Re: [virtio-dev] [RFC PATCH v2] virtio-blk: add zoned block device specification

[Dmitry Fomichev <Dmitry.Fomichev@wdc.com>]

On Sat, 2022-06-11 at 21:09 +0100, Stefan Hajnoczi wrote:
> On Thu, Jun 09, 2022 at 08:43:21PM -0400, Dmitry Fomichev wrote:
> > +Host-managed zoned block devices have their LBA range divided to Sequential
> > +Write Required (SWR) zones that require some additional handling from the
> > host
> > +for sustainable operation. All write requests to SWR zones must be
> > sequential
> > +and the zones with some data need to be reset before that data can be
> > rewritten.
> > +Host-managed devices support a set of ZBD-specific I/O requests that can be
> > used
> > +by the host to manage device zones. Host-managed devices report
> > VIRTIO_BLK_Z_HM
> > +value in the \field{model} field in \field{zoned}.
> 
> One of
> "report the VIRTIO_BLK_Z_HM value in ..."
> "report a VIRTIO_BLK_Z_HM value in ..."
> "report VIRTIO_BLK_Z_HM in ..."
> reads more naturally.

OK.

> 
> > +
> > +Host-aware zoned block devices have their LBA range divided to Sequential
> > +Write Preferred (SWP) zones that support the random write access, similar to
> > +regular non-zoned devices. However, the device I/O performance might not be
> > +optimal if SWP zones are used in a random I/O pattern. SWP zones also
> > support
> > +the same set of ZBD-specific I/O requests as host-managed devices that allow
> > +host-aware devices to be managed by any host that supports zoned block
> > devices
> > +to achieve its optimum performance. Host-aware devices report
> > VIRTIO_BLK_Z_HA
> > +value in the \field{model} field in \field{zoned}.
> 
> Same as above.

Sure.

> 
> > +
> > +During device operation, SWR and SWP zones can be in one of the following
> > states:
> > +empty, implicitly-open, explicitly-open, closed and full. The state machine
> > that
> > +governs the transitions between these states is described later in this
> > document.
> > +
> > +SWR and SWP zones consume volatile device resources while being in certain
> > +states and the device may set limits on the number of zones that can be in
> > these
> > +states simultaneously.
> > +
> > +Zoned block devices use two internal counters to account for the device
> > +resources in use, the number of currently open zones and the number of
> > currently
> > +active zones.
> > +
> > +Any zone state transition from a state that doesn't consume a zone resource
> > to a
> > +state that consumes the same resource increments the internal device counter
> > for
> > +that resource. Any zone transition out of a state that consumes a zone
> > resource
> > +to a state that doesn't consume the same resource decrements the counter.
> > Any
> > +request that causes the device to exceed the reported zone resource limits
> > is
> > +terminated by the device with a "zone resources exceeded" error as defined
> > for
> > +specific commands later.
> > +
> > Â\begin{lstlisting}
> > Âstruct virtio_blk_config {
> > ÂÂÂÂÂÂÂÂ le64 capacity;
> > @@ -4623,6 +4694,15 @@ \subsection{Device configuration
> > layout}\label{sec:Device Types / Block Device /
> > ÂÂÂÂÂÂÂÂ le32 max_secure_erase_sectors;
> > ÂÂÂÂÂÂÂÂ le32 max_secure_erase_seg;
> > ÂÂÂÂÂÂÂÂ le32 secure_erase_sector_alignment;
> > +ÂÂÂÂÂÂÂ struct virtio_blk_zoned_characteristics {
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 zone_sectors;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 max_open_zones;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 max_active_zones;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 max_append_sectors;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 write_granularity;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ u8 model;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ u8 unused2[3];
> > +ÂÂÂÂÂÂÂ } zoned;
> > Â};
> > Â\end{lstlisting}
> > Â
> > @@ -4686,6 +4766,10 @@ \subsection{Device Initialization}\label{sec:Device
> > Types / Block Device / Devic
> > ÂÂÂÂ \field{secure_erase_sector_alignment} can be used by OS when splitting a
> > ÂÂÂÂ request based on alignment.
> > Â
> > +\item If the VIRTIO_BLK_F_ZONED feature is negotiated, the fields in
> > +ÂÂÂ \field{zoned} can be read by the driver to determine the zone
> > +ÂÂÂ characteristics of the device. All \field{zoned} fields are read-only.
> > +
> > Â\end{enumerate}
> > Â
> > Â\drivernormative{\subsubsection}{Device Initialization}{Device Types / Block
> > Device / Device Initialization}
> > @@ -4701,6 +4785,33 @@ \subsection{Device Initialization}\label{sec:Device
> > Types / Block Device / Devic
> > ÂThe driver MUST NOT read \field{writeback} before setting
> > Âthe FEATURES_OK \field{device status} bit.
> > Â
> > +Drivers SHOULD NOT negotiate VIRTIO_BLK_F_ZONED feature if they are
> > incapable
> > +of supporting devices with the VIRTIO_BLK_Z_HM or VIRTIO_BLK_Z_HA zoned
> > model.
> > +
> > +Drivers MAY operate with VIRTIO_BLK_F_ZONED feature negotiated when the
> > device
> > +reports VIRTIO_BLK_Z_NONE zoned model for testing and development.
> 
> Specifying a particular use case ("testing and development") could be
> interpreted to imply that other use cases MAY NOT operate with
> VIRTIO_BLK_F_ZONED.
> 
> I suggest dropping the use case from the sentence or moving that sub-point
> outside the normative section to where the text describes VIRTIO_BLK_Z_NONE.
> Something like "Devices that offer VIRTIO_BLK_F_ZONED with VIRTIO_BLK_Z_NONE
> commonly do so for testing and development purposes".

Good point, will do.

> 
> > +Each request is of form:
> > +
> > +\begin{lstlisting}
> > +struct virtio_blk_zoned_req {
> > +ÂÂÂÂÂÂÂ le32 type;
> > +ÂÂÂÂÂÂÂ le32 reserved;
> > +ÂÂÂÂÂÂÂ le64 sector;
> > +ÂÂÂÂÂÂÂ struct {
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* ALL zone operation flag */
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 mgmt_send_all:1;
> 
> Bit-fields cannot be used in external interfaces (like hardware
> interfaces) because the C standard says (N1570 6.7.2.1 11):
> 
> Â The order of allocation of bit-fields within a unit (high-order to
> Â low-order or low-order to high-order) is implementation-defined.
> 
> So this C syntax does not define a specific binary representation, it's
> up to the compiler or system ABI.
> 

Right... I was confused by the fact that virtio_blk_discard_write_zeroes struct
in the same spec happily uses bit-fields for the unmap flag. Perhaps, that
flag could be also defined using a bit mask... :)

> Please use a le32 field and define a bit number:
> 
> Â #define VIRTIO_BLK_ZONE_MGMT_F_SEND_ALL (1u << 0)
> Â le32 flags;
> 
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ le32 reserved:31;
> > +ÂÂÂÂÂÂÂ } zone;
> > +ÂÂÂÂÂÂÂ u8 data[];
> > +ÂÂÂÂÂÂÂ le64 append_sector;
> > +ÂÂÂÂÂÂÂ u8 reserved1[3];
> > +ÂÂÂÂÂÂÂ u8 status;
> > +};
> > +\end{lstlisting}
> 

Yep, the ends up to be pretty simple. Using a le32 should be sufficient for
future command flag additions like the partial flag, etc.

> Now that the status field is at the end of the struct the zoned commands
> can be treated like any other struct virtio_blk_req.
> 
> The contents of the data[] field for ZONE_OPEN, ZONE_CLOSE, ZONE_FINISH,
> and ZONE_RESET requests is:
> 
> Â #define VIRTIO_BLK_ZONE_MGMT_F_SEND_ALL (1u << 0)
> Â struct virtio_blk_zoned_mgmt_send {
> ÂÂÂÂÂ le32 flags;
> Â };
> 
> The contents of the data[] field for ZONE_APPEND is:
> 
> Â u8 payload[];ÂÂÂÂÂÂÂÂÂÂÂ /* driver -> device */
> 
> followed by:
> 
> Â le64 append_sector;ÂÂÂÂÂ /* device -> driver */
> 
> The contents of the data[] field for ZONE_REPORT is struct
> virtio_blk_zone_report.
> 
> The reason I'm pushing for getting rid of struct virtio_blk_zoned_req is
> that the zbd-specific fields are not common to all ZONE_* requests. I
> don't see a reason to define a common request layout anymore.
> 
> Also, it would make the zoned command design similar to struct
> virtio_blk_discard_write_zeroes where request metadata is a separate
> struct that is located in struct virtio_blk_req's data[] field instead
> of a new top-level struct virtio_blk_discard_write_zeroes_request that
> duplicates struct virtio_blk_req fields.

My concern is that this approach requires quite a few of runtime checks to be
added in the hot path - if (alba), if (flags), request opcode checks... My PoC
code that uses virtio_blk_zoned_req adds zero checks, both for zoned and non-
zoned operation.

With discard, there is no other way to handle it except to introduce
virtio_blk_discard_write_zeroes because the number of incoming discard segments
is variable, but for zoned operation it should be sufficient to use a simpler
solution with the virtio_blk_zoned_req which seems to be less fragile in terms of
implementation.

DF

> 
> Stefan