All,
I rely on your greater knowledge to help me understand these questions.
Thanks in advance.
1) GPT and block size
GPT uses LBA for its data stuctures
The size of a block is historically 512B but is moving to larger sizes
(4KB). The code needs to handle this on a per device mount basis.
How does the driver know the block size used in the LBA?
1A) By querying the device
1B) Some MBR magic?
If 1A then that means to me that dd if=/dev/sdb of=/dev/sdc won't
produce a usable image on sdc if its block size is different than sdb's.
(Of course I also assume that the total space ion sdc is also == or >
than that of sdb. Which brings me to ...)
2) Can GPT be grown?
In the above example if sdc is much bigger than sdb,
I presume this is OK, at least as long as the GPT header in LBA1 passes
its CRC. Mounters won't query the drive size and refuse to mount the
GPT just because it does not cover the whole disk right?
Now what happens if LBA1 becomes corrupted? Now does the driver query
the drive size and block size and look at drive_size-block_size for the
backup GOT header? Again does it use the block size from the device or
does try something else? (I suppose to could try several block sizes
until it found a good CRC. However it does seem that it must assume
that the redundant copy is at the end of the physical disk.)
So even if the GPT is "mounted" OK, the extra space on the drive is not
usable, even for new partitions. Are there utilities that will "grow"
the GPT? Such growing would find the new end of disk and move the
redundant GPT table & header there.
3) Is it actually required that the partition array start at LBA2?
If not, then it would be possible to create a GPT assuming 512B blocks
but allow it to be "re-block sized" later by leaving 7 512B blocks free
before the table. Of course the partitions themselves should be aligned
and sized to multiples of the max block size expected. This is probibly
already done as you would want them to align to the prefered read/write
and those will almost certianly be larger than 512B.
Why?
The main case I am thinking about is:
wget http://downloads.new-wizbang-os.org/images/latest/aarch64-disk.img
dd if=aarch64-disk.img of=/dev/my-usb-sd-adapter
Then boot the image and the OS will resize the GPT and last filesystem
to cover the 16GB of my SD card even though they only require a minimum
size of 2GB.
Thanks,
Bill
----------------
William A. Mills
Chief Technologist, Open Solutions, SDO
Texas Instruments, Inc.
20450 Century Blvd
Germantown MD 20878
240-643-0836
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