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+---
+layout: post
+title: "An Overview of QEMU Storage Features"
+date: 2020-09-07 07:00:00 +0000
+categories: [storage]
+---
+This article introduces QEMU storage concepts including disk images, emulated
+storage controllers, block jobs, the qemu-img utility, and qemu-storage-daemon.
+If you are new to QEMU or want an overview of storage functionality in QEMU
+then this article explains how things fit together.
+
+## Storage technologies
+Persistently storing data and retrieving it later is the job of storage devices
+such as hard disks, solid state drives (SSDs), USB flash drives, network
+attached storage, and many others. Technologies vary in their storage capacity
+(disk size), access speed, price, and other factors but most of them follow the
+same block device model.
+
+
+
+Block devices are accessed in storage units called blocks. It is not possible
+to access individual bytes, instead an entire block must be transferred. Block
+sizes vary between devices with 512 bytes and 4KB block sizes being the most
+common.
+
+As an emulator and virtualizer of computer systems, QEMU naturally has to offer
+block device functionality. QEMU is capable of emulating hard disks, solid
+state drives (SSDs), USB flash drives, SD cards, and more.
+
+## Storage for virtual machines
+There is more to storage than just persisting data on behalf of a virtual
+machine. The lifecycle of a disk image includes several operations that are
+briefly covered below.
+
+
+
+Virtual machines consist of device configuration (how much RAM, which
+graphics card, etc) and the contents of their disks. Transferring virtual
+machines either to migrate them between hosts or to distribute them to users is
+an important workflow that QEMU and its utilities support.
+
+Much like ISO files are used to distribute operating system installer images,
+QEMU supports disk image file formats that are more convenient for transferring
+disk images than the raw contents of a disk. In fact, disk image file formats
+offer many other features such as the ability to import/export disks from other
+hypervisors, snapshots, and instantiating new disk images from a backing file.
+
+Finally, managing disk images also involves the ability to take backups and
+restore them should it be necessary to roll back after the current disk
+contents have been lost or corrupted.
+
+## Emulated storage controllers
+
+The virtual machine accesses block devices through storage controllers. These
+are the devices that the guest talks to in order to read or write blocks. Some
+storage controllers facilitate access to multiple block devices, such as a SCSI
+Host Bus Adapter that provides access to many SCSI disks.
+
+Storage controllers vary in their features, performance, and guest operating
+system support. They expose a storage interface such as virtio-blk, NVMe, or
+SCSI. Virtual machines program storage controller registers to transfer data
+between memory buffers in RAM and block devices. Modern storage controllers
+support multiple request queues so that I/O can processed in parallel at high
+rates.
+
+The most common storage controllers in QEMU are virtio-blk, virtio-scsi, AHCI
+(SATA), IDE for legacy systems, and SD Card controllers on embedded or smaller
+boards.
+
+## Disk image file formats
+
+Disk image file formats handle the layout of blocks within a host file or
+device. The simplest format is the raw format where each block is located at
+its Logical Block Address (LBA) in the host file. This simple scheme does not
+offer much in the way of features.
+
+QEMU's native disk image format is QCOW2 and it offers a number of features:
+* Compactness - the host file grows as blocks are written so a sparse disk image can be much smaller than the virtual disk size.
+* Backing files - disk images can be based on a parent image so that a master image can be shared by virtual machines.
+* Snapshots - the state of the disk image can be saved and later reverted.
+* Compression - block compression reduces the image size.
+* Encryption - the disk image can be encrypted to protect data at rest.
+* Dirty bitmaps - backup applications can track changed blocks so that efficient incremental backups are possible.
+
+A number of other disk image file formats are available for importing/exporting
+disk images for use with other software including VMware and Hyper-V.
+
+## Block jobs
+
+Block jobs are background operations that manipulate disk images:
+* Commit - merging backing files to shorten a backing file chain.
+* Backup - copying out a point-in-time snapshot of a disk.
+* Mirror - copying an image to a new destination while the virtual machine can still write to it.
+* Stream - populating a disk image from its backing file.
+* Create - creating new disk image files.
+
+These background operations are powerful tools for building storage migration
+and backup workflows.
+
+Some operations like mirror and stream can take a long time because they copy
+large amounts of data. Block jobs support throttling to limit the performance
+impact on virtual machines.
+
+## qemu-img and qemu-storage-daemon
+
+The [qemu-img utility](https://www.qemu.org/docs/master/interop/qemu-img.html) manipulates disk images. It can create, resize, snapshot,
+repair, and inspect disk images. It has both human-friendly and JSON output
+formats, making it suitable for manual use as well as scripting.
+
+qemu-storage-daemon exposes QEMU's storage functionality in a server process
+without running a virtual machine. It can export disk images over the Network
+Block Device (NBD) protocol as well as run block jobs and other storage
+commands. This makes qemu-storage-daemon useful for applications that want to
+automate disk image manipulation.
+
+## Conclusion
+
+QEMU presents block devices to virtual machines via emulated storage
+controllers. On the host side the disk image file format, block jobs, and
+qemu-img/qemu-storage-daemon utilities provide functionality for working with
+disk images. Future blog posts will dive deeper into some of these areas and
+describe best practices for configuring storage.
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I want to kick of a series of posts about storage. The first post covers high-level concepts, features, and utilities in QEMU. Later posts will discuss configuration details, architecture, and performance. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> --- _posts/2020-09-07-qemu-storage-overview.md | 122 +++++++ screenshots/2020-09-07-block-device-io.svg | 366 +++++++++++++++++++++ screenshots/2020-09-07-lifecycle.svg | 328 ++++++++++++++++++ 3 files changed, 816 insertions(+) create mode 100644 _posts/2020-09-07-qemu-storage-overview.md create mode 100644 screenshots/2020-09-07-block-device-io.svg create mode 100644 screenshots/2020-09-07-lifecycle.svg