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Re: [Qemu-devel] [Nbd] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extensio

From: Eric Blake
Subject: Re: [Qemu-devel] [Nbd] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extension
Date: Mon, 4 Apr 2016 15:17:46 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/04/2016 03:04 PM, Denis V. Lunev wrote:
> In v1 we have had 'status' field, which can have the
> following values for dirty request:
> +      - `NBD_STATE_DIRTY` (0x0), LBA extent is dirty;
> +      - `NBD_STATE_CLEAN` (0x1), LBA extent is clean.
> in the extent structure:
> +        * 64 bits, offset (unsigned)
> +        * 32 bits, length (unsigned)
> +        * 16 bits, status (unsigned)

Between v1 and v2, we dropped 64-bit offset (offset is now implied, by
adding lengths of all earlier descriptors), and widened status from 16
bits to 32 bits (so that each descriptor is now naturally 8-byte aligned
and therefore easier to make a C array).

> we could report the entire state using one query. The user could be
> able to read entire state which is useful for backup at once.
> Your current proposal is more tricky and it was misunderstood by Alex:
> +        * 32 bits, status flags
> and you describe flags as
> +      - `NBD_STATE_HOLE` (bit 0); if set, the block represents a hole

> +      - `NBD_STATE_ZERO` (bit 1), if set, the block contents read as

> +      - `NBD_STATE_CLEAN` (bit 2); if set, the block represents a

> and opinion of Alex was that all 3 bits could be set in reply to
> This confused him. This confuses me too.

There's nothing that says that NBD_STATE_CLEAN can't be reassigned to
bit 0.  Conversely, we may want to add a future NBD_CMD_FLAG_FOO that
lets you read allocation and dirty information in the same call, in
which case having the bits be distinct will make that easier; but where
we would also make it obvious that the server is allowed to reject that
command flag as unsupported (we already state the server can reject
NBD_CMD_FLAG_STATUS_DIRTY with EINVAL as unsupported; and that it if
does not reject a dirtiness request but cannot otherwise report
anything, then the entire region is reported as dirty).

I don't have any strong opinions on whether NBD_STATE_CLEAN should
remain bit 2 or be renumbered to bit 0, although renumbering it to bit 0
would make it painfully obvious that you cannot query allocation and
dirtiness at the same time.

> If allocated state is not replied to command with NBD_CMD_FLAG_STATUS_DIRTY
> then why to have different meaning of bits.

Because we still have room - no need to overlap the meaning of bits as
long as we have more bits to choose from.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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