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Virtualization Guide › Hypervisor-independent features › Disk cache modes
ContentsContents
Virtualization Guide
  1. Preface
  2. I Introduction
    1. 1 Virtualization technology
    2. 2 Virtualization scenarios
    3. 3 Introduction to Xen virtualization
    4. 4 Introduction to KVM virtualization
    5. 5 Virtualization tools
    6. 6 Installation of virtualization components
  3. II Managing virtual machines with libvirt
    1. 7 libvirt daemons
    2. 8 Preparing the VM Host Server
    3. 9 Guest installation
    4. 10 Basic VM Guest management
    5. 11 Connecting and authorizing
    6. 12 Advanced storage topics
    7. 13 Configuring virtual machines with Virtual Machine Manager
    8. 14 Configuring virtual machines with virsh
    9. 15 Xen to KVM migration guide
  4. III Hypervisor-independent features
    1. 16 Disk cache modes
    2. 17 VM Guest clock settings
    3. 18 libguestfs
    4. 19 QEMU guest agent
    5. 20 Software TPM emulator
    6. 21 Creating crash dumps of a VM Guest
  5. IV Managing virtual machines with Xen
    1. 22 Setting up a virtual machine host
    2. 23 Virtual networking
    3. 24 Managing a virtualization environment
    4. 25 Block devices in Xen
    5. 26 Virtualization: configuration options and settings
    6. 27 Administrative tasks
    7. 28 XenStore: configuration database shared between domains
    8. 29 Xen as a high-availability virtualization host
    9. 30 Xen: converting a paravirtual (PV) guest into a fully virtual (FV/HVM) guest
  6. V Managing virtual machines with QEMU
    1. 31 QEMU overview
    2. 32 Setting up a KVM VM Host Server
    3. 33 Guest installation
    4. 34 Running virtual machines with qemu-system-ARCH
    5. 35 Virtual machine administration using QEMU monitor
  7. VI Troubleshooting
    1. 36 Integrated help and package documentation
    2. 37 Gathering system information and logs
  8. Glossary
  9. A Configuring GPU Pass-Through for NVIDIA cards
  10. B GNU licenses
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Virtualization Guide › Hypervisor-independent features › Disk cache modes
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Applies to openSUSE Leap 15.5

16 Disk cache modes Edit source

16.1 What is a disk cache?
16.2 How does a disk cache work?
16.3 Benefits of disk caching
16.4 Virtual disk cache modes
16.5 Cache modes and data integrity
16.6 Cache modes and live migration

16.1 What is a disk cache? Edit source

A disk cache is a memory used to speed up the process of storing and accessing data from the hard disk. Physical hard disks have their cache integrated as a standard feature. For virtual disks, the cache uses VM Host Server's memory and you can fine-tune its behavior, for example, by setting its type.

16.2 How does a disk cache work? Edit source

Normally, a disk cache stores the most recent and frequently used programs and data. When a user or program requests data, the operating system first checks the disk cache. If the data is there, the operating system quickly delivers the data to the program instead of re-reading the data from the disk.

Caching mechanism
Figure 16.1: Caching mechanism

16.3 Benefits of disk caching Edit source

Caching of virtual disk devices affects the overall performance of guest machines. You can improve the performance by optimizing the combination of cache mode, disk image format, and storage subsystem.

16.4 Virtual disk cache modes Edit source

If you do not specify a cache mode, writeback is used by default. Each guest disk can use one of the following cache modes:

writeback

writeback uses the host page cache. Writes are reported to the guest as completed when they are placed in the host cache. Cache management handles commitment to the storage device. The guest's virtual storage adapter is informed of the writeback cache and therefore expected to send flush commands as needed to manage data integrity.

writethrough

Writes are reported as completed only when the data has been committed to the storage device. The guest's virtual storage adapter is informed that there is no writeback cache, so the guest does not need to send flush commands to manage data integrity.

none

The host cache is bypassed, and reads and writes happen directly between the hypervisor and the storage device. Because the actual storage device may report a write as completed when the data is still placed in its write queue only, the guest's virtual storage adapter is informed that there is a writeback cache. This mode is equivalent to direct access to the host's disk.

unsafe

Similar to the writeback mode, except all flush commands from the guests are ignored. Using this mode implies that the user prioritizes performance gain over the risk of data loss in case of a host failure. This mode can be useful during guest installation, but not for production workloads.

directsync

Writes are reported as completed only when the data has been committed to the storage device and the host cache is bypassed. Similar to writethrough, this mode can be useful for guests that do not send flushes when needed.

16.5 Cache modes and data integrity Edit source

writethrough, none, directsync

These modes are considered to be safest when the guest operating system uses flushes as needed. For unsafe or unstable guests, use writethough or directsync.

writeback

This mode informs the guest of the presence of a write cache, and it relies on the guest to send flush commands as needed to maintain data integrity within its disk image. This mode exposes the guest to data loss if the host fails. The reason is the gap between the moment a write is reported as completed and the time the write being committed to the storage device.

unsafe

This mode is similar to writeback caching, except the guest flush commands are ignored. This means a higher risk of data loss caused by host failure.

16.6 Cache modes and live migration Edit source

The caching of storage data restricts the configurations that support live migration. Currently, only raw and qcow2 image formats can be used for live migration. If a clustered file system is used, all cache modes support live migration. Otherwise the only cache mode that supports live migration on read/write shared storage is none.

The libvirt management layer includes checks for migration compatibility based on several factors. If the guest storage is hosted on a clustered file system, is read-only, or is marked shareable, then the cache mode is ignored when determining if migration can be allowed. Otherwise libvirt does not allow migration unless the cache mode is set to none. However, this restriction can be overridden with the “--unsafe” option to the migration APIs, which is also supported by virsh. For example 

> virsh migrate --live --unsafe
Tip
Tip

The cache mode none is required for the AIO mode setting native. If another cache mode is used, the AIO mode is silently switched back to the default threads.

→Chapter 17 VM Guest clock settings←Part III Hypervisor-independent features
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