Design proposal for supported image formats (#6308)

Proposing a new field field to get information about supported image
formats for a given SR. This information is retrieved from SMAPIv1
plugins.

Author: lindig

doc/content/design/sm-supported-image-formats.md

@@ -0,0 +1,76 @@
+---
+title: Add supported image formats in sm-list
+layout: default
+design_doc: true
+revision: 2
+status: proposed
+---
+
+# Introduction
+
+At XCP-ng, we are enhancing support for QCOW2 images in SMAPI. The primary
+motivation for this change is to overcome the 2TB size limitation imposed
+by the VHD format. By adding support for QCOW2, a Storage Repository (SR) will
+be able to host disks in VHD and/or QCOW2 formats, depending on the SR type.
+In the future, additional formats—such as VHDx—could also be supported.
+
+We need a mechanism to expose to end users which image formats are supported
+by a given SR. The proposal is to extend the SM API object with a new field
+that clients (such as XenCenter, XenOrchestra, etc.) can use to determine the
+available formats.
+
+# Design Proposal
+
+To expose the available image formats to clients (e.g., XenCenter, XenOrchestra, etc.),
+we propose adding a new field called `supported-image-formats` to the Storage Manager (SM)
+module. This field will be included in the output of the `SM.get_all_records` call.
+
+The `supported-image-formats` field will be populated by retrieving information
+from the SMAPI drivers. Specifically, each driver will update its `DRIVER_INFO`
+dictionary with a new key, `supported_image_formats`, which will contain a list
+of strings representing the supported image formats
+(for example: `["vhd", "raw", "qcow2"]`).
+
+The list designates the driver's preferred VDI format as its first entry. That
+means that when migrating a VDI, the destination storage repository will
+attempt to create a VDI in this preferred format. If the default format cannot
+be used (e.g., due to size limitations), an error will be generated.
+
+If a driver does not provide this information (as is currently the case with existing
+drivers), the default value will be an empty array. This signifies that it is the
+driver that decides which format it will use. This ensures that the modification
+remains compatible with both current and future drivers.
+
+With this new information, listing all parameters of the SM object will return:
+
+```bash
+# xe sm-list params=all
+```
+
+will output something like:
+
+```
+uuid ( RO)                         : c6ae9a43-fff6-e482-42a9-8c3f8c533e36
+name-label ( RO)                   : Local EXT3 VHD
+name-description ( RO)             : SR plugin representing disks as VHD files stored on a local EXT3 filesystem, created inside an LVM volume
+type ( RO)                         : ext
+vendor ( RO)                       : Citrix Systems Inc
+copyright ( RO)                    : (C) 2008 Citrix Systems Inc
+required-api-version ( RO)         : 1.0
+capabilities ( RO) [DEPRECATED]     : SR_PROBE; SR_SUPPORTS_LOCAL_CACHING; SR_UPDATE; THIN_PROVISIONING; VDI_ACTIVATE; VDI_ATTACH; VDI_CLONE; VDI_CONFIG_CBT; VDI_CREATE; VDI_DEACTIVATE; VDI_DELETE; VDI_DETACH; VDI_GENERATE_CONFIG; VDI_MIRROR; VDI_READ_CACHING; VDI_RESET_ON_BOOT; VDI_RESIZE; VDI_SNAPSHOT; VDI_UPDATE
+features (MRO)                    : SR_PROBE: 1; SR_SUPPORTS_LOCAL_CACHING: 1; SR_UPDATE: 1; THIN_PROVISIONING: 1; VDI_ACTIVATE: 1; VDI_ATTACH: 1; VDI_CLONE: 1; VDI_CONFIG_CBT: 1; VDI_CREATE: 1; VDI_DEACTIVATE: 1; VDI_DELETE: 1; VDI_DETACH: 1; VDI_GENERATE_CONFIG: 1; VDI_MIRROR: 1; VDI_READ_CACHING: 1; VDI_RESET_ON_BOOT: 2; VDI_RESIZE: 1; VDI_SNAPSHOT: 1; VDI_UPDATE: 1
+configuration ( RO)               : device: local device path (required) (e.g. /dev/sda3)
+driver-filename ( RO)              : /opt/xensource/sm/EXTSR
+required-cluster-stack ( RO)       :
+supported-image-formats ( RO)       : vhd, raw, qcow2
+```
+
+This change impacts the SM data model, and as such, the XAPI database version will
+be incremented.
+
+# Impact
+
+- **Data Model:** A new field (`supported-image-formats`) is added to the SM records.
+- **Client Awareness:** Clients like the `xe` CLI will now be able to query and display the supported image formats for a given SR.
+- **Database Versioning:** The XAPI database version will be updated to reflect this change.
+