[libvirt PATCH 4/4] conf: add control over COW for storage pool directories

[Peter Krempa]

On Mon, Jul 20, 2020 at 18:33:22 +0100, Daniel Berrange wrote:
> The storage pool code now attempts to disable COW by default on btrfs,
> but management applications may wish to override this behaviour. Thus we
> introduce a concept of storage pool features:
> 
>   <features>
>     <cow state='yes|no'/>
>   </features>
> 
> If the <cow> feature policy is set, it will be enforced. It will always
> return an hard error if COW cannot be explicitly set or unset.
> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
>  docs/formatstorage.html.in                   | 25 ++++++++++
>  docs/schemas/storagepool.rng                 | 30 ++++++++++++
>  src/conf/storage_conf.c                      | 49 ++++++++++++++++++++
>  src/conf/storage_conf.h                      |  8 ++++
>  src/storage/storage_util.c                   |  2 +-
>  tests/storagepoolxml2xmlin/pool-dir-cow.xml  | 10 ++++
>  tests/storagepoolxml2xmlout/pool-dir-cow.xml | 15 ++++++
>  tests/storagepoolxml2xmltest.c               |  1 +
>  8 files changed, 139 insertions(+), 1 deletion(-)
>  create mode 100644 tests/storagepoolxml2xmlin/pool-dir-cow.xml
>  create mode 100644 tests/storagepoolxml2xmlout/pool-dir-cow.xml


> diff --git a/src/conf/storage_conf.c b/src/conf/storage_conf.c
> index 65d9b33049..4e63865b39 100644
> --- a/src/conf/storage_conf.c
> +++ b/src/conf/storage_conf.c
> @@ -839,6 +839,33 @@ virStoragePoolDefRefreshFormat(virBufferPtr buf,
>  }
>  
>  
> +static int
> +virStoragePoolDefParseFeatures(virStoragePoolDefPtr def,
> +                               xmlXPathContextPtr ctxt)
> +{
> +    g_autofree char *cow = virXPathString("string(./features/cow/@state)", ctxt);
> +
> +    if (cow) {
> +        int val;
> +        if (def->type != VIR_STORAGE_POOL_FS &&
> +            def->type != VIR_STORAGE_POOL_DIR) {
> +            virReportError(VIR_ERR_NO_SUPPORT, "%s",
> +                           _("cow feature may only be used for 'fs' and 'dir' pools"));
> +            return -1;
> +        }
> +        if ((val = virTristateBoolTypeFromString(cow)) < 0) {

<= 0

> +            virReportError(VIR_ERR_XML_ERROR,
> +                           _("invalid storage pool cow feature state '%s'"),
> +                           cow);
> +            return -1;
> +        }
> +        def->features.cow = val;
> +    }
> +
> +    return 0;
> +}
> +
> +
>  virStoragePoolDefPtr
>  virStoragePoolDefParseXML(xmlXPathContextPtr ctxt)
>  {
> @@ -910,6 +937,9 @@ virStoragePoolDefParseXML(xmlXPathContextPtr ctxt)
>          }
>      }
>  
> +    if (virStoragePoolDefParseFeatures(def, ctxt) < 0)
> +        return NULL;
> +
>      if (options->flags & VIR_STORAGE_POOL_SOURCE_HOST) {
>          if (!def->source.nhost) {
>              virReportError(VIR_ERR_XML_ERROR, "%s",
> @@ -1131,6 +1161,23 @@ virStoragePoolSourceFormat(virBufferPtr buf,
>  }
>  
>  
> +static void
> +virStoragePoolDefFormatFeatures(virBufferPtr buf,
> +                                virStoragePoolDefPtr def)
> +{
> +    if (!def->features.cow)
> +        return;
> +
> +    virBufferAddLit(buf, "<features>\n");
> +    virBufferAdjustIndent(buf, 2);
> +    if (def->features.cow)

def->features.cow != VIR_TRISTATE_BOOL_ABSENT

> +        virBufferAsprintf(buf, "<cow state='%s'/>\n",
> +                          virTristateBoolTypeToString(def->features.cow));
> +    virBufferAdjustIndent(buf, -2);
> +    virBufferAddLit(buf, "</features>\n");
> +}