Re: [Qemu-devel] [Nbd] [PATCH v5] doc: Add NBD_CMD_BLOCK_STATUS extension

[Alex Bligh]

Wouter,

Some comments below:

> On 12 Dec 2016, at 20:43, Wouter Verhelst <address@hidden> wrote:
...
> +## Metadata querying
> +
> +With the availability of sparse storage formats, it is often needed to
> +query the status of a particular range and read only those blocks of
> +data that are actually present on the block device.
> +
> +Some storage formats and operations over such formats express a
> +concept of data dirtiness. Whether the operation is block device
> +mirroring, incremental block device backup or any other operation with
> +a concept of data dirtiness, they all share a need to provide a list
> +of ranges that this particular operation treats as dirty.

I'm not sure this necessarily has anything to do with sparse storage
formats. For instance the 'dirty' concept would work perfectly well
on a non-sparse format. I propose the following to replace the two
paragraphs above:

"It is often helpful for the client to be able to query the status
of a range of blocks. The nature of the status that can be
queried is in part implementation dependent. For instance,
the status might represent:

* in a sparse storage format, whether the relevant blocks are
  actually present on the backing device for the export; or

* whether the relevant blocks are 'dirty'; some storage formats
  and operations over such formats express a concept of data dirtiness.
  Whether the operation is block device mirroring, incremental block
  device backup or any other operation with a concept of data dirtiness,
  they all share a need to provide a list of ranges that this
  particular operation treats as dirty.
"

> +
> +To provide such classes of information, the NBD protocol has a generic
> +framework for querying metadata; however, its use must first be
> +negotiated, and one or more metadata contexts must be selected.
> +
> +The procedure works as follows:
> +
> +- First, during negotiation,

insert: "if the client wishes to query metadata,"

> +  the client MUST select one or more metadata
> +  contexts with the `NBD_OPT_SET_META_CONTEXT` command. If needed, the client
> +  can use `NBD_OPT_LIST_META_CONTEXT` to list contexts.

"to list contexts that the server supports"

> +- During transmission, a client can then indicate interest in metadata
> +  for a given region by way of the `NBD_CMD_BLOCK_STATUS` command, where
> +  *offset* and *length* indicate the area of interest. The server MUST
> +  then respond with the requested information, for all contexts which
> +  were selected during negotiation. For every metadata context, the
> +  server sends one set of extent chunks, where the sizes of the
> +  extents MUST be less than or equal to the length as specified in the
> +  request. Each extent comes with a *flags* field, the semantics of
> +  which are defined by the metadata context.
> +- A server MUST reply to `NBD_CMD_BLOCK_STATUS` with a structured reply
> +  of type `NBD_REPLY_TYPE_BLOCK_STATUS`.
> +
> +A client MUST NOT use `NBD_CMD_BLOCK_STATUS` unless it selected a
> +nonzero

"non-zero"

> number of metadata contexts during negotiation. Servers SHOULD
> +reply to clients doing so anyway with `EINVAL`.

I had difficulty parsing "so". I think you mean "Servers SHOULD
reply to clients sending `NBD_CMD_BLOCK_STATUS without
selecting metadata contexts with"

But actually, why do we need to be so mean? Why can't we assume
that if NBD_OPT_SET_META_CONTEXT is not sent, then all the metadata
contexts should be selected?

Of course arguably this means why might need to restore
NBD_FLAG_SEND_BLOCK_STATUS.

> +
> +The reply to the `NBD_CMD_BLOCK_STATUS` request MUST be sent by a

"be sent as a"

> +structured reply; this implies that in order to use metadata querying,
> +structured replies MUST be negotiated first.
> +
> +This standard defines exactly one metadata context; it is called
> +`base:allocation`, and it provides information on the basic allocation
> +status of extents (that is, whether they are allocated at all in a
> +sparse file context).
> +
> ## Values
> 
> This section describes the value and meaning of constants (other than
> @@ -768,8 +814,6 @@ The field has the following format:
>   to that command to the client. In the absense of this flag, clients
>   SHOULD NOT multiplex their commands over more than one connection to
>   the export.
> -- bit 9, `NBD_FLAG_SEND_BLOCK_STATUS`: defined by the experimental
> -  `BLOCK_STATUS` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-blockstatus/doc/proto.md).
> 
> Clients SHOULD ignore unknown flags.
> 
> @@ -871,6 +915,69 @@ of the newstyle negotiation.
> 
>     Defined by the experimental `INFO` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
> 
> +- `NBD_OPT_LIST_META_CONTEXT` (10)
> +
> +    Return a list of `NBD_REP_META_CONTEXT` replies, one per context,
> +    followed by an `NBD_REP_ACK`. If a server replies to such a request
> +    with no error message, clients MAY send NBD_CMD_BLOCK_STATUS

backticks: `NBD_CMD_BLOCK_STATUS`

> +    commands during the transmission phase.

"otherwise, the client MUST NOT send NBD_CMD_BLOCK_STATUS
messages."

But actually isn't the telling question whether NBD_OPT_SET_META_CONTEXT
works?

Actually, as this could be sent more than once, I think this whole
thing would be better phrased as:

"A client MUST NOT send `NBD_CMD_BLOCK_STATUS` unless
within the negotiation phase it sent `NBD_OPT_SET_META_CONTEXT`
at least once, and the final time it was sent, the server
responded without an error."

obviously this would be better under _SET_ than _LIST_,
but the sentence can go entirely from here.

Equally obviously, if we are making _SET_ optional (as
lack of _SET_ means that all the contexts are selected)
we just gate this on `NBD_FLAG_SEND_BLOCK_STATUS`

> +
> +    If the query string is syntactically invalid, the server SHOULD send
> +    `NBD_REP_ERR_INVALID`. If the query string is syntactically valid
> +    but finds no metadata contexts, the server MUST send a single
> +    reply of type `NBD_REP_ACK`.
> +
> +    This option MUST NOT be requested unless structured replies have
> +    been negotiated first. If a client attempts to do so, a server
> +    SHOULD send `NBD_REP_ERR_INVALID`.
> +
> +    Data:
> +    - 32 bits, length of export name
> +    - String, name of export for which we wish to list or select metadata
> +      contexts.

Surely as this is _LIST_ this should read "list" not "list or select"?

> +    - 32 bits, length of query
> +    - String, query to select a subset of the available metadata
> +      contexts. If this is not specified (i.e., the "length of query"
> +      field is 0 and no query is sent), then the server MUST send all
> +      the metadata contexts it knows about.


> If specified, this query
> +      string MUST start with a name that uniquely identifies a server
> +      implementation; e.g., the reference implementation that
> +      accompanies this document would support query strings starting
> +      with 'nbd-server:'

I'm not sure this is correct (particularly now you've adopted my "X-"
suggestion), as for instance "base:" does not started with a server
implementation.

Better:

"If specified, the server MUST return the zero or more contexts
whose names (including the namespace) consist of or start with
the query string. For instance a query string of 'nbd-server:'
would return all contexts within the 'nbd-server' namespace,
and a string 'base:a' would return all context within the
'base' namespace that began with 'a'"

> +
> +    The server MUST reply with a list of `NBD_REP_META_CONTEXT` replies,
> +    followed by `NBD_REP_ACK`. The metadata context ID in these replies
> +    is reserved and SHOULD be set to zero; clients SHOULD disregard it.

Why is the context ID set to zero? Surely it would be really helpful
for this to be filled in with the ID?

> +
> +- `NBD_OPT_SET_META_CONTEXT` (11)
> +
> +    Change the set of active metadata contexts. Issuing this command
> +    replaces all previously-set metadata contexts; clients must ensure
> +    that all metadata contexts they're interested in are selected with
> +    the final query that they sent.
> +
> +    Data:
> +    - 32 bits, length of query
> +    - String, query to select metadata contexts.


Again as above, with s/return/select/:

"If not specified, the server MUST select all metadata contexts.
If specified, the server MUST select the zero or more contexts
whose names (including the namespace) consist of or start with
the query string. For instance a query string of 'nbd-server:'
would select all contexts within the 'nbd-server' namespace,
and a string 'base:a' would select all context within the
'base' namespace that began with 'a'"


> The syntax of this
> +      query is implementation-defined,

Right, but I'd make this "The syntax of the metadata context
name is implementation defined" - after all it's not only
the 'query' that has the namespace, it's also the names
themselves. Also perhaps move this elsewhere as it doesn't
only apply to _SET_ but also to _LIST_.

> except that it MUST start with a
> +      namespace. This namespace may be one of the following:
> +        - `base:`, for metadata contexts defined by this document;
> +        - `nbd-server:`, for metadata contexts defined by the
> +          implementation that accompanies this document (none
> +          currently);
> +        - `x-*:`, where `*` can be replaced by any random string not
> +          containing colons, for local experiments.

Let's be a bit more restrictive:

where `*` can be replaced by an arbitrary string of non-whitespace
printable UTF-8 characters, such that the total length of the name
including namespace does not exceed 255 bytes.

> This SHOULD NOT be
> +          used by metadata contexts that are expected to e widely used.

"expected to be widely"

> +        - third-party implementations can register additional
> +          namespaces by simple request to the mailinglist.
> +
> +    The server MUST reply with a number of `NBD_REP_META_CONTEXT`
> +    replies, one for each selected metadata context, each with a unique
> +    metadata context ID

"followed by NBD_REP_ACK"

> . It is not an error if a
> +    `NBD_OPT_SET_META_CONTEXT` option does not select any metadata
> +    context, provided the client then does not attempt to issue
> +    `NBD_CMD_BLOCK_STATUS` commands.
> +
> #### Option reply types
> 
> These values are used in the "reply type" field, sent by the server
> @@ -882,7 +989,7 @@ during option haggling in the fixed newstyle negotiation.
>     information is available, or when sending data related to the option
>     (in the case of `NBD_OPT_LIST`) has finished. No data.
> 
> -* `NBD_REP_SERVER` (2)
> +- `NBD_REP_SERVER` (2)
> 
>     A description of an export. Data:
> 
> @@ -897,10 +1004,18 @@ during option haggling in the fixed newstyle 
> negotiation.
>       particular client request, this field is defined to be a string
>       suitable for direct display to a human being.
> 
> -* `NBD_REP_INFO` (3)
> +- `NBD_REP_INFO` (3)

These two probably belong in a different patch

> 
>     Defined by the experimental `INFO` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
> 
> +- `NBD_REP_META_CONTEXT` (4)
> +
> +    A description of a metadata context. Data:
> +
> +    - 32 bits, NBD metadata context ID.
> +    - String, name of the metadata context. This is not required to be
> +      a human-readable string, but it MUST be valid UTF-8 data.

"consisting of printable non-whitespace UTF-8 characters not greater
than 255 bytes in length"

Did we not say that we wanted to insert the length of the string to allow
for expansion?

> +
> There are a number of error reply types, all of which are denoted by
> having bit 31 set. All error replies MAY have some data set, in which
> case that data is an error message string suitable for display to the user.
> @@ -938,15 +1053,62 @@ case that data is an error message string suitable for 
> display to the user.
> 
>     Defined by the experimental `INFO` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
> 
> -* `NBD_REP_ERR_SHUTDOWN` (2^32 + 7)
> +* `NBD_REP_ERR_SHUTDOWN` (2^31 + 7)
> 
>     The server is unwilling to continue negotiation as it is in the
>     process of being shut down.
> 
> -* `NBD_REP_ERR_BLOCK_SIZE_REQD` (2^32 + 8)
> +* `NBD_REP_ERR_BLOCK_SIZE_REQD` (2^31 + 8)
> 
>     Defined by the experimental `INFO` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
> 

Probably should be in a different patch

> +##### Metadata contexts
> +
> +The `base:allocation` metadata context is the basic "allocated at all"
> +metadata context. If an extent is marked with `NBD_STATE_HOLE` at that
> +context, this means that the given extent is not allocated in the
> +backend storage, and that writing to the extent MAY result in the ENOSPC
> +error. This supports sparse file semantics on the server side. If a
> +server has only one metadata context (the default), then writing to an
> +extent which has `NBD_STATE_HOLE` clear MUST NOT fail with ENOSPC.

Again I'm still confused by this. I *think* you mean "If a server
supports the `base:allocation` metadata context, then writing
to an extent which has `NBD_STATE_HOLE` clear MUST NOT fail with ENOSPC.`

I say that because as currently phrased:

* If a server has one metadata context only, but it is not
  `base:allocation`, then you implying something about writing
  to an extent with a state that won't even exist.

* If a server has `base:allocation` AND another metadata context
  (for instance `qemu:dirty`) then the rule you set out will not
  apply.

> +It defines the following flags for the flags field:
> +
> +- `NBD_STATE_HOLE` (bit 0): if set, the block represents a hole (and
> +  future writes to that area may cause fragmentation or encounter an
> +  `ENOSPC` error); if clear, the block is allocated or the server could
> +  not otherwise determine its status. Note that the use of
> +  `NBD_CMD_TRIM` is related to this status, but that the server MAY
> +  report a hole even where trim has not been requested, and also that a
> +  server MAY report metadata even where a trim has been requested.

'may report that the block is allocated even where'

> +- `NBD_STATE_ZERO` (bit 1): if set, the block contents read as all
> +  zeroes; if clear, the block contents are not known. Note that the use
> +  of `NBD_CMD_WRITE_ZEROES` is related to this status, but that the
> +  server MAY report zeroes even where write zeroes has not been

'even where `NBD_CMD_WRITE_ZEROES` has not been'

> +  requested, and also that a server MAY report unknown content even
> +  where write zeroes has been requested.

'where `NBD_CMD_WRITE_ZEROES` has been requested'

> +
> +It is not an error for a server to report that a region of the
> +export has both `NBD_STATE_HOLE` set and `NBD_STATE_ZERO` clear. The
> +contents of such an area is undefined, and may not be stable;
> +clients who are aware of the existence of such a region SHOULD NOT
> +read it.

As per previous comment, there is no restriction on a client reading it.
For instance, if it was a block within a long read, it might be
sensible to read it.

How about: "the contents of such an area are undefined, and a client
reading such an area should make no assumption as to its contents
or stability."

> +
> +For the `base:allocation` context, the remainder of the flags field is
> +reserved. Servers SHOULD set it to all-zero;

Surely if we want to reserve them for extension, we need "Servers
MUST set it to all-zero"

> clients MUST ignore unknown
> +flags.
> +
> +For all other cases, this specification requires no specific semantics of
> +metadata contexts, except that all the information they provide MUST be
> +representable within the flags field as defined for
> +`NBD_REPLY_TYPE_BLOCK_STATUS`.
> +
> +Likewise, the syntax of query strings is not specified by this document.
> +
> +Server implementations SHOULD document their syntax for query strings
> +and semantics for resulting metadata contexts in a document like this
> +one.
> +
> ### Transmission phase
> 
> #### Flag fields
> @@ -983,6 +1145,11 @@ valid may depend on negotiation during the handshake 
> phase.
>    content chunk in reply.  MUST NOT be set unless the transmission
>    flags include `NBD_FLAG_SEND_DF`.  Use of this flag MAY trigger an
>    `EOVERFLOW` error chunk, if the request length is too large.
> +- bit 3, `NBD_CMD_FLAG_REQ_ONE`; valid during `NBD_CMD_BLOCK_STATUS`. If
> +  set, the client is interested in only one extent per metadata
> +  context. If this flag is present, the server SHOULD NOT send metadata

'MUST NOT'?

> +  on more than one extent in the reply. Clients SHOULD NOT use this flag
> +  on multiple requests for successive regions in the export.

Perhaps better to simply note:

"Client implementors should note that using this flag on multiple
contiguous requests is likely to be inefficient."

After all, it's no worse than multiple one block reads, which we do
not preclude or have as a 'SHOULD NOT'.

> 
> ##### Structured reply flags
> 
> @@ -1051,6 +1218,34 @@ interpret the "length" bytes of payload.
>   64 bits: offset (unsigned)
>   32 bits: hole size (unsigned, MUST be nonzero)
> 
> +- `NBD_REPLY_TYPE_BLOCK_STATUS` (5)
> +
> +    *length* MUST be 4 + (a positive integer multiple of 8).  This reply
> +    represents a series of consecutive block descriptors where the sum
> +    of the lengths of the descriptors

I think 'length fields within the descriptors' would be better, else
you might mean the length of each descriptor itself, which is 8.

> MUST not be greater than the
> +    length of the original request. This chunk type MUST appear exactly
> +    once per metadata ID in a structured reply.
> +
> +    The payload starts with:
> +
> +        * 32 bits, metadata context ID
> +
> +    and is followed by a list of one or more descriptors, each with this
> +    layout:
> +
> +        * 32 bits, length (unsigned, MUST NOT be zero)

Perhaps add: "the length of the extent to which which the status
below applies".

> +        * 32 bits, status flags
> +
> +    If the client used the `NBD_CMD_FLAG_REQ_ONE` flag in the request,
> +    then every reply chunk MUST NOT contain more than one descriptor.
> +
> +    Even if the client did not use the `NBD_CMD_FLAG_REQ_ONE` flag in
> +    its request, the server MAY return less descriptors in the reply

s/less/fewer/

> +    than would be required to fully specify the whole range of requested
> +    information to the client, if the number of descriptors would be
> +    over 16 otherwise

"otherwise be over 16"

> and looking up the information would be too
> +    resource-intensive for the server.
> +
> All error chunk types have bit 15 set, and begin with the same
> *error*, *message length*, and optional *message* fields as
> `NBD_REPLY_TYPE_ERROR`.  If non-zero, *message length* indicates
> @@ -1085,7 +1280,7 @@ remaining structured fields at the end.
>   were sent earlier in the structured reply, the server SHOULD NOT
>   send multiple distinct offsets that lie within the bounds of a
>   single content chunk.  Valid as a reply to `NBD_CMD_READ`,
> -  `NBD_CMD_WRITE`, and `NBD_CMD_TRIM`.
> +  `NBD_CMD_WRITE`, `NBD_CMD_TRIM`, and `NBD_CMD_BLOCK_STATUS`.
> 
>   The payload is structured as:
> 
> @@ -1259,6 +1454,44 @@ The following request types exist:
> 
>     Defined by the experimental `WRITE_ZEROES` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-write-zeroes/doc/proto.md).
> 
> +* `NBD_CMD_BLOCK_STATUS` (7)
> +
> +    A block status query request. Length and offset define the range of
> +    interest. Clients MUST NOT use this request unless metadata
> +    contexts have been negotiated,

I think you mean "have been selected" but see my comment re perhaps
making no contexts being selected meaning all contexts are selected.

> which in turn requires the client to
> +    first negotiate structured replies. For a successful return, the
> +    server MUST use a structured reply, containing at least one chunk of
> +    type `NBD_REPLY_TYPE_BLOCK_STATUS`, where the status field of each
> +    descriptor is determined by the flags field as defined by the
> +    metadata context.
> +
> +    The list of block status descriptors within the
> +    `NBD_REPLY_TYPE_BLOCK_STATUS` chunk represent consecutive portions
> +    of the file starting from specified *offset*, and the sum of the
> +    *length* fields of each descriptor MUST not be greater than the
> +    overall *length* of the request. This means that the server MAY
> +    return less data than required. However the server MUST return at
> +    least one status descriptor.  The server SHOULD use different
> +    *status* values between consecutive descriptors, and SHOULD use
> +    descriptor lengths that are an integer multiple of 512 bytes where
> +    possible (the first and last descriptor of an unaligned query being
> +    the most obvious places for an exception). The status flags are
> +    intentionally defined so that a server MAY always safely report a
> +    status of 0 for any block, although the server SHOULD return
> +    additional status values when they can be easily detected.
> +
> +    If an error occurs, the server SHOULD set the appropriate error
> +    code in the error field of an error chunk. However, if the error
> +    does not involve invalid usage (such as a request beyond the bounds
> +    of the file), a server MAY reply with a single block status
> +    descriptor with *length* matching the requested length, and *status*
> +    of 0 rather than reporting the error.
> +
> +    A client MAY initiate a hard disconnect if it detects that the
> +    server has sent an invalid chunk. The server SHOULD return `EINVAL`
> +    if it receives a `NBD_CMD_BLOCK_STATUS` request including one or
> +    more sectors beyond the size of the device.
> +
> * Other requests
> 
>     Some third-party implementations may require additional protocol
> 
> --
> < ron> I mean, the main *practical* problem with C++, is there's like a dozen
>       people in the world who think they really understand all of its rules,
>       and pretty much all of them are just lying to themselves too.
> -- #debian-devel, OFTC, 2016-02-12
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--
Alex Bligh

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