>> potentially unknown consequences.
>>
>> Let us know how it goes!
>> Aaron S. Joyner
>> --
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>> --
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>>
>
> --
> This message was sent to: Brent R Brian <brentrbrian at gmail.com>
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>