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When CLARiiON LUN migration is used to migrate to a larger LUN, after the migration completes, and a

rescan is performed on the VMware ESX server, additional space for the LUN is visible. Use the procedure

for expanding the VMFS volume for CLARiiON metaLUNs discussed previously.

CLARiiON Virtual Provisioning with VMFS and RDM volumes

A CLARiiON thin pool can contain multiple thin LUNs that can be assigned to multiple hosts. The space

assigned to these thin LUNs is the space that the VMware ESX server sees. This does not mean that the

space is fully allocated to the thin LUN from the thin pool. As the host writes to the thin LUN, space is

allocated on the fly from the thin pool.

A thin LUN created on a thin pool can be used to create a VMware file system (VMFS), or assigned

exclusively to a virtual machine as a raw disk mapping (RDM). Testing has shown that the VMFS datastore

is thin friendly, meaning when a VMware file system is created on Virtual Provisioning (thin) LUNs, a

minimal number of thin extents is allocated from the thin pool. Furthermore, a VMFS datastore reuses

previously allocated blocks, thus benefiting from Virtual Provisioning LUNs. When using RDM volumes,

the file system or device created on the guest OS will dictate whether the RDM volume will be thin

friendly.

Table 4 lists the allocation polices when creating new virtual disks.

Table 4. Allocation policies when creating new virtual disks on a VMware datastore

Allocation mechanism

(Virtual Disk format)

VMware kernel behavior

Zeroedthick All space is allocated at creation but is not initialized with zeroes. However, the allocated

space is wiped clean of any previous contents of the physical media. All blocks defined

by the block size of the VMFS datastore are initialized on the first write. This is the

default policy when creating new virtual disks.

Eagerzeroedthick This allocation mechanism allocates all of the space and initializes all of the blocks with

zeroes. This allocation mechanism performs a write to every block of the virtual disk,

and hence results in equivalent storage use in the thin pool.

Thick (not available with

ESX 4.0)

A thick disk has all the space allocated at creation time. If the guest operating system

performs a read from a block before writing to it, the VMware kernel may return stale

data if the blocks are reused.

Thin This allocation mechanism does not reserve any space on the VMware file system on

creation of the virtual disk. The space is allocated and zeroed on demand.

Rdm The virtual disk created in this mechanism is a mapping file that contains the pointers to

the blocks of SCSI disk it is mapping. However, the SCSI INQ information of the

physical media is virtualized. This format is commonly known as the “Virtual

compatibility mode of raw disk mapping”.

Rdmp This format is similar to the rdm format. However, the SCSI INQ information of the

physical media is not virtualized. This format is commonly known as the “Pass-through

raw disk mapping”.

Raw This mechanism can be used to address all SCSI devices supported by the kernel except

for SCSI disks.

2gbsparse The virtual disk created using this format is broken into multiple sparsely allocated

extents (if needed), with each extent no more than 2 GB in size.

For ESX 3.x, the zeroedthick (default) should be used when you create virtual disks on VMFS datastores,

since this option does not initialize or zero all blocks and claim all the space during creation. RDM volumes

are formatted by the guest operating system, hence virtual disk options like zeroedthick, thin, and

eagerzeroedthick only apply to VMFS volumes.

When the zeroedthick option is selected for virtual disks on VMFS volumes, the guest operating file system

(or writing pattern of the guest OS device) has an impact on how the space is allocated; if the guest

EMC CLARiiON Integration with

VMware ESX Server

Applied Technology 30

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