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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH 00/17] Improve qcow2 all-zero detection
Date: Wed, 5 Feb 2020 12:25:40 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

05.02.2020 12:04, Vladimir Sementsov-Ogievskiy wrote:
31.01.2020 20:44, Eric Blake wrote:
Based-on: <address@hidden>
([PATCH v2 1/2] qemu-img: Add --target-is-zero to convert)

I'm working on adding an NBD extension that reports whether an image
is already all zero when the client first connects.  I initially
thought I could write the NBD code to just call bdrv_has_zero_init(),
but that turned out to be a bad assumption that instead resulted in
this patch series.  The NBD patch will come later (and cross-posted to
the NBD protocol, libnbd, nbdkit, and qemu, as it will affect all four

I do have an RFC question on patch 13 - as implemented here, I set a
qcow2 bit if the image has all clusters known zero and no backing
image.  But it may be more useful to instead report whether all
clusters _allocated in this layer_ are zero, at which point the
overall image is all-zero only if the backing file also has that
property (or even make it two bits).  The tweaks to subsequent patches
based on what we think makes the most useful semantics shouldn't be

[repo.or.cz appears to be down as I type this; I'll post a link to a
repository later when it comes back up]

I have several ideas around it.

1. For generic block layer.
Did you consider as alternative to BDRV_ZEO_OPEN, to export the
information through normal block_status? So, if we have the
information, that disk is all-zero, we can always add _ZERO
flag to block-status result. And in generic bdrv_is_all_zeroes(),
we can just call block_status(0, disk_size), which will return
ZERO and n=disk_size if driver supports all-zero feature and is
all-zero now.
I think block-status is a native way for such information, and I
think that we anyway want to come to support of 64bit block-status
for qcow2 and nbd.

2. For NBD
Again, possible alternative is BLOCK_STATUS, but we need 64bit
commands for it. I plan to send a proposal anyway. Still, nothing
bad in two possible path of receiving all-zero information.
And even with your NBD extension, we can export this information
through block-status [1.]

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)..

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.

Best regards,

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