solomani
Level 1 (32 points)
Mac OS X

Q: Bootcamp Partition Oddness

Hi all,

I recently created a Boot Camp partition on my Fusion drive which I then had to remove as Windows failed to install (different issue).  I then see my drive looks like this which is fine:

 

elektra FUSION.png

 

 

But oddly the drive is actually two partitions:

Spilt Fusion.png

 

And when Boot Camp created the partition it put it in the middle of my Fusion drive.  Logically I assume it doesn't matter so long as the OS knows that OSX partition 1 and OSX partition 3 are one logical fusion drive (with WIN partition 2 being Boot Camp) but it does seem weird. Why would Boot Camp do this instead of putting the partition at the end?

 

For clarity, the Fusion drive was wiped and clean so it wasn't due to file fragmentation.

 

Thanks.

Posted on Aug 22, 2016 7:49 PM

by Loner T,Solvedanswer
Loner T Loner T
Level 7 (23,663 points)
Safari
A:

This is normal and expected behavior. On a Mac, when a Hybrid MBR is used to support legacy BIOS/MBR installations, it has two limitations.

 

1. It supports only four (4) entries.

2. It supports only 32-bit integers. This limits the MBR to 2TB size and 2TB addressable blocks.

 

BCA assistant locates the Windows partition in the middle of the CS LVG and then spans the two chunks into one. Use Time Machine to coalesce these into contiguous physical disks.

 

This limitation does not exist on GPT/EFI Macs (late 2013 and later models). W7 has many issues with GPT/EFI and is not used on Macs, but can be installed using GPT/EFI on PCs.

Posted on Aug 23, 2016 4:07 AM

Close
solomani

Q: Bootcamp Partition Oddness

  • All replies
  • Helpful answers

  • by Loner T,Solvedanswer

    Loner T Loner T Aug 23, 2016 4:07 AM in response to solomani
    Level 7 (23,663 points)
    Safari
    Aug 23, 2016 4:07 AM in response to solomani

    This is normal and expected behavior. On a Mac, when a Hybrid MBR is used to support legacy BIOS/MBR installations, it has two limitations.

     

    1. It supports only four (4) entries.

    2. It supports only 32-bit integers. This limits the MBR to 2TB size and 2TB addressable blocks.

     

    BCA assistant locates the Windows partition in the middle of the CS LVG and then spans the two chunks into one. Use Time Machine to coalesce these into contiguous physical disks.

     

    This limitation does not exist on GPT/EFI Macs (late 2013 and later models). W7 has many issues with GPT/EFI and is not used on Macs, but can be installed using GPT/EFI on PCs.

  • by solomani,

    solomani solomani Aug 24, 2016 12:06 AM in response to Loner T
    Level 1 (32 points)
    Mac OS X
    Aug 24, 2016 12:06 AM in response to Loner T

    Thanks Loner.  I did end up putting the BootCamp partition back with Windows 10.  Are you saying that if I do a Time Machine Restore to the HD it will coalesce the partitions without breaking the boot camp Windows 10 install?

     

    Am I right in saying that two partitions in one logical drive makes no practical difference since OSX will treat it as one drive?

  • by Loner T,Helpful

    Loner T Loner T Aug 24, 2016 5:44 AM in response to solomani
    Level 7 (23,663 points)
    Safari
    Aug 24, 2016 5:44 AM in response to solomani

    solomani wrote:

     

    I did end up putting the BootCamp partition back with Windows 10.  Are you saying that if I do a Time Machine Restore to the HD it will coalesce the partitions without breaking the boot camp Windows 10 install?

    Please do not do this. It will definitely break Windows. If you decide to remove Windows, then TM can do this.

     

     

    Am I right in saying that two partitions in one logical drive makes no practical difference since OSX will treat it as one drive?

    There is a difference (to be pedantic). A CS LVG typically uses a 'faster' disk at the head of the CS LV, and remaining one or more disks are 'slower'.  There is some block movement between the two parts. In your case, you will have this arrangement on the disk as

     

    Fusion drive without Bootcamp

     

    1. SSD (Faster)

    2. OSX (HDD slower)

     

    Fusion drive with Bootcamp

     

    1. SSD (Faster)

    2. OSX (HDD Part 1 - slower)

    3. Bootcamp (CS unaware)

    4. OSX (HDD Part 2 - slower)

     

    This can at times cause delays in fetching blocks during disk IO. In the overall scheme, this is still faster than traditional mechanical disks.

  • by solomani,

    solomani solomani Aug 24, 2016 5:44 AM in response to Loner T
    Level 1 (32 points)
    Mac OS X
    Aug 24, 2016 5:44 AM in response to Loner T

    That makes sense, thanks for the clarification.