[clug] Dumb user question: 2+TB disks, MBR & BIOS

rodneyp at pcug.org.au rodneyp at pcug.org.au
Wed Jan 2 15:22:55 MST 2013

On Wed, 2 Jan 2013 00:23:49 Brett Worth <brett at worth.id.au> wrote:
> Message: 7
> Date: Wed, 02 Jan 2013 18:08:00 +1100
> From: Brett Worth <brett at worth.id.au>
> To: Hal Ashburner <hal at ashburner.info>
> Cc: linux at lists.samba.org
> Subject: Re: [clug] Dumb user question: 2+TB disks, MBR & BIOS...
> Message-ID: <50E3DCD0.5010407 at worth.id.au>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> On 02/01/13 17:18, Hal Ashburner wrote:
> > Plugged a 3TB usb drive into the box. Formatted it to ext4 and started
> > using it. Didn't know there might be an issue with that and didn't find
> > one. Haven't tried to boot of it though.
> Hal,
> Did you partition that drive?  This is a bit of a grey area.  i.e. should I
> be just using  drives with a 4K block size (The Microsoft Windows approach)
> or should I be using a GPT partition table?
Rod Smith has most of the bases covered on GPT/UEFI.


His *tests* show potential *huge* loss of I/O performance on AF formatted HDD 
if 4k sector size is not used for partitioning.  HDD manufacturers are using 
an interim work-around of presenting the physical 4k sectors as 8 logical 512 
B sectors and this allows adverse alignment of partitions to physical sectors, 
if older partitioning tools are used.

Recent releases of Rod's gdisk optimise alignment by default.  (g)parted now 
does also but does not implement some of the useful features of GPT, 
particularly non-sequential partition numbering nor do partition table cloning 
or MBR2GPT conversion etc.

I have used MBR2GPT conversion on a couple of drives.  Probably only useful if 
you have inadvertently formatted an AF HDD with MBR and don't want to go back 
to scratch.  Obviously not for mission-critical systems.

> I've only every gone the GPT route but I've also never tried using a >2TB
> drive  to boot  from.  Obviously I've never tried to boot from a GPT
> partitioned drive.
> Also the 4K block size approach only gets you to 16TB.  We'll be using
> drives that size in  a fortnight or two.
That is true only for MBR partiton table.  GPT handles some much larger number 
of sectors which won't be an issue any time soon.  It's quoted somewhare - 
what comes after PetaBytes :-)

> Brett


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