Partitioning

Disks or any other block devices with partition tables have their LMI_StorageExtent or its subclass associated to LMI_DiskPartitionConfigurationCapabilities using LMI_InstalledPartitionTable.

A GPT partition present on a block device are represented as LMI_GenericDiskPartition.

A MS-DOS partition present on a block device are represented as LMI_DiskPartition.

Both MS-DOS and GPT partitions are associated to the parent device using LMI_PartitionBasedOn. This BasedOn association contains also start and end sectors of the partitions. Note that logical partitions are associated with the extended partition where they are located, see the diagram below.

Following instance diagram shows /dev/sda disk with MS-DOS partition table and:

• 3 primary partitions
• 1 extended partition
  • 2 logical partitions

Especially note that the extended partition /dev/sda4 contains an extended partition table and all logical partitions are based on this extended partition. This is for compatibility with SMI-S and also it better illustrates physical composition of the partitions on the disk.

However, to create a partition on the device, applications can use both /dev/sda or /dev/sda4 as value of Extent parameter in LMI_CreateOrModifyPartition, call.

Useful methods

LMI_CreateOrModifyPartition

Creates a partition on a device with GPT or MS-DOS partition table. Currently no special setting can be set and the method automatically creates extended and logical partitions when the primary partition table is getting full.

SetPartitionStyle

Creates partition table on a device of requested size. If the size is not specified, the largest possible partition is created.

FindPartitionLocation

Finds start and end sector where a partition would be created and returns size of the partition.

LMI_DeletePartition

Destroys a partition.

The partitioning support is basic and has several limitations. Currently it is not possible to explicitly set partition position. OpenLMI chooses the best available free space for requested partitions. Typically, on empty disks, the partitions are created one after another.

Currently it is not possible to explicitly create extended or logical partitions, these are created automatically when there are too much primary partitions:

• If there are less than 3 primary partitions on the device, primary partition is created.
• If there are exactly 3 primary partitions on the device and one of them extended, primary partition is created.
• If there are exactly 3 primary partitions on the device and none of them extended, one extended partition is created over the largest continuous unpartitioned space on the device. Then, requested logical partition is created on it. Therefore, single call to LMI_CreateOrModifyPartition can create one extended and one logical partition.
• If there is extended partition present on the device and there are 4 primary partitions, a logical partition is created.

Use cases

List supported partition table types

Currently GPT and MS-DOS partition tables are supported. More types can be added later. Enumerate instances of LMI_DiskPartitionConfigurationCapabilities class to get list of all of them, together with their basic properties like partition table size and maximum number of partitions:

part_styles = root.LMI_DiskPartitionConfigurationCapabilities.instances()
for style in part_styles:
    print style.Caption
    print "Partition table size:", style.PartitionTableSize, "block(s)"

Create partition table

Use SetPartitionStyle method.

Sample code to create GPT partition table on /dev/sda:

# Find the disk
sda = root.LMI_StorageExtent.first_instance(
        Key="DeviceID",
        Value="/dev/sda")
# Find the partition table style we want to create there
gpt_caps = root.LMI_DiskPartitionConfigurationCapabilities.first_instance(
        Key="InstanceID",
        Value="LMI:LMI_DiskPartitionConfigurationCapabilities:GPT")
# Create the partition table
partitioning_service.SetPartitionStyle(
        Extent=sda,
        PartitionStyle = gpt_caps)

MS-DOS partition tables are created with the same code, just using different LMI_DiskPartitionConfigurationCapabilities instance.

Create partition

Use LMI_CreateOrModifyPartition method.

Following code creates several partitions on /dev/sda. The code is the same for GPT and MS-DOS partitions:

# Define helper function
def print_partition(partition_name):
    partition = partition_name.to_instance()
    print("Created partition", partition.DeviceID,
            "with", partition.NumberOfBlocks * partition.BlockSize, "bytes.")

# Find the disk
sda = root.LMI_StorageExtent.first_instance(
        Key="DeviceID",
        Value="/dev/sda")

# create 4 partitions with 100 MB each
for i in range(4):
    (ret, outparams, err) = partitioning_service.LMI_CreateOrModifyPartition(
            Extent=sda,
            Size = 100 * MEGABYTE)
print_partition(outparams['partition'])

# Create partition with the whole remaining space - just omit 'Size' parameter
(ret, outparams, err) = partitioning_service.LMI_CreateOrModifyPartition(
        Extent=sda)
print_partition(outparams['partition'])

On an empty disk with GPT partition table this code creates:

• 4 partitions with 100 MB each.
• One partition with the largest continuous unpartitioned space on the disk.

On an empty disk with MS-DOS partition table, the code creates:

• 3 primary partitions, 100 MB each.
• One extended partition with the largest continuous unpartitioned space.
• One 100 MB logical partitions.
• One logical partition with the largest continuous free space on the extended partition.

The resulting partitions can be seen in the diagram above.

List all partitions on a disk

Enumerate LMI_PartitionBasedOn associations of the disk.

Following code lists all partitions on /dev/sda, together with their location:

# Find the disk
sda = root.LMI_StorageExtent.first_instance(
        Key="DeviceID",
        Value="/dev/sda")

based_ons = sda.references(ResultClass="LMI_PartitionBasedOn")
for based_on in based_ons:
print "Found partition", based_on.Dependent['DeviceID'], \
        "at sectors", based_on.StartingAddress, based_on.EndingAddress
# TODO: check extended partition

Find the largest continuous unpartitioned space on a disk

Using side-effect of FindPartitionLocation, we can find size of the largest partition that can be created on /dev/sda:

# Find the disk
sda = root.LMI_StorageExtent.first_instance(
        Key="DeviceID",
        Value="/dev/sda")
# Find LMI_DiskPartitionConfigurationCapabilities associated to the disk
sda_partition_capabilities = sda.associators(
        AssocClass='LMI_InstalledPartitionTable') [0]
# Call its FindPartitionLocation without 'Size' parameter
# - the largest available space is returned.
(ret, outparams, err) = sda_partition_capabilities.FindPartitionLocation(
        Extent = sda)
print "Largest space for a partition:", outparams['size']

Delete partition

Call LMI_DeletePartition:

sda1 = root.CIM_StorageExtent.first_instance(
        Key="DeviceID",
        Value="/dev/sda1")
(ret, outparams, err) = partitioning_service.LMI_DeletePartition(
        Partition = sda1.path)

Future direction

In future, we might implement:

• CreateOrModifyPartition method to meet SMI-S requirements. Then it will be possible to set partition exact start/end address.
• LMI_CreateOrModifyPartition would also modify existing partitions, for example resize them.
• Using LMI_CreateOrModifyPartition it should be possible to set exact type of partition to create (primary/logical/extended).