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Re: [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-c

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting
Date: Fri, 22 Dec 2017 14:01:39 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0

15.12.2017 19:38, Max Reitz wrote:
Hi everyone,

Kevin, Markus, and me had a small personal meeting over the last 1.5
days and discussed a couple of things about the block layer and its QAPI

Here's a rather rough sketch on what we talked about:


== Single block job for backup/commit/mirror (blockdev-copy) ==

- Special things:
   - mirror: Ready state and block-job-complete
   - mirror: Always replaces some node by the target
   - backup: copy before write and synchronous
     (mirror/commit are copy after write and asynchronous (except
   - backup: Has @compressed, that may not work with mirror right now
     (because block drivers assume you only write once)
   - commit: Resizes target if smaller than source
   - commit: Does not share write permissions, maybe because it doesn't
     want to use a dirty bitmap (a unified job would just use a dirty
     bitmap so then it could just generally share write permissions)
   - active commit: Can read from source even if the backing chain is
     garbage because of the ongoing commit (mirror) job
     - block_job_add_bdrv() on nearly whole backing chain so that
       READ_CONSISTENT is not being shared
   - non-active commit: Write target's (base's) filename into all of
     source's (top's) overlays as their backing filename

At least some of these differences would require blockdev-copy runtime

blockdev-copy runtime parameters:
- everything blockdev-mirror has, plus
- @compressed? -- could be done as a block filter that converts normal
   writes to compressed writes
- Some option to suppress node replacement
   (in theory you could put block-job-finalize and bdrv_reopen into a
    transaction to do that replacement yourself)
- Some option to suppress READY event and complete automatically
   (compatibility to backup and non-active commit)
- Options to control the exact copying behavior (before/after write,

also, I think, for backup we need options to control the following:
- it there is a write notifier, which waits for copy, if copy fails, what to do:
  fail guest write or fail the backup?
- should we handle guest writes when block-job is paused (now they are handled)

   (passive before-write is impossible)
- Option to resize target if smaller than source
   (maybe just internal and not visible in the interface)
   (compatibility to block-commit)
- Several other commit thingies (like permissions) which can be
   automatically deduced and set if the target is in source's backing
- Option to specify the target's filename (this is written into all
   overlays of @to_replace as their backing filename; if omitted, the
   filename QEMU knows will be used)

Best regards,

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