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Re: [Qemu-devel] [PATCH V4 2/9] snapshot: add paired functions for inter

From: Wenchao Xia
Subject: Re: [Qemu-devel] [PATCH V4 2/9] snapshot: add paired functions for internal snapshot id and name
Date: Wed, 10 Jul 2013 22:43:41 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130620 Thunderbird/17.0.7

于 2013-7-10 22:22, Kevin Wolf 写道:
Am 10.07.2013 um 15:54 hat Wenchao Xia geschrieben:
于 2013-7-10 21:10, Kevin Wolf 写道:
Am 08.07.2013 um 10:00 hat Wenchao Xia geschrieben:
Internal snapshot's ID and name concept are both visible in general
block level, they are observed by user in "info snapshots", so it is
possible to have conflict. Although we can separate the two concept in
programming, but if they can be distinguished in string itself, things
will be simple and clear, so introduce two functions to do it.

The implemention, qcow2 snapshot calls snapshot_id_string_generate() to
make sure it follows the rule in driver. If caller or user give a check
with snapshot_name_wellformed() before create snapshot, then the ID
and name will never conflict. The check can be also taken in
qcow2_snapshot_create(), but require it to return error reason.

I'm not sure how useful this is. While we can restrict what IDs we allow
for creating new snapshots, we cannot take any advantage from it because
existing snapshots could already be named with only digits (they could
also use a non-numeric ID). At the end of the day we're limiting the
   Qcow2's ID seems always numeric, do you mean sheepdog may have
non-numeric ID?

That's not true, qcow2 has an ID string rather than a numeric ID.

  But there is no way to specify non-numeric ID in public interface,
such as savevm, qemu-img, and in qcow2_snapshot_create(),
find_new_snapshot_id() will always fill in a numeric ID string, so
result is that, qcow2's snapshot ID string is always a numeric string.

choice of IDs that can be generated with the QMP command without a real

   This patch may limit the choice of snapshot name. Maybe we can't
benefit from it now because existing code, but shouldn't it be improved
in any new interface to alleviate the problem? I guess you idea
is drop this check in the new interface, isn't it?

Well, the thought is that limiting it in QMP is of no use if there is no
corresponding limitation in the backend. So yes, I think, as long as
block drivers can deal with numeric snapshots names, QMP should allow
dealing with them.

  OK, I'll drop this limit.


Best Regards

Wenchao Xia

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