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Re: [Qemu-block] [PATCH 00/17] block: transactionless incremental backup

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 00/17] block: transactionless incremental backup series
Date: Fri, 27 Feb 2015 17:34:57 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 02/27/2015 04:47 PM, John Snow wrote:
> New topic, new version, same great incremental backup taste.
> This series relies on [Qemu-devel] [PULL 30/69] blkdebug: fix "once" rule,
> part of Stefan's 69 patch pull request submitted 2015-02-27.
> This patchset enables the in-memory part of the incremental backup
> feature, without transactional support.
> Support for transactions will come at a later date, but getting this
> portion upstream will help stabilize work on bitmap persistence and
> bitmap migration.
> Transactional support is being postponed to allow more development
> to create a transactional callback system that will allow objects used
> by the drive backup routines to perform cleanup actions conditionally
> based on the outcome of all backups in the transaction group.

Food for thought, and not necessarily fatal to this series:

What if this series makes 2.3, but transactions do not?  In the past,
we've had the benefit that if a standalone command exists, then the
matching transaction command also exists in the same release, such that
'query-commands' was a sufficient witness for what transactions we could
run.  But if we delay transactional incremental backup to 2.4, we'll
need some sort of introspection other than 'query-commands' to learn
what transaction elements are supported.

> Deletions:
>  - Removed Transactions, to be added later.
>  - Removed Transaction tests, as above.
> Changes:
> 01: Indentation fixes.
>     Removed enable/disable documentation.
>     Added a note that transactions aren't implemented yet.
>     Removed my needless commas
>     Added error case documentation.
> 07: QMP enable/disable commands are deleted.
> 14: Some comments concerning assertions.
>     Scrub re-alloc memory if we expand the array.
>     Do not attempt to scrub memory if fix_count is 0
> Changes made with Reviews kept:
> 02: Since 2.4

Oh, I didn't read far enough in my comments above.  If this entire
series is waiting for 2.4, then we stand a much better chance of getting
transaction support in the same release, negating all my worry above :)

> 04: Since 2.4
>     Demingled the QMP command documentation.
> 08: Additions to what was qmp_block_dirty_enable/disable
>     are no longer present as those function no longer exist.
> 09: Since 2.4
> 10: Since 2.4
>     Demingled QMP command documentation.
> 11: Since 2.4
> 15: Test 112 --> 124
> 17: Number of tests altered. (Only 4, now.)
> Fam Zheng (1):
>   qapi: Add optional field "name" to block dirty bitmap

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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