Red Hat Training

A Red Hat training course is available for Red Hat Enterprise Linux

7.2. Caching

Caching options can be configured with virt-manager during guest installation, or on an existing guest virtual machine by editing the guest XML configuration.

Table 7.1. Caching options

Caching OptionDescription
Cache=noneI/O from the guest is not cached on the host, but may be kept in a writeback disk cache. Use this option for guests with large I/O requirements. This option is generally the best choice, and is the only option to support migration.
Cache=writethroughI/O from the guest is cached on the host but written through to the physical medium. This mode is slower and prone to scaling problems. Best used for small number of guests with lower I/O requirements. Suggested for guests that do not support a writeback cache (such as Red Hat Enterprise Linux 5.5 and earlier), where migration is not needed.
Cache=writebackI/O from the guest is cached on the host.
Cache=directsyncSimilar to writethrough, but I/O from the guest bypasses the host page cache.
Cache=unsafeThe host may cache all disk I/O, and sync requests from guest are ignored.
Cache=defaultIf no cache mode is specified, the system's default settings are chosen.
In virt-manager, the caching mode can be specified under Virtual Disk. For information on using virt-manager to change the cache mode, see Section 3.3, “Virtual Disk Performance Options”
To configure the cache mode in the guest XML, edit the cache setting inside the driver tag to specify a caching option. For example, to set the cache as writeback:
<disk type='file' device='disk'>
          <driver name='qemu' type='raw' cache='writeback'/>