[syslinux] Linux syslinux-3.72 does not update boot block for SD devices

[Ferenc Wagner]

"H. Peter Anvin" <hpa at zytor.com> writes:

> Ferenc Wagner wrote:
>
>> "H. Peter Anvin" <hpa at zytor.com> writes:
>> 
>>> dd if=/usr/lib/syslinux/mbr.bin of=/dev/sdb bs=440 count=1
>>>
>>> The reason this isn't automatic is that this is sometimes not
>>> possible under Linux.
>> 
>> That makes me wonder.  When is it not possible?
>
> Linux is a little too flexible some times; when dealing with things
> like loopback devices you can't necessarily find the master device.
> Worse, with older Linux kernels there are some conditions in which
> you could end up poking the wrong device.

Oh, that feels better.  It may well not be possible to decide what the
master device is.  I misunderstood your statement as not being possible
to carry out the action.

>> Also, most docs recommend cat mbr.bin >/dev/sdb.  Is there any
>> difference?
>
> I prefer the dd= statement since it will prevent overwriting more
> than the 440 bytes of the MBR proper in case the file is corrupt.
> Just a safety.

Ack.

Speaking of such, I realised recently that the first 512 bytes of LVM
physical volumes is always zeroed by pvcreate (just experience, I
couldn't find a definition of the on-disk format).  Wouldn't it be
possible to place some boot code there and boot from a linear LV
located on that single PV (for starter :)?  Nowadays I use partitions
for nothing else but keeping /boot separate...  Just a random thought.
-- 
Cheers,
Feri.