Aligning MBR for ZFS boot help
Cody Ritts
cr at caltel.com
Wed Mar 13 21:45:58 UTC 2013
Holy crap, there is so much baggage with CHS, I had no idea we were
still dragging it along to this extent. I really appreciate you
emphasizing its importance, I was ready to just blow off the incorrect
geometry and be happy with my hacked sector start.
So here is my drive...
> root@:/root # diskinfo -v ada0
> ada0
> 512 # sectorsize
> 64023257088 # mediasize in bytes (59G)
> 125045424 # mediasize in sectors
> 0 # stripesize
> 0 # stripeoffset
> 124053 # Cylinders according to firmware.
> 16 # Heads according to firmware.
> 63 # Sectors according to firmware.
So, if I now want to create an aligned single partition, here are the
steps I think I should be taking:
Sectors should be < 64
Heads should be < 256
for OLD OLD stuff, cylinders should be < 1024
if you want boundaries on a power of 2, those the number of sectors and
heads should also be a power of 2.
So, would all of these be potential valid values?
s32 h128
512*32*128 = 2097152B = 2MB cylinder
s32 h64
512*32*64 = 1048576B = 1MB cylinder
s16 h128
512*16*128 = 1048576B = 1MB cylinder
s4 h8
512*4*4 = 8192B = 8K cylinder
I am assuming that once I know my cylinder size, I just divide the total
size of my hard drive to come up with cylinder count?
s4 h8
64023257088 / 8192 = 7815339c
(8k is the largest power of 2 that the drive will evenly divide into)
s32 h64
64023257088 / 1048576 = 61057.3359375
Round down to 61057.
(does the cylinder need to end on the end of the disk?)
So, here is what i calculated:
c61057 h64 s32
I want an offset of 2M, file system should be reduced to 61055M
(61055 * 1024 * 1024)/512 = 125040640s)
Here are the commands that I ran:
> cat << EOF > command
> g c61057 h64 s32
> p 1 0xa5 4096 125040640
> a 1
> EOF
> root@:/root # fdisk -f command ada0
> ******* Working on device /dev/ada0 *******
> fdisk: WARNING line 1: number of cylinders (61057) may be out-of-range
> (must be within 1-1024 for normal BIOS operation, unless the entire disk
> is dedicated to FreeBSD)
> root@:/root # fdisk -p ada0
> # /dev/ada0
> g c124053 h16 s63
> p 1 0xa5 4096 125040640
> a 1
note, it auto goes back when exporting
> root@:/root # gpart show ada0
> => 63 125045361 ada0 MBR (59G)
> 63 4033 - free - (2M)
> 4096 125040640 1 freebsd [active] (59G)
> 125044736 688 - free - (344k)
> root@:/root # gpart delete -i 1 ada0
> root@:/root # gpart add -t freebsd -b 4096 -s 125040640 ada0
> ada0s1 added
> root@:/root # gpart show ada0
> => 63 125045361 ada0 MBR (59G)
> 63 4095 - free - (2M)
> 4158 125040573 1 freebsd (59G)
> 125044731 693 - free - (346k)
gpart does not care
> root@:/root # fdisk -f command ada0
> ******* Working on device /dev/ada0 *******
> fdisk: WARNING line 1: number of cylinders (61057) may be out-of-range
> (must be within 1-1024 for normal BIOS operation, unless the entire disk
> is dedicated to FreeBSD)
> root@:/root # fdisk ada0
> ******* Working on device /dev/ada0 *******
> parameters extracted from in-core disklabel are:
> cylinders=124053 heads=16 sectors/track=63 (1008 blks/cyl)
>
> Figures below won't work with BIOS for partitions not in cyl 1
> parameters to be used for BIOS calculations are:
> cylinders=124053 heads=16 sectors/track=63 (1008 blks/cyl)
>
> Media sector size is 512
> Warning: BIOS sector numbering starts with sector 1
> Information from DOS bootblock is:
> The data for partition 1 is:
> sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
> start 4096, size 125040640 (61055 Meg), flag 80 (active)
> beg: cyl 2/ head 0/ sector 1;
> end: cyl 640/ head 63/ sector 32
> The data for partition 2 is:
> <UNUSED>
> The data for partition 3 is:
> <UNUSED>
> The data for partition 4 is:
> <UNUSED>
So, setting the geom simply does this:
>> sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
>> start 4096, size 125040640 (61055 Meg), flag 80 (active)
>> beg: cyl 2/ head 0/ sector 1;
>> end: cyl 640/ head 63/ sector 32
I cannot set geom in my bios, nor does not show me what it thinks geom
is. Obviously anything that only supports 1024 cylinders will not think
it is very funny.
I feel like I am missing some part of this puzzle, or is that all there
is to this to correct geom for proper alignment on an MBR?
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?
Thanks,
Cody
On 3/13/13 6:33 AM, Bruce Evans wrote:
> On Tue, 12 Mar 2013, Cody Ritts wrote:
>
>> On 3/12/13 3:25 AM, Bruce Evans wrote:
>>>> Update --
>>>>
>>>> fdisk WILL allow you to align without regards to drive geometry
>>>>
>>>> It can only be done in interactive mode:
>>>> http://lists.freebsd.org/pipermail/freebsd-geom/2011-May/004780.html
>>>
>>> It can be set in all modes. At least according to the man page:
>>
>> In interactive mode, you can simply set the start and size of your
>> partition be done with it and boot away.
>
> That will usually give nonsense beginning and ending CHS values. This will
> usually prevent BIOSes and non-broken versions of FreeBSD from detecting
> the geometry. It would also prevent booting on BIOSes that uses the CHS
> values; this is not usually a problem now. So this misconfiguration will
> mainly mess up printing of the partition table in utilities that print
> the CHS utilities, and cause warnings in utilities that want the CHS values
> to be correct, and propagate the misconfiguratation.
>
>> If you then export that config file, and re-run it, you are back to
>> being aligned to CHS.
>
> Only if the config file is broken.
>
>> I would imagine that adjusting your CHS ~correctly~ in the config file
>> will allow you to to do it, but I have not found myself motivated to
>> really learn about adjusting those values. I will have to pick that
>> up someday I suppose.
>
> Yes this is necessary and very easy. Just type in the same geometry that
> you need to type in at the start of interactive fdisk to avoid the above
> problems.
>
>> For informational purposes here is
>> 1) partition w/ offset
>> 2) show results
>> 3) export/import config
>> 4) show results with adjusted offset
>>
>>> root@:/root # fdisk -i ada0
>>> Do you want to change our idea of what BIOS thinks ? [n]
>
> You do want the change this. Use something like 32 sectors 64 heads
> (1MB cylinders).
>
>>> Do you want to change it? [n] y
>>> Supply a decimal value for "sysid (165=FreeBSD)" [165]
>>> Supply a decimal value for "start" [63] 4096
>>> Supply a decimal value for "size" [125045361] 125041328
>>> Correct this automatically? [n]
>
> After changing "what BIOS thinks" to something like the above, fdisk
> shouldn't see anything to correct. The start of 4096 is a multiple
> of 32*64 = 2048.
>
> I just noticed that despite being too chatty, "what BIOS thinks"
> has bad grammar.
>
>>> Explicitly specify beg/end address ? [n]
>>> Are we happy with this entry? [n] y
>>> Do you want to change it? [n]
>>> Do you want to change it? [n]
>>> Do you want to change it? [n]
>>> Do you want to change the active partition? [n]
>>> Should we write new partition table? [n] y
>>>
>>> root@:/root # gpart show ada0
>>> => 63 125045361 ada0 MBR (59G)
>>> 63 4033 - free - (2M)
>>> 4096 125041328 1 freebsd [active] (59G)
>
> Looks like it got a bogus 63 from the same place as fdisk (from the
> "firmware" goemetry.
>
> The free area isn't really 4033 block sectors at 63, but 4095 blocks
> starting at 1 (for non-broken BIOSes and OSes). I used to start
> partitions at offset 1, but now use 63 for portability. However, 63
> isn't portable either. The BIOS or another OS might have a different
> idea of the geometry.
>
>>> root@:/root # fdisk -p ada0
>>> # /dev/ada0
>>> g c124053 h16 s63
>>> p 1 0xa5 4096 125041328
>>> a 1
>
> This shows fdisk -p using the same garbage default geometry as interactive
> fdisk. So fdisk -p output is not directly usable.
>
> There is no place in the partition table to store the geometry directly.
> It can sometimes be determined indirectly, but neither the kernel nor
> fdisk does so. Old kernels did so, and fdisk depended on this.
> However, fdisk shouldn't depend on this, so that fdisk can work on
> images of partition tables. Linux fdisk (fdisk-linux in ports) does
> this correctly. Not using OS-specific ioctls for this also improves
> portability. The kernel support for this was mainly to ensure that
> all FreeBSD utilities got a consistent view of the geometry, back when
> consistent views mattered. It's still confusing when the views are
> different.
>
>>> root@:/root # fdisk -p ada0 > command
>>>
>>> root@:/root # fdisk -f command ada0
>>> ******* Working on device /dev/ada0 *******
>>> fdisk: WARNING line 2: number of cylinders (124053) may be out-of-range
>>> (must be within 1-1024 for normal BIOS operation, unless the
>>> entire disk
>>> is dedicated to FreeBSD)
>>> fdisk: WARNING: adjusting start offset of partition 1
>>> from 4096 to 4158, to fall on a head boundary
>>> fdisk: WARNING: adjusting size of partition 1 from 125041328 to
>>> 125041266
>>> to end on a cylinder boundary
>
> This is broken. The non-interactive version should not adjust anything.
> Even the interactive version defaults to not adjusting.
>
>>> root@:/root # fdisk -p ada0
>>> # /dev/ada0
>>> g c124053 h16 s63
>>> p 1 0xa5 4158 125041266
>>> a 1
>>>
>>> root@:/root # gpart show ada0
>>> => 63 125045361 ada0 MBR (59G)
>>> 63 4095 - free - (2M)
>>> 4158 125041266 1 freebsd [active] (59G)
>
> So the bugs of fdisk -p producing wrong geometry, not editing its output
> to fix this, and the broken adjustment done by fdisk -f, result in the
> partiton offsets being corrupted by fdisk -f.
>
> Bruce
>
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