Date: Thu, 14 Mar 2013 12:47:02 +0100 From: Peter Maloney <peter.maloney@brockmann-consult.de> To: Bruce Evans <brde@optusnet.com.au> Cc: freebsd-fs@FreeBSD.org Subject: Re: Aligning MBR for ZFS boot help Message-ID: <5141B8B6.4010209@brockmann-consult.de> In-Reply-To: <20130314195715.Y909@besplex.bde.org> References: <513C1629.50501@caltel.com> <alpine.BSF.2.00.1303101006490.5989@wonkity.com> <513CD9AB.5080903@caltel.com> <alpine.BSF.2.00.1303101326530.7218@wonkity.com> <513CE369.4030303@caltel.com> <alpine.BSF.2.00.1303101349540.7637@wonkity.com> <1362951595.99445.2.camel@btw.pki2.com> <alpine.BSF.2.00.1303101807550.8481@wonkity.com> <CABXB=RTt-j0SGxktWMfLcgLAEN6Vi%2Bf=psBuN0jQaJthk_3cbw@mail.gmail.com> <513E1208.5020804@caltel.com> <20130312203745.A1130@besplex.bde.org> <513F8F04.60206@caltel.com> <20130313232247.B1078@besplex.bde.org> <5140F373.1010907@caltel.com> <20130314195715.Y909@besplex.bde.org>
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On 2013-03-14 10:41, Bruce Evans wrote:
> On Wed, 13 Mar 2013, Cody Ritts wrote:
>
>> So, by setting those CHS values I am:
>> making the partition table more compatible with other operating
>> systems and BIOSes?
>> and giving some utilities the CHS stuff they need to function right?
>
> It's not completely clear that S=32 H=64 is portable, but it is what most
> old SCSI BIOSes used.
>
> Also, if the disk already has some partitions with a certain geometry,
> use
> the same geometry for other partitions and don't use fdisk's defaults if
> they differ.
>
> Bruce
Oh man... I thought yeah that -a 1 or -a 2048 should work, but it
doesn't. And then I thought I'd be extra crafty and use dd to directly
write the partition table myself and send that as a solution to you
guys, but even that fails!
Here's writing a 63 alignment mbr to the disk, just to prove dd can do this:
# gdd if=mbr.img of=/dev/md10 bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 16.8709 s, 0.0 kB/s
# gpart show md10
=> 63 4194241 md10 MBR (2.0G)
63 40950 1 freebsd (20M)
41013 4153291 - free - (2G)
Here's changing the start sector on the first partition to 2048 ;)
Writing to the device works with bs=512, but not bs=1, so we use a file
and bs=1 to do our edits, and then bs=512 to the disk.
# gdd if=<(echo -ne "\x00\x08" ) of=mbr.img bs=1 seek=454
2+0 records in
2+0 records out
2 bytes (2 B) copied, 0.000112023 s, 17.9 kB/s
Here's writing the new 2048 aligned mbr to the disk:
# gdd if=mbr.img of=/dev/md10 bs=1 count=1
gdd: writing `/dev/md10': *Invalid argument*
1+0 records in
0+0 records out
0 bytes (0 B) copied, 21.0247 s, 0.0 kB/s
:O
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