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Re: [qemu-web PATCH] Add QEMU storage overview blog post
|
From: |
Stefan Hajnoczi |
|
Subject: |
Re: [qemu-web PATCH] Add QEMU storage overview blog post |
|
Date: |
Mon, 14 Sep 2020 11:35:07 +0100 |
On Mon, Sep 07, 2020 at 04:04:21PM +0100, Stefan Hajnoczi wrote:
> 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
Ping? Thomas, you would you be able to merge this?
> diff --git a/_posts/2020-09-07-qemu-storage-overview.md
> b/_posts/2020-09-07-qemu-storage-overview.md
> new file mode 100644
> index 0000000..79f60c4
> --- /dev/null
> +++ b/_posts/2020-09-07-qemu-storage-overview.md
> @@ -0,0 +1,122 @@
> +---
> +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.
> diff --git a/screenshots/2020-09-07-block-device-io.svg
> b/screenshots/2020-09-07-block-device-io.svg
> new file mode 100644
> index 0000000..b1effcc
> --- /dev/null
> +++ b/screenshots/2020-09-07-block-device-io.svg
> @@ -0,0 +1,366 @@
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