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Re: Cross-project NBD extension proposal: NBD_INFO_INIT_STATE

From: Eric Blake
Subject: Re: Cross-project NBD extension proposal: NBD_INFO_INIT_STATE
Date: Mon, 10 Feb 2020 16:29:53 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

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

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