Some background information on drive partitioning

Jun 25, '02 01:13:23AM

Contributed by: Anonymous

For those planning to partition their drives (e.g. for Linux installation side by side with MacOSX, or to create separate partitions for /Users, Swap, Scratch and so on...), take care to not exceed six to eight partitions (on a single disk, that is).

The pdisk man page (% man pdisk) recommends:

Creating more than fifteen partitions is not advised. There is currently a bug in the some (all?) of the kernels which causes access to the whole disk fail if more than fifteen partitions are in the map.
I can confirm this bug for the Linux kernel, and others have observed the same under Darwin. Remember that Apple partition maps and drivers take the first eight to ten partitions.

Read the rest of the article to see a typical partition map and for a bit more info on this subject...

[Editor's note: I have no personal knowledge of this potential issue, so I can't confirm this article. I'm publishing it in the hopes that it inspires some comments either confirming or denying the presence of the bug in the OS X kernel.]

Here is how it looks on the (only) 20GB disk in my iBook600:

/dev/disk0  map block size=512
#: type name length base ( size )
1: Apple_partition_map Apple 63 @ 1
2: Apple_Driver43*Macintosh 54 @ 64
3: Apple_Driver43*Macintosh 74 @ 118
4: Apple_Driver_ATA*Macintosh 54 @ 192
5: Apple_Driver_ATA*Macintosh 74 @ 246
6: Apple_FWDriver Macintosh 200 @ 320
7: Apple_Driver_IOKit Macintosh 512 @ 520
8: Apple_Patches Patch Partition 512 @ 1032
9: Apple_HFS MacOSX 18432000 @ 1544 ( 8.8G)
10: Apple_HFS MacOS9 10240000 @ 18433544 ( 4.9G)
11: Apple_HFS Scratch 1331200 @ 28673544 (650.0M)
12: Apple_HFS Apple_Bootstrap 65536 @ 30004744 ( 32.0M)
13: Apple_HFS Linux_Swap 524288 @ 30070280 (256.0M)
14: Apple_HFS Linux_Root 6511418 @ 30594568 ( 3.1G)
15: Apple_HFS Linux_Home 1964094 @ 37105986 (959.0M)
Also, if you partition your driver with Apple Drive Setup, the program will create an unallocated partition at the end of the drive (Apple_Free type) to top the total number of blocks (so that there won't be spare blocks). This will impact the partition process in two ways:
  1. It will subtract one partition out of 15 :-(
  2. It will steal valuable disk space (in my case 959.0M)
As you can see above, wit pdisk it is possible to rescue this unallocated partition and to convert it to a usable partition ;-)

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Mac OS X Hints
http://hints.macworld.com/article.php?story=2002062501132315