GPT and disk alignment

Jeremy Chadwick freebsd at
Wed Jun 8 07:55:28 UTC 2011

On Wed, Jun 08, 2011 at 01:29:43AM -0400, Robert Simmons wrote:
> On Wed, Jun 8, 2011 at 12:29 AM, Mark Felder <feld at> wrote:
> > On Tue, 07 Jun 2011 22:27:24 -0500, Robert Simmons <rsimmons0 at>
> > wrote:
> >> Do all HDDs that have 4KB per LBA present themselves to the OS as
> >> having 512 bytes per LBA?
> >
> > No
> Ok, but can I assume that all HDDs of this type expand each of the 4K
> sectors so that physically they take up the same space as eight 512
> byte LBAs?  AFAIK, the new 4K LBA has a smaller ECC area than the sum
> of 8 ECC areas in 512 byte LBAs, so if the data area was _not_
> expanded slightly, you would never really be aligned except every x
> LBAs as the shifting approaches an LBA boundary, right?
> For any HDDs, do I need to worry about cylinder boundaries at all?
> Has the reported "disk geometry" become divorced from the physical
> reality in modern disks?  If I do still need to worry about cylinder
> boundaries, should I basically ignore every reported geometry (BIOS,
> OS) and use what is written on the sticker on the drive?
> >> What about SSDs that have 1024 bytes per LBA?
> >
> > Not sure, but I do know that not all flash media have the same bytes per LBA
> > internally. Some are 1K, some 4K, some even 8K. GPT is definitely the way to
> > go if you want to make sure you're aligned.
> Ok, is there some way to tell gpart(8) what the LBA size is, or do I
> have to calculate the offset of each partition manually?  In Linux it
> would be "fdisk -b 1024" for the example of SSDs or "fdisk -b 4096"
> for 4K HDDs.

I would think you'd just use "gpart -b" to specify the base offset.
For example, on an Intel 320-series SSD (which uses a NAND flash cell
size of 8192 bytes), "gpart -b 8" should end up at byte 65536 within the
flash itself.

I'm not sure if using 8 is correct though -- that is to say, I believe
there is other space near the beginning of the drive which is used for
things like the boot loader (I don't mean boot0, I mean boot2/loader and
friends), or for the GPT loader or GPT ZFS loader.  I could be wrong on
this part -- need someone to correct me.  All these different loaders
and GPT support on FreeBSD seriously makes my head spin.

Anyway back to SSDs:

I have yet to see anyone list off all the *actual* NAND flash cell sizes
of SSDs.  For example, everyone said "4KBytes" for Intel SSDs, but come
to find out it's actually 8KBytes.

Don't confuse NAND flash cell size with NAND erase page size.  They're
two different things.  Multiple cells make up (fit into) a single erase
page.  The alignment issue only applies to the cell part, not the erase
page size.  (I just had a discussion with an end-user on Intel's forum
about this; someone had lead him to believe the erase page size was what
he should align to).

For example, on Intel 320-series drives, the NAND erase page size is 256
cells, thus 256*8192 = 2097152, or 2MBytes.  Just a technical FYI bit
for those curious.

| Jeremy Chadwick                                   jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                   Mountain View, CA, US |
| Making life hard for others since 1977.               PGP 4BD6C0CB |

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