Re: ATA 4 KiB sector issues.

[James Bottomley]

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.

James


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