Decrease DMG Size to Fit on a Single Layer DVD

I wanted to create a recovery DVD for my Mac OS X install and all I had was an old DMG of Mac OS X version 10.6.

hdiutil Image Information

Here is the information regarding the DMG file:

kelatov@kmac:~$hdiutil imageinfo OS_X-10.6.dmg
Format Description: UDIF read-only compressed (zlib)
Class Name: CUDIFDiskImage
Checksum Type: CRC32
Size Information:
    Compressed Ratio: 0.84183833618791892
    Total Empty Bytes: 83627520
    Sector Count: 13158216
    Total Bytes: 6737006592
    CUDIFEncoding-bytes-wasted: 0
    Total Non-Empty Bytes: 6653379072
    CUDIFEncoding-bytes-in-use: 5601069940
    Compressed Bytes: 5601069940
    CUDIFEncoding-bytes-total: 5601069940
Checksum Value: $977CE522
Segments:
    0: /Users/kelatov/OS_X-10.6.dmg
Partition Information:
    0:
        Name: whole disk (Apple_HFS : 0)
        Partition Number: 0
        Checksum Type: CRC32
        Checksum Value: $D528D8A8
Format: UDZO
Backing Store Information:
    URL: file://localhost/Users/kelatov/OS_X-10.6.dmg
    Name: OS_X-10.6.dmg
    Class Name: CUDIFEncoding
    Backing Store Information:
        URL: file://localhost/Users/kelatov/OS_X-10.6.dmg
        Name: OS_X-10.6.dmg
        Class Name: CBSDBackingStore
partitions:
    partition-scheme: none
    block-size: 512
    appendable: true
    partitions:
        0:
            partition-name: whole disk
            partition-start: 0
            partition-synthesized: true
            partition-length: 13158216
            partition-hint: Apple_HFS
            partition-filesystems:
                HFS+:
    burnable: true
udif-ordered-chunks: true
Properties:
    Encrypted: false
    Kernel Compatible: true
    Checksummed: true
    Software License Agreement: false
    Partitioned: false
    Compressed: true
Resize limits (per hdiutil resize -limits):
 min     cur     max
13158216    13158216    13158216

You can see that the total size is 6737006592 bytes (about 6.3Gib). That is not going to fit on a Single Layer DVD. Here is the size information of an empty SL-DVD:

$drutil status
 Vendor   Product           Rev
 MATSHITA DVD-R   UJ-8A8    HA13

           Type: DVD-R                Name: /dev/disk1
   Write Speeds: 2x, 4x, 8x
   Overwritable:  510:38:38         blocks:  2297888 /   4.71GB /   4.38GiB
     Space Free:  510:38:38         blocks:  2297888 /   4.71GB /   4.38GiB
     Space Used:   00:00:00         blocks:        0 /   0.00MB /   0.00MiB
    Writability: appendable, blank, overwritable
      Book Type: DVD-R (v5)
       Media ID: RITEKF1

We can see that we can fit about 4.3GiB onto a SL-DVD. When I mount the DMG, I see the following size:

kelatov@kmac:~$hdiutil attach -nobrowse OS_X-10.6.dmg
expected   CRC32 $977CE522
/dev/disk2                                              /Volumes/Mac OS X Install DVD

Here is the df output:

kelatov@kmac:~$df -Ph /Volumes/Mac\ OS\ X\ Install\ DVD/
Filesystem   Size   Used  Avail Capacity  Mounted on
/dev/disk2  6.3Gi  6.2Gi   81Mi    99%    /Volumes/Mac OS X Install DVD

Since the data is compressed the actual file is 5.2GiB:

kelatov@kmac:~$ls -lh OS_X-10.6.dmg
-rw-r--r--@ 1 kelatov  staff   5.2G Jul 17 18:34 OS_X-10.6.dmg

Create a Sparse Image

Let’s create a sparse image and copy the contents of the DMG to that. To create the sparse image we can run the following:

kelatov@kmac:~$hdiutil create -size 7g -type SPARSE -fs HFS+ -volname copy image -layout NONE
created: /Users/kelatov/image.sparseimage

I noticed that the DMG didn’t have a partition schema so I did the same thing for the Sparse Image.

Restore DMG to a Sparse Image

Now we can mount the sparse file. First let’s unmount the DMG:

kelatov@kmac:~$hdiutil detach /Volumes/Mac\ OS\ X\ Install\ DVD/
"disk2" unmounted.
"disk2" ejected.

Now let’s mount the Sparse Image:

kelatov@kmac:~$hdiutil attach -nobrowse image.sparseimage
/dev/disk2                                              /Volumes/copy

Now we can use asr (Apple Software Restore) to basically do a block level copy of the DMG onto the Sparse Image. First let’s scan our DMG:

kelatov@kmac:~$sudo asr -imagescan OS_X-10.6.dmg
Block checksum: ....10....20....30....40....50....60....70....80....90....100
asr: successfully scanned image "/Users/kelatov/OS_X-10.6.dmg"

Now for the copy/restore:

kelatov@kmac:~$asr -s OS_X-10.6.dmg -t /Volumes/copy -erase
    Validating target...done
    Validating source...done
    Erase contents of /dev/disk2 (/Volumes/copy)? [ny]: y
    Retrieving scan information...done
    Validating sizes...done
    Restoring  ....10....20....30....40....50....60....70....80....90....100
    Verifying  ....10....20....30....40....50....60....70....80....90....100
    Remounting target volume...done

Checking the size of the Sparse Image:

kelatov@kmac:~$du -sh image.sparseimage
6.2G    image.sparseimage

After the copy the Sparse was mounted and checking the maximum size:

kelatov@kmac:~$df -Ph /Volumes/Mac\ OS\ X\ Install\ DVD/
Filesystem   Size   Used  Avail Capacity  Mounted on
/dev/disk2  7.0Gi  6.2Gi  824Mi    89%    /Volumes/Mac OS X Install DVD

Notice the maximum size is 7.0GiB, which is what we created the Sparse File to be. Notice that since we did a block level copy the Volume Name was also copied.

Clean Up Un-necessary Files in the New Sparse Image

The Sparse Image is already mounted so let’s clean it up. The Mac OS X install CD contains a lot of printer Drivers, Language Packages, and X11 tools. For recovery purposes we don’t need those. To clean up the Sparse Image, I ran the following:

kelatov@kmac:~$cd /Volumes/Mac\ OS\ X\ Install\ DVD/System/Installation/Packages/
kelatov@kmac:/Volumes/Mac OS X Install DVD/System/Installation/Packages$rm -rf Norwegian.pkg Russian.pkg Polish.pkg Portuguese.pkg SimplifiedChinese.pkg Spanish.pkg Swedish.pkg TraditionalChinese.pkg Japanese.pkg Italian.pkg German.pkg French.pkg Finnish.pkg Danish.pkg Dutch.pkg Korean.pkg HP_* Canon_* Lexmark_* X11User.pkg Samsung_Common.pkg BrazilianPortuguese.pkg Brother_* EPSON_*
kelatov@kmac:/Volumes/Mac OS X Install DVD/System/Installation/Packages$rm -rf ../../../Optional\ Installs.localized/Packages/

After you are done cleaning up superflous files, the used size should be about 4.2GiB:

kelatov@kmac:~$df -Ph /Volumes/Mac\ OS\ X\ Install\ DVD/
Filesystem   Size   Used  Avail Capacity  Mounted on
/dev/disk2  7.0Gi  4.2Gi  2.8Gi    61%    /Volumes/Mac OS X Install DVD

Compact Sparse Image

Now we can reclaim the unused space in the Sparse image and resize it:

kelatov@kmac:~$hdiutil compact image.sparseimage
Starting to compact…
Reclaiming free space…
...............................................................................
Finishing compaction…
...............................................................................
Reclaimed 1.9 GB out of 2.8 GB possible.

Notice we didn’t get all of the possible space. Looking over the size information of the image, we see the following:

hdiutil imageinfo image.sparseimage | grep "Size Information" -A 6
Size Information:
    Total Bytes: 7516192768
    Compressed Ratio: 1
    Sector Count: 14680064
    Total Non-Empty Bytes: 4639948800
    Compressed Bytes: 7516192768
    Total Empty Bytes: 2876243968

we can also check the limits set on the Sparse file:

kelatov@kmac:~$hdiutil resize -limits image.sparseimage
 min     cur     max
13158272    14680064    34359738368

the values are in 512 sized sectors. So our minimum possible size is (13158272 * 512) 6737035264 bytes. So I can resize the image to that by running the following:

kelatov@kmac:~$hdiutil resize -size min image.sparseimage
kelatov@kmac:~$hdiutil imageinfo image.sparseimage | grep "Size Information" -A 6
Size Information:
    Total Bytes: 6737035264
    Compressed Ratio: 1
    Sector Count: 13158272
    Total Non-Empty Bytes: 4638900224
    Compressed Bytes: 6737035264
    Total Empty Bytes: 2098135040

Notice the size has changed, but it didn’t go as low as 4.3GiB (as shown by Total Non-Empty Bytes). The reason for this is because the sparse image is not defragmented. This is discussed in How to reclaim all/most free space from a sparsebundle on OS X.

From here we have two options. If you have access to iDefrag then you can use that to defragment the Sparse Image to reclaim more space, or we can create a smaller Sparse Image and copy the contents manually.

Use iDefrag to Defragment the Sparse Image

Luckily, I had iDefrag :). Mount your Sparse image:

kelatov@kmac:~$hdiutil attach image.sparseimage

then start up iDefrag and you will see you drive there. Click “Go” and it will start defragmenting:

iDefrag started Decrease DMG Size to Fit on a Single Layer DVD

after it’s done you will the following message:

iDefrag finished Decrease DMG Size to Fit on a Single Layer DVD

at this point you can quit iDefrag and unmount the Sparse Image:

kelatov@kmac:~$hdiutil detach /dev/disk2
"disk2" unmounted.
"disk2" ejected.

Then we can try to compact again and we will see the following:

kelatov@kmac:~$hdiutil compact image.sparseimage
Starting to compact…
Reclaiming free space…
...............................................................................
Finishing compaction…
...............................................................................
Reclaimed 2.0 GB out of 2.0 GB possible.

Checking the new size limits:

kelatov@kmac:~$hdiutil resize -limits image.sparseimage
 min     cur     max
8862888 13158272    34359738368

we can go to (8862888 * 512) 4537798656 bytes (less than 4.3 GiB). Now for the actual resize:

kelatov@kmac:~$hdiutil resize -size min image.sparseimage

and here is the new size:

kelatov@kmac:~$hdiutil imageinfo image.sparseimage  | grep "Size Information" -A 6
Size Information:
    Total Bytes: 4537798656
    Compressed Ratio: 1
    Sector Count: 8862888
    Total Non-Empty Bytes: 4537798656
    Compressed Bytes: 4537798656
    Total Empty Bytes: 0

that looks much better.

Copy Contents of Sparse Image into a Smaller Sparse Image

If you don’t have access to iDefrag, you can create a smaller sparse image:

kelatov@kmac:~$hdiutil create -size 4.3g -type SPARSE -fs HFS+ -volname Small_Sparse image2 -layout NONE
created: /Users/kelatov/image2.sparseimage

Now we can mount both our images:

kelatov@kmac:~$hdiutil attach -nobrowse image.sparseimage
/dev/disk2                                              /Volumes/Mac OS X Install DVD
kelatov@kmac:~$hdiutil attach -nobrowse image2.sparseimage
/dev/disk3                                              /Volumes/Small_Sparse

Now we can just copy from the first sparse image to the second (smaller) sparse file. First check to make sure the source is smaller than the destination:

kelatov@kmac:~$df -Ph /Volumes/*
Filesystem        Size   Used  Avail Capacity  Mounted on
/dev/disk2       4.2Gi  4.2Gi    0Bi   100%    /Volumes/Mac OS X Install DVD
/dev/disk3       4.3Gi   72Mi  4.2Gi     2%    /Volumes/Small_Sparse

asr used to be able to do file-level copies, but now only block level copies/restores are allowed. From the man page:

When run in its first form above, the --erase option must always be used,
as asr no longer supports file copying.  Such functionality is done bet-
ter by ditto(1).

I prefer rsync over ditto (ditto vs. rsync), so here is what I ran to copy the contents onto the smaller sparse image:

kelatov@kmac:~$sudo rsync -avzP /Volumes/Mac\ OS\ X\ Install\ DVD/. /Volumes/Small_Sparse/.
...
...
usr/standalone/i386/boot.efi
      319152 100%  301.72kB/s    0:00:01 (xfer#16938, to-check=0/28139)

sent 3435096577 bytes  received 439862 bytes  6850521.31 bytes/sec
total size is 4458227175  speedup is 1.30

From here we can either convert the Sparse Image back to a DMG or an ISO/CDR. Let’s unmount both:

kelatov@kmac:~$hdiutil detach /Volumes/Mac\ OS\ X\ Install\ DVD/
"disk2" unmounted.
"disk2" ejected.
kelatov@kmac:~$hdiutil detach /Volumes/Small_Sparse/
"disk3" unmounted.
"disk3" ejected.

Convert Sparse Image to DMG

We can use this command to convert our sparse image to a DMG:

kelatov@kmac:~$hdiutil convert image.sparseimage -format UDBZ -o Mac_OS_10_6_Custom.dmg
Preparing imaging engine…
Reading whole disk (Apple_HFS : 0)…
...............................................................................
   (CRC32 $9B980201: whole disk (Apple_HFS : 0))
Adding resources…
...............................................................................
Elapsed Time:  3m 53.244s
File size: 3404005521 bytes, Checksum: CRC32 $FD3402E5
Sectors processed: 8862888, 8862881 compressed
Speed: 18.6Mbytes/sec
Savings: 25.0%
created: /Users/kelatov/Mac_OS_10_6_Custom.dmg

Just for reference here are the available formats:

UDRW - UDIF read/write image
UDRO - UDIF read-only image
UDCO - UDIF ADC-compressed image
UDZO - UDIF zlib-compressed image
UDBZ - UDIF bzip2-compressed image (OS X 10.4+ only)
UFBI - UDIF entire image with MD5 checksum
UDRo - UDIF read-only (obsolete format)
UDCo - UDIF compressed (obsolete format)
UDTO - DVD/CD-R master for export
UDxx - UDIF stub image
UDSP - SPARSE (grows with content)
UDSB - SPARSEBUNDLE (grows with content; bundle-backed)
RdWr - NDIF read/write image (deprecated)
Rdxx - NDIF read-only image (Disk Copy 6.3.3 format)
ROCo - NDIF compressed image (deprecated)
Rken - NDIF compressed (obsolete format)
DC42 - Disk Copy 4.2 image

taken from the man page. With bzip2 compression the end file will be even smaller:

kelatov@kmac:~$du -sh Mac_OS_10_6_Custom.dmg
3.2G    Mac_OS_10_6_Custom.dmg

Now we can burn the file, by just running:

kelatov@kmac:~$hdiutil burn Mac_OS_10_6_Custom.dmg
Preparing data for burn
Opening session
Opening track
Writing track
..............................................................................
Closing track
..............................................................................
Closing session
Finishing burn
Verifying burn…
Verifying
...............................................................................
Burn completed successfully
...............................................................................
hdiutil: burn: completed

Convert Sparse Image to ISO

This can be accomplished with hdiutil in multiple ways. The first one is with the following command:

kelatov@kmac:~$hdiutil convert image.sparseimage -format UDTO -o MAC_OS_10_6_Custom.cdr
Reading whole disk (Apple_HFS : 0)…
...............................................................................
Elapsed Time:  4m 38.798s
Speed: 15.5Mbytes/sec
Savings: 0.0%
created: /Users/kelatov/MAC_OS_10_6_Custom.cdr

the second way is like this:

kelatov@kmac:~$sudo hdiutil makehybrid -iso -joliet -o MAC_OS_10_6_Custom.iso image.sparseimage
Creating hybrid image...
..............................................................................

both can be burned by using the ‘hdiutil burn’ command or with any burning software.

Mac OS X 10.8 Recovery Options

Since I was on Mac OS 10.8 there are a couple of other options for recovery media.

1. Resize OS X Mountain Lion installer to fit on a 4.7 GB DVD

The 10.8 Installer is downloadable via App Store and even though it’s doesn’t fit on a regular DVD you just have to copy the contents into a smaller image. No deletion of any files is necessary. The instructions on how to accomplish that are here:

From the bottom link (Mac OS X hints), here is a snippet of the code that can accomplishes our goal:

#!/bin/bash
rm -f /private/tmp/Mountain\ Lion\ DVD\ Image\ read-write.dmg # Remove any old copies of the DVD image before we begin.

echo "Creating DVD Image..."
hdiutil create -size 4.2g -volname "Mac OS X Install ESD" /private/tmp/Mountain\ Lion\ DVD\ Image\ read-write.dmg -fs HFS+ -layout SPUD

hdiutil attach -nobrowse ~/Desktop/InstallESD.dmg
hdiutil attach -nobrowse /private/tmp/Mountain\ Lion\ DVD\ Image\ read-write.dmg

echo "Copying Mountain Lion to new image..."
cp -pRv /Volumes/Mac\ OS\ X\ Install\ ESD/* /Volumes/Mac\ OS\ X\ Install\ ESD\ 1/

hdiutil detach /Volumes/Mac\ OS\ X\ Install\ ESD\ 1
hdiutil detach /Volumes/Mac\ OS\ X\ Install\ ESD

echo "Converting to read-only..."
hdiutil convert /private/tmp/Mountain\ Lion\ DVD\ Image\ read-write.dmg -format UDZO -o ~/Desktop/Mountain\ Lion\ DVD\ Image.dmg

rm -f /private/tmp/Mountain\ Lion\ DVD\ Image\ read-write.dmg

echo "Image Creation Complete. Please burn '~/Desktop/Mountain Lion DVD Image.dmg' to a DVD using Disk Utility."
open ~/Desktop/

2. Mac OS X 10.8 Has a Built-in Recovery Partition

From “OS X Recovery”:

The new Mac safety net. OS X Recovery lets you repair disks or reinstall OS X without the need for a physical install disc. Since OS X Recovery is built into your Mac, it’s always there when you need it. Even if you don’t need it, it’s good to know it’s there. And you don’t have to search through original packaging to find install DVDs to get your Mac back up and running.

Command-R to the rescue. Just hold down Command-R during startup and OS X Recovery springs into action. It lets you choose from common utilities: You can run Disk Utility to check or repair your hard drive, erase your hard drive and reinstall a fresh copy of OS X, or restore your Mac from a Time Machine backup. You can even use Safari to get help from Apple Support online. And if OS X Recovery encounters problems, it will automatically connect to Apple over the Internet.

So I can just reboot the Mac and hold Command-R to get into recovery mode.

3. Use OS X Lion’s Recovery Disk Assistant to create a bootable USB recovery Disk

Starting with OS X Lion, you can create a recovery USB disk to help with recovery. Instructions on how to do that are here:

From “OS X Recovery Disk Assistant v1.0”:

The OS X Recovery Disk Assistant lets you create OS X Recovery on an external drive that has all of the same capabilities as the built-in OS X Recovery: reinstall Lion or Mountain Lion, repair the disk using Disk Utility, restore from a Time Machine backup, or browse the web with Safari.

Well at least I have a cheap bootable OS 10.6 DVD now :)