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diff --git a/qemu/docs/bitmaps.md b/qemu/docs/bitmaps.md new file mode 100644 index 000000000..fa87f077f --- /dev/null +++ b/qemu/docs/bitmaps.md @@ -0,0 +1,352 @@ +<!-- +Copyright 2015 John Snow <jsnow@redhat.com> and Red Hat, Inc. +All rights reserved. + +This file is licensed via The FreeBSD Documentation License, the full text of +which is included at the end of this document. +--> + +# Dirty Bitmaps and Incremental Backup + +* Dirty Bitmaps are objects that track which data needs to be backed up for the + next incremental backup. + +* Dirty bitmaps can be created at any time and attached to any node + (not just complete drives.) + +## Dirty Bitmap Names + +* A dirty bitmap's name is unique to the node, but bitmaps attached to different + nodes can share the same name. + +## Bitmap Modes + +* A Bitmap can be "frozen," which means that it is currently in-use by a backup + operation and cannot be deleted, renamed, written to, reset, + etc. + +## Basic QMP Usage + +### Supported Commands ### + +* block-dirty-bitmap-add +* block-dirty-bitmap-remove +* block-dirty-bitmap-clear + +### Creation + +* To create a new bitmap, enabled, on the drive with id=drive0: + +```json +{ "execute": "block-dirty-bitmap-add", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } +} +``` + +* This bitmap will have a default granularity that matches the cluster size of + its associated drive, if available, clamped to between [4KiB, 64KiB]. + The current default for qcow2 is 64KiB. + +* To create a new bitmap that tracks changes in 32KiB segments: + +```json +{ "execute": "block-dirty-bitmap-add", + "arguments": { + "node": "drive0", + "name": "bitmap0", + "granularity": 32768 + } +} +``` + +### Deletion + +* Bitmaps that are frozen cannot be deleted. + +* Deleting the bitmap does not impact any other bitmaps attached to the same + node, nor does it affect any backups already created from this node. + +* Because bitmaps are only unique to the node to which they are attached, + you must specify the node/drive name here, too. + +```json +{ "execute": "block-dirty-bitmap-remove", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } +} +``` + +### Resetting + +* Resetting a bitmap will clear all information it holds. + +* An incremental backup created from an empty bitmap will copy no data, + as if nothing has changed. + +```json +{ "execute": "block-dirty-bitmap-clear", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } +} +``` + +## Transactions (Not yet implemented) + +* Transactional commands are forthcoming in a future version, + and are not yet available for use. This section serves as + documentation of intent for their design and usage. + +### Justification + +Bitmaps can be safely modified when the VM is paused or halted by using +the basic QMP commands. For instance, you might perform the following actions: + +1. Boot the VM in a paused state. +2. Create a full drive backup of drive0. +3. Create a new bitmap attached to drive0. +4. Resume execution of the VM. +5. Incremental backups are ready to be created. + +At this point, the bitmap and drive backup would be correctly in sync, +and incremental backups made from this point forward would be correctly aligned +to the full drive backup. + +This is not particularly useful if we decide we want to start incremental +backups after the VM has been running for a while, for which we will need to +perform actions such as the following: + +1. Boot the VM and begin execution. +2. Using a single transaction, perform the following operations: + * Create bitmap0. + * Create a full drive backup of drive0. +3. Incremental backups are now ready to be created. + +### Supported Bitmap Transactions + +* block-dirty-bitmap-add +* block-dirty-bitmap-clear + +The usages are identical to their respective QMP commands, but see below +for examples. + +### Example: New Incremental Backup + +As outlined in the justification, perhaps we want to create a new incremental +backup chain attached to a drive. + +```json +{ "execute": "transaction", + "arguments": { + "actions": [ + {"type": "block-dirty-bitmap-add", + "data": {"node": "drive0", "name": "bitmap0"} }, + {"type": "drive-backup", + "data": {"device": "drive0", "target": "/path/to/full_backup.img", + "sync": "full", "format": "qcow2"} } + ] + } +} +``` + +### Example: New Incremental Backup Anchor Point + +Maybe we just want to create a new full backup with an existing bitmap and +want to reset the bitmap to track the new chain. + +```json +{ "execute": "transaction", + "arguments": { + "actions": [ + {"type": "block-dirty-bitmap-clear", + "data": {"node": "drive0", "name": "bitmap0"} }, + {"type": "drive-backup", + "data": {"device": "drive0", "target": "/path/to/new_full_backup.img", + "sync": "full", "format": "qcow2"} } + ] + } +} +``` + +## Incremental Backups + +The star of the show. + +**Nota Bene!** Only incremental backups of entire drives are supported for now. +So despite the fact that you can attach a bitmap to any arbitrary node, they are +only currently useful when attached to the root node. This is because +drive-backup only supports drives/devices instead of arbitrary nodes. + +### Example: First Incremental Backup + +1. Create a full backup and sync it to the dirty bitmap, as in the transactional +examples above; or with the VM offline, manually create a full copy and then +create a new bitmap before the VM begins execution. + + * Let's assume the full backup is named 'full_backup.img'. + * Let's assume the bitmap you created is 'bitmap0' attached to 'drive0'. + +2. Create a destination image for the incremental backup that utilizes the +full backup as a backing image. + + * Let's assume it is named 'incremental.0.img'. + + ```sh + # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + ``` + +3. Issue the incremental backup command: + + ```json + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + ``` + +### Example: Second Incremental Backup + +1. Create a new destination image for the incremental backup that points to the + previous one, e.g.: 'incremental.1.img' + + ```sh + # qemu-img create -f qcow2 incremental.1.img -b incremental.0.img -F qcow2 + ``` + +2. Issue a new incremental backup command. The only difference here is that we + have changed the target image below. + + ```json + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.1.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + ``` + +## Errors + +* In the event of an error that occurs after a backup job is successfully + launched, either by a direct QMP command or a QMP transaction, the user + will receive a BLOCK_JOB_COMPLETE event with a failure message, accompanied + by a BLOCK_JOB_ERROR event. + +* In the case of an event being cancelled, the user will receive a + BLOCK_JOB_CANCELLED event instead of a pair of COMPLETE and ERROR events. + +* In either case, the incremental backup data contained within the bitmap is + safely rolled back, and the data within the bitmap is not lost. The image + file created for the failed attempt can be safely deleted. + +* Once the underlying problem is fixed (e.g. more storage space is freed up), + you can simply retry the incremental backup command with the same bitmap. + +### Example + +1. Create a target image: + + ```sh + # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + ``` + +2. Attempt to create an incremental backup via QMP: + + ```json + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + ``` + +3. Receive an event notifying us of failure: + + ```json + { "timestamp": { "seconds": 1424709442, "microseconds": 844524 }, + "data": { "speed": 0, "offset": 0, "len": 67108864, + "error": "No space left on device", + "device": "drive1", "type": "backup" }, + "event": "BLOCK_JOB_COMPLETED" } + ``` + +4. Delete the failed incremental, and re-create the image. + + ```sh + # rm incremental.0.img + # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + ``` + +5. Retry the command after fixing the underlying problem, + such as freeing up space on the backup volume: + + ```json + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + ``` + +6. Receive confirmation that the job completed successfully: + + ```json + { "timestamp": { "seconds": 1424709668, "microseconds": 526525 }, + "data": { "device": "drive1", "type": "backup", + "speed": 0, "len": 67108864, "offset": 67108864}, + "event": "BLOCK_JOB_COMPLETED" } + ``` + +<!-- +The FreeBSD Documentation License + +Redistribution and use in source (Markdown) and 'compiled' forms (SGML, HTML, +PDF, PostScript, RTF and so forth) with or without modification, are permitted +provided that the following conditions are met: + +Redistributions of source code (Markdown) must retain the above copyright +notice, this list of conditions and the following disclaimer of this file +unmodified. + +Redistributions in compiled form (transformed to other DTDs, converted to PDF, +PostScript, RTF and other formats) must reproduce the above copyright notice, +this list of conditions and the following disclaimer in the documentation and/or +other materials provided with the distribution. + +THIS DOCUMENTATION IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" +AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE +DISCLAIMED. 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