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Date: Sat, 16 Nov 2013 07:05:07 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Shane Ambler <FreeBSD@ShaneWare.Biz>
Subject: Re: gpart, to -a 4k or to -b 1M
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Cc: FreeBSD Questions <freebsd-questions@freebsd.org>,
 aurfalien <aurfalien@gmail.com>
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On Sat, 16 Nov 2013, Shane Ambler wrote:

> On 16/11/2013 10:46, Warren Block wrote:
>> On Fri, 15 Nov 2013, aurfalien wrote:
>>>
>>> Curious, for best practice in ensuring 4K alignment, should one do;
>>>
>>> gpart add ... -b 1M
>>>
>>> or
>>>
>>> gpart add ... -a 4k
>>>
>>> Thank in advance,
>>
>> They do different things.  '-b 1M' starts the added partition at 1M but
>> does not force it to be a multiple of 4K in size.
>>
>> '-a 4k' forces the added partition to start at the next 4K-aligned
>> block, and forces the size to be a multiple of 4K.
>>
>> I recommend both for the first filesystem partition, and '-a 4k' for
>> later ones.
>
> I believe it was related to windows co-existence but I heard a while
> back that -a 1M is a good option to use.

It doesn't hurt anything.  By rounding both starting location and length 
to 1M multiples, it still forces 4K alignment.