Re: [PATCH v5 28/31] qcow2: Add the 'extended_l2' option and the QCOW2_INCOMPAT_EXTL2 bit

[Eric Blake]

On 5/5/20 12:38 PM, Alberto Garcia wrote:
> Now that the implementation of subclusters is complete we can finally
> add the necessary options to create and read images with this feature,
> which we call "extended L2 entries".
>
> Signed-off-by: Alberto Garcia <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
> ---

What you have looks good, but I didn't notice anything affecting amend. 
The simplest option: amend can reject attempts to toggle the extended L2 
option (the zstd compression patches take that path).   More complicated 
is actually supporting it (in either direction, turning it on or off), 
which requires rewriting ALL L2 tables in the entry (including any in 
internal snapshots), which could be quite time-intensive, and where you 
must be careful to stage things so that failures during partial 
conversion merely leave leaked clusters rather than a header pointing to 
a half-converted state.  Either way, one of the iotests should probably 
add coverage on what happens when you attempt to amend the bit on or off.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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