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Zoned Block Device User Interface

User applications can access the zone information of a zoned block device and can manage the zones of a zoned block device by using two types of interfaces.

  1. sysfs attribute files, accessible either directly from applications as regular files or from scripted languages (shell scripts, python, etc).

  2. ioctl() system calls, suitable for use from C programs or other programming languages that have an equivalent system-call binding.

The sysfs files and ioctl() commands available to applications have evolved since the introduction of zoned block device support in Kernel 4.10. The availability of files and commands per kernel version is detailed in the following sections.

Sysfs Interface#

Programs that use scripting languages (e.g. bash scripts) can access zoned device information through sysfs attribute files. The attribute files provided are shown in the following table.

FileKernel versionDescription
/sys/block/dev name/queue/zoned4.10.0Device zoned model
/sys/block/dev name/queue/chunk_sectors4.10.0Device zone size
/sys/block/dev name/queue/nr_zones4.20.0Total number of zones
/sys/block/dev name/queue/zone_append_max_bytes5.8.0Maximum size in bytes of a zone append write operation
/sys/block/dev name/queue/max_open_zones5.9.0Maximum number of open zones
/sys/block/dev name/queue/max_active_zones5.9.0Maximum number of active zones

Device Zoned Model#

The zone model of a zoned device can be discovered by using the zoned device queue attribute file. For example: for a zoned block device named sdb, the following shell command displays the device zoned model.

# cat /sys/block/sdb/queue/zoned
host-managed

The possible values of the zoned attribute file are shown in the table below.

ValueDescription
noneRegular block device, including drive-managed SMR disks
host-awareHost-aware device model
host-managedHost-managed device model

Device Zone Size#

The device zone size can be read from the sysfs queue attribute file that is named chunk_sectors. For a device named sdb (the same device as in the previous example), the following command gives the device zone size.

# cat /sys/block/sdb/queue/chunk_sectors
524288

The value is displayed as a number of 512B sectors, regardless of the actual logical and physical block size of the device. In this example, the device zone size is 524288 x 512 = 256 MiB.

Number of Zones#

The sysfs queue attribute file nr_zones was introduced in Linux kernel version 4.20.0, and is available to obtain the total number of zones of a zoned device.

# cat /sys/block/sdb/queue/nr_zones
55880

This attribute value is always 0 for a regular block device.

ioctl() Application Programming Interface#

The C header file /usr/include/linux/blkzoned.h contains macro definitions and data structure definitions that allow application developers to obtain information about zoned block devices and to manage the zones of the devices.

Zone Information Data Structures#

The data structure struct blk_zone defines a zone-descriptor structure that contains a complete description of a zone: this includes the zone's location on the device, the zone type, its condition (state), and the position of the zone write pointer (for sequential zones). For kernels Up to kernel version 5.8, this data structure is as shown below.

/**
* struct blk_zone - Zone descriptor for BLKREPORTZONE ioctl.
*
* @start: Zone start in 512 B sector units
* @len: Zone length in 512 B sector units
* @wp: Zone write pointer location in 512 B sector units
* @type: see enum blk_zone_type for possible values
* @cond: see enum blk_zone_cond for possible values
* @non_seq: Flag indicating that the zone is using non-sequential resources
* (for host-aware zoned block devices only).
* @reset: Flag indicating that a zone reset is recommended.
* @reserved: Padding to 64 B to match the ZBC/ZAC defined zone descriptor size.
*
* start, len, capacity and wp use the regular 512 B sector unit, regardless
* of the device logical block size. The overall structure size is 64 B to
* match the size of the ZBC-, ZAC- and ZNS-defined zone descriptor and to
* allow support for future additional zone information.
*/
struct blk_zone {
__u64 start; /* Zone start sector */
__u64 len; /* Zone length in number of sectors */
__u64 wp; /* Zone write pointer position */
__u8 type; /* Zone type */
__u8 cond; /* Zone condition */
__u8 non_seq; /* Non-sequential write resources active */
__u8 reset; /* Reset write pointer recommended */
__u8 reserved[36];
};

As indicated in the comments to this data structure definition, the unit that is used to indicate (1) the zone start position, (2) the zone's size and (3) the write pointer position is 512B sector size. This holds true regardless of the actual logical block size of the device. Even for a device with a 4KB logical sector, the above zone descriptor fields use a 512-byte sector size unit.

The capacity field was added to struct blk_zone in kernel version 5.9. With this change, the data structure is as follows.

/**
* struct blk_zone - Zone descriptor for BLKREPORTZONE ioctl.
*
* @start: Zone start in 512 B sector units
* @len: Zone length in 512 B sector units
* @wp: Zone write pointer location in 512 B sector units
* @type: see enum blk_zone_type for possible values
* @cond: see enum blk_zone_cond for possible values
* @non_seq: Flag indicating that the zone is using non-sequential resources
* (for host-aware zoned block devices only).
* @reset: Flag indicating that a zone reset is recommended.
* @resv: Padding for 8B alignment.
* @capacity: Zone usable capacity in 512 B sector units
* @reserved: Padding to 64 B to match the ZBC, ZAC and ZNS defined zone
* descriptor size.
*
* start, len, capacity and wp use the regular 512 B sector unit, regardless
* of the device logical block size. The overall structure size is 64 B to
* match the ZBC, ZAC and ZNS defined zone descriptor and allow support for
* future additional zone information.
*/
struct blk_zone {
__u64 start; /* Zone start sector */
__u64 len; /* Zone length in number of sectors */
__u64 wp; /* Zone write pointer position */
__u8 type; /* Zone type */
__u8 cond; /* Zone condition */
__u8 non_seq; /* Non-sequential write resources active */
__u8 reset; /* Reset write pointer recommended */
__u8 resv[4];
__u64 capacity; /* Zone capacity in number of sectors */
__u8 reserved[24];
};

The capacity field indicates the usable zone capacity of a zone in units of 512B sectors. The presence, or validity, of this field within the structure is indicated using a zone report flag. See Obtaining Zone Information below for details.

Zone Type#

The type field of a zone descriptor can have only one of the values defined by the enumeration enum blk_zone_type.

/**
* enum blk_zone_type - Types of zones allowed in a zoned device.
*
* @BLK_ZONE_TYPE_CONVENTIONAL: The zone has no write pointer and can be writen
* randomly. Zone reset has no effect on the zone.
* @BLK_ZONE_TYPE_SEQWRITE_REQ: The zone must be written sequentially
* @BLK_ZONE_TYPE_SEQWRITE_PREF: The zone can be written non-sequentially
*
* Any other value not defined is reserved and must be considered as invalid.
*/
enum blk_zone_type {
BLK_ZONE_TYPE_CONVENTIONAL = 0x1,
BLK_ZONE_TYPE_SEQWRITE_REQ = 0x2,
BLK_ZONE_TYPE_SEQWRITE_PREF = 0x3,
};

Zone Condition#

The cond field of the struct blkzone data structure defines the current condition of a zone. The possible condition (state) values of this field are defined by the blk_zone_cond enumeration.

/**
* enum blk_zone_cond - Condition [state] of a zone in a zoned device.
*
* @BLK_ZONE_COND_NOT_WP: The zone has no write pointer, it is conventional.
* @BLK_ZONE_COND_EMPTY: The zone is empty.
* @BLK_ZONE_COND_IMP_OPEN: The zone is open, but not explicitly opened.
* @BLK_ZONE_COND_EXP_OPEN: The zones was explicitly opened by an
* OPEN ZONE command.
* @BLK_ZONE_COND_CLOSED: The zone was [explicitly] closed after writing.
* @BLK_ZONE_COND_FULL: The zone is marked as full, possibly by a zone
* FINISH ZONE command.
* @BLK_ZONE_COND_READONLY: The zone is read-only.
* @BLK_ZONE_COND_OFFLINE: The zone is offline (sectors cannot be read/written).
*
* The Zone Condition state machine in the ZBC/ZAC standards maps the above
* deinitions as:
* - ZC1: Empty | BLK_ZONE_EMPTY
* - ZC2: Implicit Open | BLK_ZONE_COND_IMP_OPEN
* - ZC3: Explicit Open | BLK_ZONE_COND_EXP_OPEN
* - ZC4: Closed | BLK_ZONE_CLOSED
* - ZC5: Full | BLK_ZONE_FULL
* - ZC6: Read Only | BLK_ZONE_READONLY
* - ZC7: Offline | BLK_ZONE_OFFLINE
*
* Conditions 0x5 to 0xC are reserved by the current ZBC/ZAC spec and should
* be considered invalid.
*/
enum blk_zone_cond {
BLK_ZONE_COND_NOT_WP = 0x0,
BLK_ZONE_COND_EMPTY = 0x1,
BLK_ZONE_COND_IMP_OPEN = 0x2,
BLK_ZONE_COND_EXP_OPEN = 0x3,
BLK_ZONE_COND_CLOSED = 0x4,
BLK_ZONE_COND_READONLY = 0xD,
BLK_ZONE_COND_FULL = 0xE,
BLK_ZONE_COND_OFFLINE = 0xF,
};

Under a device's normal operation, some of these conditions cannot result directly from host-initiated operations. These conditions are BLK_ZONE_COND_OFFLINE and BLK_ZONE_COND_READONLY. They can be set only by the device itself, to indicate zones with capabilities that have been limited by a hardware defect.

Transitions to other conditions result from user operations, either write operations or zone management commands. Zone management commands can be issued by an application using the kernel ioctl() interface (see ioctl() Commands.

The SCSI Zoned Block Command specification (ZBC), the ATA Zoned Device ATA Command Set specification (ZAC) and the NVM Express Zoned Namespace Command Set specification (ZNS) define a zone condition state machine governing the possible transitions of a zone from one condition to another depending on the commands executed.

ioctl() Commands#

Several ioctl() commands are defined to manipulate and obtain information and manipulate the zones of a zoned block device. All supported commands are shown below.

/**
* Zoned block device ioctl's:
*
* @BLKREPORTZONE: Get zone information. Takes a zone report as argument.
* The zone report will start from the zone containing the
* sector specified in the report request structure.
* @BLKRESETZONE: Reset the write pointer of the zones in the specified
* sector range. The sector range must be zone aligned.
* @BLKGETZONESZ: Get the device zone size in number of 512 B sectors.
* @BLKGETNRZONES: Get the total number of zones of the device.
* @BLKOPENZONE: Open the zones in the specified sector range.
* The 512 B sector range must be zone aligned.
* @BLKCLOSEZONE: Close the zones in the specified sector range.
* The 512 B sector range must be zone aligned.
* @BLKFINISHZONE: Mark the zones as full in the specified sector range.
* The 512 B sector range must be zone aligned.
*/
#define BLKREPORTZONE _IOWR(0x12, 130, struct blk_zone_report)
#define BLKRESETZONE _IOW(0x12, 131, struct blk_zone_range)
#define BLKGETZONESZ _IOR(0x12, 132, __u32)
#define BLKGETNRZONES _IOR(0x12, 133, __u32)
#define BLKOPENZONE _IOW(0x12, 134, struct blk_zone_range)
#define BLKCLOSEZONE _IOW(0x12, 135, struct blk_zone_range)
#define BLKFINISHZONE _IOW(0x12, 136, struct blk_zone_range)

Not all commands are available on all kernel versions. The following table shows the kernel version that introduced each command.

CommandKernel versionDescription
BLKREPORTZONE4.10.0Get zone information
BLKRESETZONE4.10.0Reset a zone write pointer
BLKGETZONESZ4.20.0Get a device zone size
BLKGETNRZONES4.20.0Get the total number of zones of a device
BLKOPENZONE5.5.0Explicitly open a zone
BLKCLOSEZONE5.5.0Close a zone
BLKFINISHZONE5.5.0Finish a zone

Obtaining Zone Information#

The BLKREPORTZONE command allows an application to obtain a device's zone information in the form of an array of zone descriptors. The data argument that is passed to the ioctl() must be the address of a memory area large enough to store one struct blk_zone_report header structure, followed by an array of zone descriptors.

The zone report header structure blk_zone_report is as shown below.

/**
* struct blk_zone_report - BLKREPORTZONE ioctl request/reply
*
* @sector: starting sector of report
* @nr_zones: IN maximum / OUT actual
* @reserved: padding to 16 byte alignment
* @zones: Space to hold @nr_zones @zones entries on reply.
*
* The array of at most @nr_zones must follow this structure in memory.
*/
struct blk_zone_report {
__u64 sector;
__u32 nr_zones;
__u8 reserved[4];
struct blk_zone zones[0];
};

The header indicates the 512-byte sector from which the report should start as well as the number of zone descriptors in the array following the header. A typical use of the BLKREPORTZONE command to obtain information on all the zones of a device is as shown below.

#include <stdlib.h>
#include <linux/blkzoned.h>
unsigned long long start_sector = 0;
struct blk_zone_report *hdr;
size_t hdr_len;
int nr_zones = 256;
hdr_len = sizeof(struct blk_zone_report) + nr_zones * sizeof(struct blkzone);
hdr = malloc(hdr_len);
if (!hdr)
return -1;
while (1) {
hdr->sector = start_sector;
hdr->nr_zones = nr_zones;
ret = ioctl(fd, BLKREPORTZONE, hdr);
if (ret)
goto error;
if (!hdr->nr_zones) {
/* Done */
break;
}
printf("Got %u zone descriptors\n", hdr->nr_zones);
...
/* The next report must start after the last zone reported */
start_sector = hdr->zones[hdr->nr_zones - 1].start +
hdr->zones[hdr->nr_zones - 1].len;
}

The number of zone descriptors obtained is returned to the user in the nr_zones field of the report header structure blk_zone_report.

With the introduction of zone capacity support for NVMe Zoned Namepsaces in kernel version 5.9, zone descriptors gained the capacity field. The presence of this field is indicated by the new flag field added to struct blk_zone_report.

/**
* enum blk_zone_report_flags - Feature flags of reported zone descriptors.
*
* @BLK_ZONE_REP_CAPACITY: Zone descriptor has capacity field.
*/
enum blk_zone_report_flags {
BLK_ZONE_REP_CAPACITY = (1 << 0),
};
/**
* struct blk_zone_report - BLKREPORTZONE ioctl request/reply
*
* @sector: starting sector of report
* @nr_zones: IN maximum / OUT actual
* @flags: one or more flags as defined by enum blk_zone_report_flags.
* @zones: Space to hold @nr_zones @zones entries on reply.
*
* The array of at most @nr_zones must follow this structure in memory.
*/
struct blk_zone_report {
__u64 sector;
__u32 nr_zones;
__u32 flags;
struct blk_zone zones[0];
};

If the flags field of struct blk_zone_report has the flag BLK_ZONE_REP_CAPACITY set, then the zone descriptor's structure will have a valid value set in the capacity field of sturct blk_zone. Otherwise, this field can be ignored as it will show a value of 0.

The example code below, extracted from the code of the libzbd library, illustrates how applications can implement backward-compatible support for zone capacity information by using the autotools build environment.

# less configure.ac
...
AC_CHECK_HEADER(linux/blkzoned.h, [],
[AC_MSG_ERROR([Couldn't find linux/blkzoned.h. Kernel too old ?])],
[[#include <linux/blkzoned.h>]])
AC_CHECK_MEMBER([struct blk_zone.capacity],
[AC_DEFINE(HAVE_BLK_ZONE_REP_V2, [1], [report zones includes zone capacity])],
[], [[#include <linux/blkzoned.h>]])
...
# less lib/zbd.h
...
/*
* Handle kernel zone capacity support
*/
#ifndef HAVE_BLK_ZONE_REP_V2
#define BLK_ZONE_REP_CAPACITY (1 << 0)
struct blk_zone_v2 {
__u64 start; /* Zone start sector */
__u64 len; /* Zone length in number of sectors */
__u64 wp; /* Zone write pointer position */
__u8 type; /* Zone type */
__u8 cond; /* Zone condition */
__u8 non_seq; /* Non-sequential write resources active */
__u8 reset; /* Reset write pointer recommended */
__u8 resv[4];
__u64 capacity; /* Zone capacity in number of sectors */
__u8 reserved[24];
};
#define blk_zone blk_zone_v2
struct blk_zone_report_v2 {
__u64 sector;
__u32 nr_zones;
__u32 flags;
struct blk_zone zones[0];
};
#define blk_zone_report blk_zone_report_v2
#endif /* HAVE_BLK_ZONE_REP_V2 */
...

With this method, the main code responsible for issuing and parsing zone reports always has access to the capacity field of struct blk_zone regardless of the kernel version the code is executed on. For kernels before kernel version 5.9, the zone capacity field will always be equal to 0, meaning that the zone capacity should be ignored and that the zone size should be used in its place. Different coding techniques can also be used to always return a zone capacity equal to the zone size for kernels lacking support for this field.

The command line utility blkzone, which is part of the util-linux project, uses the BLKREPORTZONE command to implement its report function. Its code was modified similarly to the above method to allow its correct compilation and execution regardless of the version of the kernel being used.

Resetting a Zone Write Pointer#

The write pointer of a single sequential zone or of a range of contiguous sequential zones can be reset using the BLKRESETZONE command. Resetting a sequential zone write pointer position will also transition the zone to the Empty condition (BLK_ZONE_COND_EMPTY).

The range of zones to reset is defined using the data structure blk_zone_range shown below.

/**
* struct blk_zone_range - BLKRESETZONE/BLKOPENZONE/
* BLKCLOSEZONE/BLKFINISHZONE ioctl
* requests
* @sector: Starting sector of the first zone to operate on.
* @nr_sectors: Total number of sectors of all zones to operate on.
*/
struct blk_zone_range {
__u64 sector;
__u64 nr_sectors;
};

The sector field must specify the start sector of the first zone to reset. The nr_sectors field specifies the total length of the range of zones to reset. This length must be at least as large as one zone.

As indicated in comments describing the blk_zone_range structure, the commands BLKOPENZONE, BLKCLOSEZONE and BLKFINISHZONE also use this data structure to define the range of zones on which the command will operate.

The following code shows an example use of the BLKRESETZONE command to reset a single zone starting at sector 274726912 with a zone size of 256 MiB (524288 sectors of 512B).

#include <linux/blkzoned.h>
struct blk_zone_range zrange;
int ret;
zrange.sector = 274726912;
zrange.nr_sectors = 524288;
ret = ioctl(fd, BLKRESETZONE, &zrange);
if (ret)
goto error;
...

The device file descriptor fd must be open for writing in order for this command to succeed.

The command line utility blkzone uses the BLKRESETZONE command to implement its reset functionality.

Opening, Closing and Finishing Zones#

Explicitly opening a zone or a range of zones can be done using the BLKOPENZONE command. This command uses the same arguments as the BLKRESETZONE command. It takes a pointer to a data structure blk_zone_range, which specifies the range of zones to operate on.

Closing a zone is done using the command BLKCLOSEZONE. Finishing a zone--that is, transitioning the zone to the full condition (BLK_ZONE_COND_FULL), is done using the BLKFINISHZONE command. Both of these commands also take as arguments a pointer to the blk_zone_range data structure to specify the range of zones to operate on.

The BLKOPENZONE, BLKCLOSEZONE and BLKFINISHZONE commands were introduced in kernel version 5.5.0.

Zone Size and Number of Zones#

Linux® kernel version 4.20 introduced two new commands: one to obtain a zoned device's zone size (BLKGETZONESZ), and one to obtain the total number of zones of the device (BLKGETNRZONES). Both commands take a pointer to an unsigned 32-bit integer variable as an argument, and the zone-size value or the number of zones will be returned. The following sample C code illustrates the use of these commands.

#include <linux/blkzoned.h>
#include <stdio.h>
unsigned int nr_zones, zone_size;
int ret;
ret = ioctl(fd, ,BLKGETZONESZ, &zone_size);
if (ret)
goto error;
ret = ioctl(fd, ,BLKGETNRZONES, &nr_zones);
if (ret)
goto error;
printf("Device has %u zones of %u 512-Bytes sectors\n",
nr_zones, zone_size);
...

The command BLKGETNRZONES is especially useful for allocating an array of zone descriptors large enough for a zone report on all the zones of a device.