On 03/07/2010 11:00 PM, James Bottomley wrote:
Just a quick note:
The 2TB size for msdos partitions is a problem independent of the 4k
sector issue. Traditional 512 byte sector drives are now available in
those sizes. It looks like we're going to have to move to a new
partitioning label to solve this.
There's actually another barrier at 8 or 16TB, which is where a 4k
logical sector filesystem tops out using 32 bit block offsets (it's 8TB
if the fs hasn't been proof checked against sign extension problems).
However, for 4k sectors, the main issues which have shown up in testing
by others (mostly Martin) are
1. In native 4k mode, we work perfectly fine. *however*, most
BIOSs can't boot native 4k drives.
2. Even if the BIOS can boot native 4k, our own boot loaders seem
to be hard coded for 512 byte sectors in several places.
3. If we run in the 512 byte sector emulation mode, we end up with
the partition alignment problems you allude to.
4. The aligment problem is made more complex by drives that make
use of the offset exponent feature (what you refer to as offset
by one) ... fortunately very few of these have been seen in the
wild and we're hopeful they can be shot before they breed.
5. I'm really, really sorry to have to mention it, but it looks
like uefi is going to be the only way we can boot non-msdos
partitioned devices with native 4k sectors.
so the bottom line seems to be that if you want the device as a non boot
disk, use native 4k sectors and a non-msdos partition label. If you
want to boot from the drive and your bios won't book 4k natively,
partition everything using the 512 emulation and try to align the
partitions correctly. If your bios/uefi will boot 4k natively, just use
it and whatever partition label the bios/uefi supports.
Martin can fill in the pieces I've left out.
I would very much like a reference for a platform which has firmware
which can successfully boot from 4K-logical media. It would be very
useful for bootloader testing.
Aligning partitions is something we should have done long ago. It
affects RAID and many flash drives just as much or more than 4K-sectored
disks.
Legacy BIOS doesn't care at all how the disk is partitioned, so as long
as the BIOS can read the disk at all the rest is up to the bootloader.
Of course, since there hasn't been the opportunity to test, bootloaders
generally don't handle it correctly (early versions of Syslinux
supported any sector size, but that bitrotted, and for the lack of
testing I eventually ended up hard-coding the number. Now I'd like to
get it working properly.)
As far as partitioning... I believe we should be using GPT partition
tables where possible. Even on non-EFI systems, it's simply a much
better partition table format.
-hpa
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