Advanced Format Drive ?

[Warren Block]

On Thu, 15 Nov 2012, Ronald F. Guilmette wrote:

> In message <alpine.BSF.2.00.1211150828040.62459 at wonkity.com>,
> Warren Block <wblock at wonkity.com> wrote:
>
>> On Thu, 15 Nov 2012, Ronald F. Guilmette wrote:
>>> In your tutorial document, you say:
>>>
>>>     "Create a boot partition to hold the loader, size of 512K."
>>>
>>> How big is that thing (gpart boot loader), actually?  Half a megabyte
>>> seems rather a bit large-ish, certainly relative to ye olde MBR loader,
>>> which I gather was limited to... what?  32KB (minus a little for the
>>> partition table) ?
>>
>> /boot/gptboot is 15K, /boot/gptzfsboot is 39K.  A code limitation makes
>> 512K the largest this partition can be made.  So I make it that big so
>> it won't have to be increased for bigger boot loaders later.
>
> It wouldn't hurt to add the above info to your tutorial page.

The problem with that sort of detail is that too much of it obscures the 
point, which in this case is just trying to show the right way to set up 
disks without overwhelming the reader.

>> And the space is not wasted because of the next partition...
>
> Huh?
>
> Oh, oh, OK.  I read down to the end of your message and now understand.
> You meant that the space is not wasted because you will subsequently
> arrange to have the next following partition begin at 1MB, yes?

Yes.

> Besides which, you probably want to get your readers in the habit of
> doing things generally on 4KB boundaries, because (as I have just learned)
> they are probably going to need to start doing that before too long, even
> if they don't already need to just yet.

It does do that, although it's not overt.

>> Starting the first filesystem partition at 1M is a semi-standard, used
>> by various vendors including Microsoft.
>
> Yeabut why or how does Microsoft get involved at all with the position
> of my *FreeBSD* partition?!?

There are other vendors and some RAID systems that also use 1M as a 
starting point.  Sticking to that de facto standard helps keep us as 
compatible as possible with other systems and partitioning software. 
The cost in space is tiny, and it's a lot easier to do when setting up 
the disk than after the filesystems are populated.

> If I began my FreeBSD partition at, say 768KB, would anything from
> Microsoft be even likely to even notice?

I can't say I've tested it.  I see this as low-cost insurance.  For less 
than 1M of space, try to be as compatible with other systems and 
software that exists.

>> Besides being aligned to 4K, it's also aligned to bigger values
>> that can be important for performance on devices like SSDs.
>
> I see.  That's also another useful tidbit of knowledge that you may also
> wish to impart to readers of your tutorial.  I can only speak for myself,
> but I for one (perhaps because I have never owned an SSD myself) was
> totally unaware that those had any such additional alignment issues.

Again, I'm trying to avoid too much of that type of detail in that 
particular article.  I've considered writing a separate SSD article, but 
have not done it yet.

> P.S.  I really do think that it is a serious omission that the gpart(8)
> man page doesn't really say anything regarding proper or desirable use
> of the "add -a" option.  If it were up to me, I'd put in just a couple
> of short notes that would say at least something about 4K being Good and
> Desirable for modern drives, and 1M being Good and Desirable for SSDs.
>
> But maybe that kind of info does really belong in something more like
> an actual tutorial document... you know... something like, um, your's.

I can see it both ways.  A short mention of those values in that section 
of gpart(8) would be helpful.  The 1M value is controversial to some 
people.  Of course, some people think that calling bare bsdlabel disks 
"dangerously dedicated" or using an MBR is controversial.