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Re: [RFC PATCH 09/22] nbd: Add writethrough to block-export-add


From: Eric Blake
Subject: Re: [RFC PATCH 09/22] nbd: Add writethrough to block-export-add
Date: Wed, 19 Aug 2020 15:05:58 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0

On 8/17/20 7:56 AM, Max Reitz wrote:
On 13.08.20 18:29, Kevin Wolf wrote:
qemu-nbd allows use of writethrough cache modes, which mean that write
requests made through NBD will cause a flush before they complete.
Expose the same functionality in block-export-add.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---

+++ b/qapi/block-export.json
@@ -167,10 +167,15 @@
  # Describes a block export, i.e. how single node should be exported on an
  # external interface.
  #
+# @writethrough: If true, caches are flushed after every write request to the
+#                export before completion is signalled. (since: 5.2;
+#                default: false)
+#
  # Since: 4.2
  ##
  { 'union': 'BlockExportOptions',
-  'base': { 'type': 'BlockExportType' },
+  'base': { 'type': 'BlockExportType',
+            '*writethrough': 'bool' },
    'discriminator': 'type',
    'data': {
        'nbd': 'BlockExportOptionsNbd'

Hm.  I find it weird to have @writethrough in the base but @device in
the specialized class.

I think everything that will be common to all block exports should be in
the base, and that probably includes a node-name.  I’m aware that will
make things more tedious in the code, but perhaps it would be a nicer
interface in the end.  Or is the real problem that that would create
problems in the storage daemon’s command line interface, because then
the specialized (legacy) NBD interface would no longer be compatible
with the new generalized block export interface?

Anyway, @writable might be a similar story.  A @read-only may make sense
in general, I think.

And maybe even auto-read-only. As for @writable vs. @read-only, that's a choice of spelling, but we don't want both; there's also the question of which default is saner (having to opt-in to allowing writes is more verbose than defaulting to allowing writes when possible, but arguably saner as it is less risk of unintended writes when you forgot to specify the option; @auto-read-only can help in choosing nicer defaults).


Basically, I think that the export code should be separate from the code
setting up the BlockBackend that should be exported, so all options
regarding that BB should be common; and those options are @node-name,
@writethrough, and @read-only.  (And perhaps other things like
@resizable, too, even though that isn’t something to consider for NBD.)

NBD isn't resizable yet, but extending the protocol to let it become so is on my TODO list.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org




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