Disk Image Modes
Bochs can handle independent disk image format for each disk present on the ata interfaces. The disk image type is selected in the configuration file by the "mode" option of the ataX-xxx directives. Example:
ata0-master: type=disk, mode=flat, path=10M.sample, cylinders=306, heads=4, spt=17 |
Note: If unspecified, the default "mode" is flat.
Table 8-3. Supported Disk Modes
| Name | Description | Features |
|---|---|---|
| flat | one file, flat layout | accessible with mtools or winimage-like tools |
| concat | multiple files, concatenated | mappable to contained partitions |
| external | accessed through an external C++ class | developer specific, needs a C++ class at compile time |
| dll | accessed through a DLL | developer specific, windows only |
| sparse | up to 10 layers stackable files | commitable, rollbackable, growing |
| vmware3 | vmware3 disk support | vmware3 compatibility |
| undoable | flat file with a commitable redolog | commitable, rollbackable |
| growing | one growing file | growing |
| volatile | flat file with a volatile redolog | always rollbacked |
flat
image creation
Flat disk images can be created with the bximage utility (see the Section called Using the bximage tool for more information).
path
The "path" option of the ataX-xxx directive in the configuration file must point to the flat image file.
external tools
Flat images content can be accessed from the host by the following tools :
mtools (see the Section called Use mtools to manipulate disk images)
mount with a loopback (see the Section called Linux: Mounting a disk image using the loop device)
Winimage / DiskExplorer (see the Section called Win32 only: Tools to manipulate disk images)
Bochs Tools (see the Section called Bochs GNU/Linux DiskTools)
concat
text from old enable-split-hd option, to be completed
When enabled, this allows a series of partial hard disk image files to be
treated as if it was one large file. The .bochsrc specifies the first
partial HD image (example win95-1) and then bochs searches for the other
partial images in as a sequence (win95-2, win95-3, etc.) and opens them
all. Then, it treats the series as if there was a single large file
created by "cat win95-1 win95-2 win95-3".
All files must be a multiple of 512 bytes. |
// This option enables "split hard drive" support, which means // that a series of partial hard disk images can be treated // as a single large image. If you set up the partition sizes and // file sizes correctly, this allows you to store each partition // in a separate file, which is very convenient if you want to operate // on a single partition (e.g. mount with loopback, create filesystem, // fsck, etc.). // [[Provide example of partitioning]] |
description
In concat mode, all sectors of the harddisk are stored in several flat files, in lba order.
sparse
description
Sparse disk support has been added by JustinSB. Sparse disk features are:
Large hard drive can be created, and only used space will be stored in the file. In practice, on Unix, this is not a large gain as it is done anyway.
Multiple sparse drive images can be mounted on top of each other. Writes go to the top image. This allows several similar configurations to share a master "base" file, and also allows filesystem rollback or no-write options. Up to 10 disk images can be layered on top of each other.
image creation
Sparse disk images must be created with the bximage utility (see the Section called Using the bximage tool for more information). Be sure to enter "sparse" when selecting the image type.
path
The "path" option of the ataX-xxx directive in the configuration file must point to the top layered file. The lower layer files names are found by substracting 1 from the last character (must be a digit)
typical use
Space Saving
Create a sparse disk image using bximage. Set size to eg 10GB. Only allocated space will be stored, so your drive image should be only about as large as the files stored on it.
Disk Rollback
Create a sparse disk image called "c.img.0". Point .bochsrc at "c.img.0". In bochs, install your favourite OS. Switch off bochs.
Create a sparse disk image (of the same size) and name it "c.img.1". Point .bochsrc at "c.img.1" "c.img.0" is visible, but all writes go to "c.img.1". After using bochs, you can simply delete "c.img.1" to undo changes and go back to a clean OS install.
Disk Optional Commit
Create a sparse disk image called "c.img.0". Point .bochsrc at "c.img.0". In bochs, install your favourite OS. Switch off bochs.
Create a sparse disk image (of the same size) and name it "c.img.1". Point .bochsrc at "c.img.1" "c.img.0" is visible, but all writes go to "c.img.1". After using bochs, if you want to keep the changes, use the (currently non-existant) merge utility to make a single unified drive image.
Alternatively simply create a new partition on top called "c.img.2".
Common Base
Create a sparse disk image called "base.img". Point .bochsrc at "base.img". In bochs, install your favourite OS. Switch off bochs.
Create a sparse disk image (of the same size) and name it "www.img.1". Make "wwww.img.0" a symlink to "base.img". Point .bochsrc at "www.img.1". Using bochs, install a webserver.
Create a symlink to "base.img" called "db.img.0". Create a sparse disk image (of the same size) and name it "db.img.1". Point .bochsrc at "db.img.1". Using bochs, install a database server.
Now both a database server and webserver can be run in separate virtual machines, but they share the common OS image, saving drive space.
vmware3
description
Sharvil Nanavati has added vmware3 disk image support into Bochs for Net Integration Technologies, Inc. You should be able to use disk images created by vmware3.
undoable
description
Undoable disks are commitable/rollbackable disk images. An undoable disk is based on a read-only flat image (see the Section called flat), associated with a growing redolog, that contains all changes (writes) made to the flat image content.
This redolog is dynamically created at runtime, if it does not previously exists.
All writes go to the redolog, reads are done from the redolog if previously written, or from the flat file otherwise.
If unspecified with the "journal" option of the ataX-xxx directive, the redolog file name is created by adding a ".redolog" suffix to the flat image name.
File size of the redolog can grow up to the total disk size plus a small overhead due to internal data managment (about 3% for a 32MiB disk, less than 0.5% for a 2GiB disk).
After a run, the redolog will still be present, so the changes are still visible the next time you run Bochs with this disk image.
After a run, the redolog can be committed (merged) to the flat image with the bxcommit utility.
After a run, the redolog can be rollbacked (discarded) by simply deleting the redolog file.
Note: In this mode, the flat file is always open in read-only mode, so it can safely be stored on a read-only medium (for example on a cdrom).
image creation
The flat disk images must be created with the bximage utility (see the Section called Using the bximage tool for more information). The growing redolog is created automatically if needed.
path
The "path" option of the ataX-xxx directive in the configuration file must be the flat image name. The redolog name can be set with the "journal" option of the same directive. If not set, the redolog name is created by adding the ".redolog" suffix to the flat image name.
external tools
See the Section called external tools for tools to access the flat disk image content.
Note: The up-to-date content can only be seen after you commit the redolog to the flat file with the bxcommit utility.
growing
description
Growing disk images start as a small files, and grow whenever new data is written to them.
Once a sector is written in the growing file, subsequent writes to the same sector will happen in place.
File size of Growing disk images can go up to the total disk size plus a small overhead due to internal data managment. (about 3% for a 32MiB disk, less than 0.5% for a 2GiB disk).
image creation
Growing disk images must be created with the bximage utility (see the Section called Using the bximage tool for more information). Be sure to enter "growing" when selecting the image type.
path
The "path" option of the ataX-xxx directive in the configuration file must be the growing image name.
volatile
description
Volatile disks are always-rollbacked disk images. An volatile disk is based on a read-only flat image (see the Section called flat), associated with a growing temporary redolog, that contains all changes (writes) made to the flat image content. All data written to the disk image are lost at the end of the Bochs session.
The redolog is dynamically created at runtime, when Bochs starts, and is deleted when Bochs closes (win32) or just after it has been created (unix).
All writes go to the redolog, reads are done from the redolog if previously written, or from the flat file otherwise.
If unspecified with the "journal" option of the ataX-xxx directive, the redolog file name is created by adding a ".redolog" suffix to the flat image name.
File size of the redolog can grow up to the total disk size plus a small overhead due to internal data managment (about 3% for a 32MiB disk, less than 0.5% for a 2GiB disk).
After a run, the redolog is not any more present, so the changes are discarded.
Note: In this mode, the flat file is always open in read-only mode, so it can safely be stored on a read-only medium (for example on a cdrom).
image creation
The flat disk images must be created with the bximage utility (see the Section called Using the bximage tool for more information). The growing redolog is created automatically.
path
The "path" option of the ataX-xxx directive in the configuration file must be the flat image name. The redolog name can be set with the "journal" option of the same directive. If not set, the redolog name is created by adding the ".redolog" suffix to the flat image name. A random suffix is also appended to the redolog name.
external tools
See the Section called external tools for tools to access the flat disk image content.