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Re: [Qemu-block] [PATCH V5 10/10] block/qcow2: add compress info to imag

From: Peter Lieven
Subject: Re: [Qemu-block] [PATCH V5 10/10] block/qcow2: add compress info to image specific info
Date: Wed, 26 Jul 2017 11:05:34 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

Am 25.07.2017 um 23:55 schrieb Eric Blake:
> On 07/25/2017 09:41 AM, Peter Lieven wrote:
>> Signed-off-by: Peter Lieven <address@hidden>
>> ---
>>  block/qcow2.c        | 7 +++++++
>>  qapi/block-core.json | 6 +++++-
>>  2 files changed, 12 insertions(+), 1 deletion(-)
>> +++ b/qapi/block-core.json
>> @@ -68,6 +68,9 @@
>>  # @encrypt: details about encryption parameters; only set if image
>>  #           is encrypted (since 2.10)
>>  #
>> +# @compress: details about parameters for compressed clusters; only set if
>> +#            the compress format header extension is present (since 2.11)
> Thinking aloud:
> I still think this is better as "only set if the image uses compressed
> headers" (similar to encrypt).  That is, I would like this output to be
> present even when opening legacy deflate/0/12 images.
> But thinking more, what I am REALLY asking for is "if any cluster is
> compressed, then this information is present; if nothing is compressed,
> the choice of compression header doesn't matter".  But we don't have a
> quick-and-dirty way to tell if an image has any compressed clusters,
> short of examining every L2 map (including maps inside internal snapshots).
> Hmm - we already have 'Dirty bit' and 'Corrupt bit' as
> incompatible_features that can quickly identify certain image
> properties; do we want another bit set to 1 for images known to use
> compressed clusters?  Or even make it an auto-clear bit (or even a pair
> of auto-clear bits: one to designate that this image was written by a
> new-enough qemu that promises to mark the image if any compressed
> clusters exist, and the other is set only if such clusters do exist;
> that way, if both bits are clear, we know we have to walk L2 tables to
> look for compressed clusters, but if the first bit is set, then the
> second bit is reliable)?
> So maybe this means we are still debating spec ideas.  Kevin, do you
> want to chime in?

I would also vote for adding an information to the image if it contains
compressed clusters. 2 bits sounds reasonable. One to promise we set
the compressed bit if we write any compressed cluster and one to tell
that there are actually compressed cluster. If we set the "promise" bit
at create time we can simply hook up in qcow2_co_pwritev_comperssed
and set the bit there as soon as we write any compressed cluster.

The compression info I would display unconditionally. Because the compress
settings are made at create time and are there even if we don't have compressed
clusters yet.


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