Status of support for 4KB disk sectors

Jeremy Chadwick freebsd at
Mon Jul 18 23:41:26 UTC 2011

On Mon, Jul 18, 2011 at 03:50:15PM -0700, Kevin Oberman wrote:
> I just want to check on the status of 4K sector support in FreeBSD.  I read
> a long thread on the topic from a while back and it looks like I might hit some
> issues if I'm not REALLY careful. Since I will be keeping the existing Windows
> installation, I need to be sure that I can set up the disk correctly without
> screwing up Windows 7.
> I was planning on just DDing the W7 slice over, but I am not sure how well this
> would play with GPT. Or should I not try to use GPT at all? I'd like
> to as this laptop
> spreads Windows 7 over two slices and adds a third for the recovery
> system, leaving
> only one for FreeBSD and I'd like to put my files in a separate slice.
> GPT would offer
> that fifth slice.
> I have read the handbook and don't see any reference to 4K sectors and only a
> one-liner about gpart(8) and GPT. Oncew I get this all figured out,
> I'll see about writing
> an update about this as GPT looks like the way to go in e future.

When you say "4KB sector support", what do you mean by this?  All
drives on the market as of this writing, that I've seen, all claim a
physical/logical sector size of 512 bytes -- yes, even SSDs, and EARS
drives which we know use 4KB sectors.  They do this to guarantee full
compatibility with existing software.

Since you're talking about gpart and "4KB sector support", did you mean
to ask "what's the state of FreeBSD and aligned partition support to
ensure decent performance with 4KB-sector drives?"

If so: there have been some commits in recent days to RELENG_8 to help
try to address the shortcomings of the existing utilities and GEOM
infrastructure.  Read the most recent commit text carefully:

But the currently "known method" is to use gnop(8).  Here's an example:

Now, that's for ZFS, but I'm under the impression the exact same is
needed for FFS/UFS.

<rant> Do I bother doing this with my SSDs?  No.  Am I suffering in
performance?  Probably.  Why do I not care?  Because the level of
annoyance is extremely high -- remember, all of this has to be done from
within the installer environment (referring to "Emergency Shell"), which
on FreeBSD lacks an incredible amount of usability, and is even worse to
deal with when doing a remote install via PXE/serial.  Fixit is the only
decent environment.  Given that floppies are more or less gone, I don't
understand why the Fixit environment doesn't replace the "Emergency
Shell". </rant>

| 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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