Status of support for 4KB disk sectors

Kevin Oberman kob6558 at
Tue Jul 19 06:04:16 UTC 2011

On Mon, Jul 18, 2011 at 4:41 PM, Jeremy Chadwick
<freebsd at> wrote:
> 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.


Yes, this is exactly what I mean. If I shell out for a high-end 7200
RPM drive for
my laptop only to have it run like a dog.

Thanks for the pointers. The first is good news. I'm a bit confused as to what I
will need gnop(8) for, but I'll be spending some time reading both the article
and the gnop man page and hopefully that will make it clear. I've only
used gpart
once and to took way too long to figure out what I needed to do, but it did work
fine. I just wish FreeBSD had some decent documentation on such a fundamental
operation. Fortunately there are some pretty good articles folks have
written, but
they did leave me with several questions.
R. Kevin Oberman, Network Engineer - Retired
E-mail: kob6558 at

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