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Re: Why 262144 blocks next to each partition?
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Re: Why 262144 blocks next to each partition?



They're for future use by Apple, and are populated as needed for some boot methods, such as AppleRAID booting on PowerPC systems

Shantonu

On Aug 1, 2006, at 10:11 AM, Howard Gayle wrote:

On 10.4.7, diskutil (or Disk Utility) places an area of 262144
512-byte blocks after each GPT partition, except the last
partition, which gets 262151 512-byte blocks.

Here's an example of partitioning a 320 GB disk into 7
partitions:
diskutil partitionDisk disk1 7 GPTFormat \
   'Journaled HFS+' s01b_root0 53.9G \
   UFS s01b_utils0 43.2G \
   'Journaled HFS+' s01b_root1 53.9G \
   UFS s01b_utils1 43.2G \
   'Journaled HFS+' s01b_root2 53.9G \
   UFS s01b_utils2 43.2G \
   'Journaled HFS+' s01b_rest 1G

Finished partitioning on disk disk1
/dev/disk1
   #:                   type name               size      identifier
   0:  GUID_partition_scheme                    *298.1 GB disk1
   1:                    EFI                    200.0 MB  disk1s1
   2:              Apple_HFS s01b_root0         53.8 GB   disk1s2
   3:              Apple_UFS s01b_utils0        41.7 GB   disk1s3
   4:              Apple_HFS s01b_root1         53.8 GB   disk1s4
   5:              Apple_UFS s01b_utils1        41.7 GB   disk1s5
   6:              Apple_HFS s01b_root2         53.8 GB   disk1s6
   7:              Apple_UFS s01b_utils2        41.7 GB   disk1s7
   8:              Apple_HFS s01b_rest          6.5 GB    disk1s8

Now look what gpt(8) shows:
sudo gpt -r -v show /dev/disk1
gpt show: /dev/disk1: mediasize=320072933376; sectorsize=512; blocks=625142448
start size index contents
0 1 PMBR
1 1 Pri GPT header
2 32 Pri GPT table
34 6
40 409600 1 GPT part - C12A7328-F81F-11D2- BA4B-00A0C93EC93B
409640 112774352 2 GPT part - 48465300-0000-11AA- AA11-00306543ECAC
113183992 262144
113446136 90334824 3 GPT part - 55465300-0000-11AA- AA11-00306543ECAC
203780960 262144
204043104 112774352 4 GPT part - 48465300-0000-11AA- AA11-00306543ECAC
316817456 262144
317079600 90334824 5 GPT part - 55465300-0000-11AA- AA11-00306543ECAC
407414424 262144
407676568 112774352 6 GPT part - 48465300-0000-11AA- AA11-00306543ECAC
520450920 262144
520713064 90334824 7 GPT part - 55465300-0000-11AA- AA11-00306543ECAC
611047888 262144
611310032 13570232 8 GPT part - 48465300-0000-11AA- AA11-00306543ECAC
624880264 262151
625142415 32 Sec GPT table
625142447 1 Sec GPT header


With an Apple partition map it looks like this:
diskutil partitionDisk disk1 7 APMFormat \
   'Journaled HFS+' s01b_root0 53.9G \
   UFS s01b_utils0 43.2G \
   'Journaled HFS+' s01b_root1 53.9G \
   UFS s01b_utils1 43.2G \
   'Journaled HFS+' s01b_root2 53.9G \
   UFS s01b_utils2 43.2G \
   'Journaled HFS+' s01b_rest 1G
Finished partitioning on disk disk1
/dev/disk1
   #:                   type name               size      identifier
   0: Apple_partition_scheme                    *298.1 GB disk1
   1:    Apple_partition_map                    31.5 KB   disk1s1
   2:              Apple_HFS s01b_root0         53.8 GB   disk1s3
   3:             Apple_Boot                    8.5 MB    disk1s4
   4:              Apple_UFS s01b_utils0        41.9 GB   disk1s5
   5:              Apple_HFS s01b_root1         53.8 GB   disk1s7
   6:             Apple_Boot                    8.5 MB    disk1s8
   7:              Apple_UFS s01b_utils1        41.9 GB   disk1s9
   8:              Apple_HFS s01b_root2         53.8 GB   disk1s11
   9:             Apple_Boot                    8.5 MB    disk1s12
  10:              Apple_UFS s01b_utils2        41.9 GB   disk1s13
  11:              Apple_HFS s01b_rest          6.7 GB    disk1s15
3:17

sudo pdisk
e /dev/rdisk1
p
Partition map (with 512 byte blocks) on '/dev/rdisk1'
#: type name length base ( size )
1: Apple_partition_map Apple 63 @ 1
2: Apple_Free 262144 @ 64 (128.0M)
3: Apple_HFS Apple_HFS_Untitled_1 112774352 @ 262208 ( 53.8G)
4: Apple_Boot eXternal booter 17408 @ 113036560 ( 8.5M)
5: Apple_UFS Apple_UFS_Untitled_2 90579560 @ 113053968 ( 43.2G)
6: Apple_Free 262144 @ 203633528 (128.0M)
7: Apple_HFS Apple_HFS_Untitled_3 112774352 @ 203895672 ( 53.8G)
8: Apple_Boot eXternal booter 17408 @ 316670024 ( 8.5M)
9: Apple_UFS Apple_UFS_Untitled_4 90579560 @ 316687432 ( 43.2G)
10: Apple_Free 262144 @ 407266992 (128.0M)
11: Apple_HFS Apple_HFS_Untitled_5 112774352 @ 407529136 ( 53.8G)
12: Apple_Boot eXternal booter 17408 @ 520303488 ( 8.5M)
13: Apple_UFS Apple_UFS_Untitled_6 90579560 @ 520320896 ( 43.2G)
14: Apple_Free 262144 @ 610900456 (128.0M)
15: Apple_HFS Apple_HFS_Untitled_7 13979832 @ 611162600 ( 6.7G)
16: Apple_Free 16 @ 625142432


Device block size=512, Number of Blocks=625142448 (298.1G)

This time, the 262144-block area comes before each partition, and
the one at the end is the same size.

I didn't notice anything in IOApplePartitionScheme or
IOGUIDPartitionScheme that seemed to be looking for these areas.

What are they for?
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References: 
 >Why 262144 blocks next to each partition? (From: Howard Gayle <email@hidden>)



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