Libguestfs - Feb 2020 - Re: Cross-project NBD extension proposal: NBD_INFO_INIT_STATE

On 2/10/20 4:12 PM, Richard W.M. Jones wrote:
> On Mon, Feb 10, 2020 at 03:37:20PM -0600, Eric Blake wrote:
>> For now, only 2 of those 16 bits are defined: NBD_INIT_SPARSE (the
>> image has at least one hole) and NBD_INIT_ZERO (the image reads
>> completely as zero); the two bits are orthogonal and can be set
>> independently, although it is easy enough to see completely sparse
>> files with both bits set.
> 
> I think I'm confused about the exact meaning of NBD_INIT_SPARSE.  Do
> you really mean the whole image is sparse; or (as you seem to have
> said above) that there exists a hole somewhere in the image but we're
> not saying where it is and there can be non-sparse parts of the image?
As implemented:

NBD_INIT_SPARSE - there is at least one hole somewhere (allocation would 
be required to write to that part of the file), but there may b 
allocated data elsewhere in the image.  Most disk images will fit this 
definition (for example, it is very common to have a hole between the 
MBR or GPT and the first partition containing a file system, or for file 
systems themselves to be sparse within the larger block device).

NBD_INIT_ZERO - all bytes read as zero.

The combination NBD_INIT_SPARSE|NBD_INIT_ZERO is common (generally, if 
you use lseek(SEEK_DATA) to prove the entire image reads as zeroes, you 
also know the entire image is sparse), but NBD_INIT_ZERO in isolation is 
also possible (especially with the qcow2 proposal of a persistent 
autoclear bit, where even with a fully preallocated qcow2 image you 
still know it reads as zeroes but there are no holes).  But you are also 
right that for servers that can advertise both bits efficiently, 
NBD_INIT_SPARSE in isolation may be more common than 
NBD_INIT_SPARSE|NBD_INIT_ZERO (the former for most disk images, the 
latter only for a freshly-created image that happens to create with zero 
initialization).

What's more, in my patches, I did NOT patch qemu to set or consume 
INIT_SPARSE; so far, it only sets/consumes INIT_ZERO.  Of course, if we 
can find a reason WHY qemu should track whether a qcow2 image is 
fully-allocated, by demonstrating a qemu-img algorithm that becomes 
easier for knowing if an image is sparse (even if our justification is: 
"when copying an image, I want to know if the _source_ is sparse, to 
know whether I have to bend over backwards to preallocate the 
destination"), then using that in qemu makes sense for my v2 patches. 
But for v1, my only justification was "when copying an image, I can skip 
holes in the source if I know the _destination_ already reads as 
zeroes", which only needed INIT_ZERO.

Some of the nbdkit patches demonstrate the some-vs.-all nature of the 
two bits; for example, in the split plugin, I initialize h->init_sparse 
= false; h->init_zero = true; then in a loop over each file change 
h->init_sparse to true if at least one file was sparse, and change 
h->init_zero to false if at least one file had non-zero contents.

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

On Mon, Feb 10, 2020 at 04:29:53PM -0600, Eric Blake
wrote:
> On 2/10/20 4:12 PM, Richard W.M. Jones wrote:
> >On Mon, Feb 10, 2020 at 03:37:20PM -0600, Eric Blake wrote:
> >>For now, only 2 of those 16 bits are defined: NBD_INIT_SPARSE (the
> >>image has at least one hole) and NBD_INIT_ZERO (the image reads
> >>completely as zero); the two bits are orthogonal and can be set
> >>independently, although it is easy enough to see completely sparse
> >>files with both bits set.
> >
> >I think I'm confused about the exact meaning of NBD_INIT_SPARSE. 
Do
> >you really mean the whole image is sparse; or (as you seem to have
> >said above) that there exists a hole somewhere in the image but
we're
> >not saying where it is and there can be non-sparse parts of the image?
> 
> As implemented:
> 
> NBD_INIT_SPARSE - there is at least one hole somewhere (allocation
> would be required to write to that part of the file), but there may
> b allocated data elsewhere in the image.  Most disk images will fit
> this definition (for example, it is very common to have a hole
> between the MBR or GPT and the first partition containing a file
> system, or for file systems themselves to be sparse within the
> larger block device).
I think I'm still confused about why this particular flag would be
useful for clients (I can completely understand why clients need
NBD_INIT_ZERO).

But anyway ... could a flag indicating that the whole image is sparse
be useful, either as well as NBD_INIT_SPARSE or instead of it?  You
could use it to avoid an initial disk trim, which is something that
mke2fs does:

 
https://github.com/tytso/e2fsprogs/blob/0670fc20df4a4bbbeb0edb30d82628ea30a80598/misc/mke2fs.c#L2768

and which is painfully slow over NBD for very large devices because of
the 32 bit limit on request sizes - try doing mke2fs on a 1E nbdkit
memory disk some time.
> NBD_INIT_ZERO - all bytes read as zero.
> 
> The combination NBD_INIT_SPARSE|NBD_INIT_ZERO is common (generally,
> if you use lseek(SEEK_DATA) to prove the entire image reads as
> zeroes, you also know the entire image is sparse), but NBD_INIT_ZERO
> in isolation is also possible (especially with the qcow2 proposal of
> a persistent autoclear bit, where even with a fully preallocated
> qcow2 image you still know it reads as zeroes but there are no
> holes).  But you are also right that for servers that can advertise
> both bits efficiently, NBD_INIT_SPARSE in isolation may be more
> common than NBD_INIT_SPARSE|NBD_INIT_ZERO (the former for most disk
> images, the latter only for a freshly-created image that happens to
> create with zero initialization).
> 
> What's more, in my patches, I did NOT patch qemu to set or consume
> INIT_SPARSE; so far, it only sets/consumes INIT_ZERO.  Of course, if
> we can find a reason WHY qemu should track whether a qcow2 image is
> fully-allocated, by demonstrating a qemu-img algorithm that becomes
> easier for knowing if an image is sparse (even if our justification
> is: "when copying an image, I want to know if the _source_ is
> sparse, to know whether I have to bend over backwards to preallocate
> the destination"), then using that in qemu makes sense for my v2
> patches. But for v1, my only justification was "when copying an
> image, I can skip holes in the source if I know the _destination_
> already reads as zeroes", which only needed INIT_ZERO.
> 
> Some of the nbdkit patches demonstrate the some-vs.-all nature of
> the two bits; for example, in the split plugin, I initialize
> h->init_sparse = false; h->init_zero = true; then in a loop over
> each file change h->init_sparse to true if at least one file was
> sparse, and change h->init_zero to false if at least one file had
> non-zero contents.
Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html

On 2/10/20 4:52 PM, Richard W.M. Jones wrote:
> On Mon, Feb 10, 2020 at 04:29:53PM -0600, Eric Blake wrote:
>> On 2/10/20 4:12 PM, Richard W.M. Jones wrote:
>>> On Mon, Feb 10, 2020 at 03:37:20PM -0600, Eric Blake wrote:
>>>> For now, only 2 of those 16 bits are defined: NBD_INIT_SPARSE
(the
>>>> image has at least one hole) and NBD_INIT_ZERO (the image reads
>>>> completely as zero); the two bits are orthogonal and can be set
>>>> independently, although it is easy enough to see completely
sparse
>>>> files with both bits set.
>>>
>>> I think I'm confused about the exact meaning of
NBD_INIT_SPARSE.  Do
>>> you really mean the whole image is sparse; or (as you seem to have
>>> said above) that there exists a hole somewhere in the image but
we're
>>> not saying where it is and there can be non-sparse parts of the
image?
>>
>> As implemented:
>>
>> NBD_INIT_SPARSE - there is at least one hole somewhere (allocation
>> would be required to write to that part of the file), but there may
>> b allocated data elsewhere in the image.  Most disk images will fit
>> this definition (for example, it is very common to have a hole
>> between the MBR or GPT and the first partition containing a file
>> system, or for file systems themselves to be sparse within the
>> larger block device).
> 
> I think I'm still confused about why this particular flag would be
> useful for clients (I can completely understand why clients need
> NBD_INIT_ZERO).
> 
> But anyway ... could a flag indicating that the whole image is sparse
> be useful, either as well as NBD_INIT_SPARSE or instead of it?  You
> could use it to avoid an initial disk trim, which is something that
> mke2fs does:
> 
>   
https://github.com/tytso/e2fsprogs/blob/0670fc20df4a4bbbeb0edb30d82628ea30a80598/misc/mke2fs.c#L2768
I'm open to suggestions on how many initial bits should be provided.  In 
fact, if we wanted, we could have a pair mutually-exclusive bits, 
advertising:
00 - no information known
01 - image is completely sparse
10 - image is completely allocated
11 - error

The goal of providing a 16-bit answer (or we could mandate 32 or 64 
bits, if we think we will ever want to extend that far) was to make it 
easier to add in whatever other initial-state extensions that someone 
could find useful.  Until we're happy with the design, the size or any 
given bit assignment is not locked down; once we do start committing any 
of this series, we've locked in what interoperability will demand but 
still have spare bits as future extensions.

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

Hi,

On Mon, Feb 10, 2020 at 10:52:55PM +0000, Richard W.M. Jones
wrote:
> But anyway ... could a flag indicating that the whole image is sparse
> be useful, either as well as NBD_INIT_SPARSE or instead of it?  You
> could use it to avoid an initial disk trim, which is something that
> mke2fs does:
Yeah, I think that could definitely be useful. I honestly can't see a
use for NBD_INIT_SPARSE as defined in this proposal; and I don't think
it's generally useful to have a feature if we can't think of a use case
for it (that creates added complexity for no benefit).

If we can find a reasonable use case for NBD_INIT_SPARSE as defined in
this proposal, then just add a third bit (NBD_INIT_ALL_SPARSE or
something) that says "the whole image is sparse". Otherwise, I think
we
should redefine NBD_INIT_SPARSE to say that.

-- 
To the thief who stole my anti-depressants: I hope you're happy

  -- seen somewhere on the Internet on a photo of a billboard