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Re: [PATCH v7 29/47] blockdev: Use CAF in external_snapshot_prepare()

From: Andrey Shinkevich
Subject: Re: [PATCH v7 29/47] blockdev: Use CAF in external_snapshot_prepare()
Date: Fri, 24 Jul 2020 13:37:22 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 24.07.2020 12:23, Max Reitz wrote:
On 20.07.20 18:08, Andrey Shinkevich wrote:
On 25.06.2020 18:21, Max Reitz wrote:
This allows us to differentiate between filters and nodes with COW
backing files: Filters cannot be used as overlays at all (for this

Signed-off-by: Max Reitz <mreitz@redhat.com>
   blockdev.c | 7 ++++++-
   1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 1eb0fcdea2..aabe51036d 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1549,7 +1549,12 @@ static void
external_snapshot_prepare(BlkActionState *common,
           goto out;
   -    if (state->new_bs->backing != NULL) {
+    if (state->new_bs->drv->is_filter) {

Is there a chance to get a filter here? If so, is that when a user
specifies the file name of such a kind “filter[filter-name]:foo.qcow2”
or somehow else?
It would be with blockdev-snapshot and by specifying a filter for
@overlay.  Technically that’s already caught by the check whether the
overlay supports backing images (whether drv->supports_backing is true),
but we might as well give a nicer error message.







Thank you for the example.


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