Re: [PATCH 00/17] Improve qcow2 all-zero detection

[Eric Blake <eblake@redhat.com>]

On 2/5/20 3:25 AM, Vladimir Sementsov-Ogievskiy wrote:

>> 3. For qcow2
>> Hmm. Here, as I understand, than main case is freshly created qcow2,
>> which is fully-unallocated. To understand that it is empty, we
>> need only to check all L1 entries. And for empty L1 table it is fast.
>> So we don't need any qcow2 format improvement to check it.
>>
> 
> Ah yes, I forget about preallocated case. Hmm. For preallocated clusters,
> we have zero bits in L2 entries. And with them, we even don't need
> preallocated to be filled by zeros, as we never read them (but just return
> zeros on read)..

Scanning all L2 entries is O(n), while an autoclear bit properly 
maintained is O(1).

> 
> Then, may be we want similar flag for L1 entry (this will enable large
> fast write-zero). And may be we want flag which marks the whole image
> as read-zero (it's your flag). So, now I think, my previous idea
> of "all allocated is zero" is worse. As for fully-preallocated images
> we are sure that all clusters are allocated, and it is more native to
> have flags similar to ZERO bit in L2 entry.

Right now, we don't have any L1 entry flags.  Adding one would require 
adding an incompatible feature flag (if older qemu would choke to see 
unexpected flags in an L1 entry), or at best an autoclear feature flag 
(if the autoclear bit gets cleared because an older qemu opened the 
image and couldn't maintain L1 entry flags correctly, then newer qemu 
knows it cannot trust those L1 entry flags).  But as soon as you are 
talking about adding a feature bit, then why add one that still requires 
O(n) traversal to check (true, the 'n' in an O(n) traversal of L1 tables 
is much smaller than the 'n' in an O(n) traversal of L2 tables), when 
you can instead just add an O(1) autoclear bit that maintains all_zero 
status for the image as a whole?

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