Citrix Systems 6 User Manual

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2. This command suspends the VM (unless it is already powered down), initiates the space reclamation process,

and then resumes the VM.

Citrix recommends that, before executing the off-line coalesce tool, you shutdown or suspend
the VM manually (using either XenCenter or the XenServer CLI). If you execute the coalesce
tool on a VM that is running, the tool automatically suspends the VM, performs the required
VDI coalesce operation(s), and resumes the VM.

If the Virtual Disk Images (VDIs) to be coalesced are on shared storage, you must execute the
off-line coalesce tool on the pool master.

If the VDIs to be coalesced are on local storage, you must execute the off-line coalesce tool
on the server to which the local storage is attached.

Virtual disks have an optional I/O priority Quality of Service (QoS) setting. This setting can be applied to existing
virtual disks using the xe CLI as described in this section.

In the shared SR case, where multiple hosts are accessing the same LUN, the QoS setting is applied to VBDs
accessing the LUN from the same host. QoS is not applied across hosts in the pool.

Before  configuring  any  QoS  parameters  for  a  VBD,  ensure  that  the  disk  scheduler  for  the  SR  has  been  set
appropriately. See 

 for details on how to adjust the scheduler.

The scheduler parameter must be set to 

cfq

 on the SR for which the QoS is desired.

Remember to set the scheduler to 

cfq

 on the SR, and to ensure that the PBD has been re-

plugged in order for the scheduler change to take effect.

The first parameter is 

qos_algorithm_type

. This parameter needs to be set to the value ionice, which is

the only type of QoS algorithm supported for virtual disks in this release.

The  QoS  parameters  themselves  are  set  with  key/value  pairs  assigned  to  the 

qos_algorithm_param

parameter. For virtual disks, 

qos_algorithm_param

 takes a sched key, and depending on the value, also

requires a class key.

Possible values of 

qos_algorithm_param:sched

 are:

•

sched=rt

 or 

sched=real-time

 sets the QoS scheduling parameter to real time priority, which requires

a class parameter to set a value

•

sched=idle

 sets the QoS scheduling parameter to idle priority, which requires no class parameter to set

any value

•

sched=

<anything>

  sets  the  QoS  scheduling  parameter  to  best  effort  priority,  which  requires  a  class

parameter to set a value

The possible values for 

class

 are:

• One of the following keywords: highest, high, normal, low, lowest
• an integer between 0 and 7, where 7 is the highest priority and 0 is the lowest, so that, for example, I/O requests

with a priority of 5, will be given priority over I/O requests with a priority of 2.

To enable the disk QoS settings, you also need to set the 

other-config:scheduler

 to 

cfq

 and replug

PBDs for the storage in question.

For example, the following CLI commands set the virtual disk's VBD to use real time priority 

5

: