Block Device User Guide


The SPDK block device layer, often simply called bdev, is a C library intended to be equivalent to the operating system block storage layer that often sits immediately above the device drivers in a traditional kernel storage stack. Specifically, this library provides the following functionality:

  • A pluggable module API for implementing block devices that interface with different types of block storage devices.
  • Driver modules for NVMe, malloc (ramdisk), Linux AIO, virtio-scsi, Ceph RBD, Pmem and Vhost-SCSI Initiator and more.
  • An application API for enumerating and claiming SPDK block devices and then performing operations (read, write, unmap, etc.) on those devices.
  • Facilities to stack block devices to create complex I/O pipelines, including logical volume management (lvol) and partition support (GPT).
  • Configuration of block devices via JSON-RPC.
  • Request queueing, timeout, and reset handling.
  • Multiple, lockless queues for sending I/O to block devices.

Bdev module creates abstraction layer that provides common API for all devices. User can use available bdev modules or create own module with any type of device underneath (please refer to Writing a Custom Block Device Module for details). SPDK provides also vbdev modules which creates block devices on existing bdev. For example Logical volumes or SPDK GPT partition table


This guide assumes that you can already build the standard SPDK distribution on your platform. The block device layer is a C library with a single public header file named bdev.h. All SPDK configuration described in following chapters is done by using JSON-RPC commands. SPDK provides a python-based command line tool for sending RPC commands located at scripts/ User can list available commands by running this script with -h or --help flag. Additionally user can retrieve currently supported set of RPC commands directly from SPDK application by running scripts/ rpc_get_methods. Detailed help for each command can be displayed by adding -h flag as a command parameter.

General Purpose RPCs


List of currently available block devices including detailed information about them can be get by using bdev_get_bdevs RPC command. User can add optional parameter name to get details about specified by that name bdev.

Example response

"num_blocks": 32768,
"assigned_rate_limits": {
"rw_ios_per_sec": 10000,
"rw_mbytes_per_sec": 20
"supported_io_types": {
"reset": true,
"nvme_admin": false,
"unmap": true,
"read": true,
"write_zeroes": true,
"write": true,
"flush": true,
"nvme_io": false
"driver_specific": {},
"claimed": false,
"block_size": 4096,
"product_name": "Malloc disk",
"name": "Malloc0"


Users can use the bdev_set_qos_limit RPC command to enable, adjust, and disable rate limits on an existing bdev. Two types of rate limits are supported: IOPS and bandwidth. The rate limits can be enabled, adjusted, and disabled at any time for the specified bdev. The bdev name is a required parameter for this RPC command and at least one of rw_ios_per_sec and rw_mbytes_per_sec must be specified. When both rate limits are enabled, the first met limit will take effect. The value 0 may be specified to disable the corresponding rate limit. Users can run this command with -h or --help for more information.


The bdev_enable_histogram RPC command allows to enable or disable gathering latency data for specified bdev. Histogram can be downloaded by the user by calling bdev_get_histogram and parsed using scripts/ script.

Example command bdev_enable_histogram Nvme0n1 --enable

The command will enable gathering data for histogram on Nvme0n1 device. bdev_get_histogram Nvme0n1 |

The command will download gathered histogram data. The script will parse the data and show table containing IO count for latency ranges. bdev_enable_histogram Nvme0n1 --disable

The command will disable histogram on Nvme0n1 device.

Ceph RBD

The SPDK RBD bdev driver provides SPDK block layer access to Ceph RADOS block devices (RBD). Ceph RBD devices are accessed via librbd and librados libraries to access the RADOS block device exported by Ceph. To create Ceph bdev RPC command bdev_rbd_create should be used.

Example command bdev_rbd_create rbd foo 512

This command will create a bdev that represents the 'foo' image from a pool called 'rbd'.

To remove a block device representation use the bdev_rbd_delete command. bdev_rbd_delete Rbd0

To resize a bdev use the bdev_rbd_resize command. bdev_rbd_resize Rbd0 4096

This command will resize the Rbd0 bdev to 4096 MiB.

Compression Virtual Bdev Module

The compression bdev module can be configured to provide compression/decompression services for an underlying thinly provisioned logical volume. Although the underlying module can be anything (i.e. NVME bdev) the overall compression benefits will not be realized unless the data stored on disk is placed appropriately. The compression vbdev module relies on an internal SPDK library called reduce to accomplish this, see SPDK "Reduce" Block Compression Algorithm for detailed information.

The vbdev module relies on the DPDK CompressDev Framework to provide all compression functionality. The framework provides support for many different software only compression modules as well as hardware assisted support for Intel QAT. At this time the vbdev module supports the DPDK drivers for ISAL and QAT.

Persistent memory is used to store metadata associated with the layout of the data on the backing device. SPDK relies on PMDK to interface persistent memory so any hardware supported by PMDK should work. If the directory for PMEM supplied upon vbdev creation does not point to persistent memory (i.e. a regular filesystem) performance will be severely impacted. The vbdev module and reduce libraries were designed to use persistent memory for any production use.

Example command bdev_compress_create -p /pmem_files -b myLvol

In this example, a compression vbdev is created using persistent memory that is mapped to the directory pmem_files on top of the existing thinly provisioned logical volume myLvol. The resulting compression bdev will be named COMP_LVS/myLvol where LVS is the name of the logical volume store that myLvol resides on.

The logical volume is referred to as the backing device and once the compression vbdev is created it cannot be separated from the persistent memory file that will be created in the specified directory. If the persistent memory file is not available, the compression vbdev will also not be available.

By default the vbdev module will choose the QAT driver if the hardware and drivers are available and loaded. If not, it will revert to the software-only ISAL driver. By using the following command, the driver may be specified however this is not persistent so it must be done either upon creation or before the underlying logical volume is loaded to be honored. In the example below, 0 is telling the vbdev module to use QAT if available otherwise use ISAL, this is the default and if sufficient the command is not required. Passing a value of 1 tells the driver to use QAT and if not available then the creation or loading the vbdev should fail to create or load. A value of '2' as shown below tells the module to use ISAL and if for some reason it is not available, the vbdev should fail to create or load. compress_set_pmd -p 2

To remove a compression vbdev, use the following command which will also delete the PMEM file. If the logical volume is deleted the PMEM file will not be removed and the compression vbdev will not be available. bdev_compress_delete COMP_LVS/myLvol

To list compression volumes that are only available for deletion because their PMEM file was missing use the following. The name parameter is optional and if not included will list all volumes, if used it will return the name or an error that the device does not exist. bdev_compress_get_orphans --name COMP_Nvme0n1

Crypto Virtual Bdev Module

The crypto virtual bdev module can be configured to provide at rest data encryption for any underlying bdev. The module relies on the DPDK CryptoDev Framework to provide all cryptographic functionality. The framework provides support for many different software only cryptographic modules as well hardware assisted support for the Intel QAT board. The framework also provides support for cipher, hash, authentication and AEAD functions. At this time the SPDK virtual bdev module supports cipher only as follows:

  • AESN-NI Multi Buffer Crypto Poll Mode Driver: RTE_CRYPTO_CIPHER_AES128_CBC
  • Intel(R) QuickAssist (QAT) Crypto Poll Mode Driver: RTE_CRYPTO_CIPHER_AES128_CBC (Note: QAT is functional however is marked as experimental until the hardware has been fully integrated with the SPDK CI system.)

In order to support using the bdev block offset (LBA) as the initialization vector (IV), the crypto module break up all I/O into crypto operations of a size equal to the block size of the underlying bdev. For example, a 4K I/O to a bdev with a 512B block size, would result in 8 cryptographic operations.

For reads, the buffer provided to the crypto module will be used as the destination buffer for unencrypted data. For writes, however, a temporary scratch buffer is used as the destination buffer for encryption which is then passed on to the underlying bdev as the write buffer. This is done to avoid encrypting the data in the original source buffer which may cause problems in some use cases.

Example command bdev_crypto_create NVMe1n1 CryNvmeA crypto_aesni_mb 0123456789123456

This command will create a crypto vbdev called 'CryNvmeA' on top of the NVMe bdev 'NVMe1n1' and will use the DPDK software driver 'crypto_aesni_mb' and the key '0123456789123456'.

To remove the vbdev use the bdev_crypto_delete command. bdev_crypto_delete CryNvmeA

Delay Bdev Module

The delay vbdev module is intended to apply a predetermined additional latency on top of a lower level bdev. This enables the simulation of the latency characteristics of a device during the functional or scalability testing of an SPDK application. For example, to simulate the effect of drive latency when processing I/Os, one could configure a NULL bdev with a delay bdev on top of it.

The delay bdev module is not intended to provide a high fidelity replication of a specific NVMe drive's latency, instead it's main purpose is to provide a "big picture" understanding of how a generic latency affects a given application.

A delay bdev is created using the bdev_delay_create RPC. This rpc takes 6 arguments, one for the name of the delay bdev and one for the name of the base bdev. The remaining four arguments represent the following latency values: average read latency, average write latency, p99 read latency, and p99 write latency. Within the context of the delay bdev p99 latency means that one percent of the I/O will be delayed by at least by the value of the p99 latency before being completed to the upper level protocol. All of the latency values are measured in microseconds.

Example command: bdev_delay_create -b Null0 -d delay0 -r 10 --nine-nine-read-latency 50 -w 30 --nine-nine-write-latency 90

This command will create a delay bdev with average read and write latencies of 10 and 30 microseconds and p99 read and write latencies of 50 and 90 microseconds respectively.

A delay bdev can be deleted using the bdev_delay_delete RPC

Example command: bdev_delay_delete delay0

GPT (GUID Partition Table)

The GPT virtual bdev driver is enabled by default and does not require any configuration. It will automatically detect SPDK GPT partition table on any attached bdev and will create possibly multiple virtual bdevs.

SPDK GPT partition table

The SPDK partition type GUID is 7c5222bd-8f5d-4087-9c00-bf9843c7b58c. Existing SPDK bdevs can be exposed as Linux block devices via NBD and then can be partitioned with standard partitioning tools. After partitioning, the bdevs will need to be deleted and attached again for the GPT bdev module to see any changes. NBD kernel module must be loaded first. To create NBD bdev user should use nbd_start_disk RPC command.

Example command nbd_start_disk Malloc0 /dev/nbd0

This will expose an SPDK bdev Malloc0 under the /dev/nbd0 block device.

To remove NBD device user should use nbd_stop_disk RPC command.

Example command nbd_stop_disk /dev/nbd0

To display full or specified nbd device list user should use nbd_get_disks RPC command.

Example command nbd_stop_disk -n /dev/nbd0

Creating a GPT partition table using NBD

# Expose bdev Nvme0n1 as kernel block device /dev/nbd0 by JSON-RPC nbd_start_disk Nvme0n1 /dev/nbd0
# Create GPT partition table.
parted -s /dev/nbd0 mklabel gpt
# Add a partition consuming 50% of the available space.
parted -s /dev/nbd0 mkpart MyPartition '0%' '50%'
# Change the partition type to the SPDK GUID.
# sgdisk is part of the gdisk package.
sgdisk -t 1:7c5222bd-8f5d-4087-9c00-bf9843c7b58c /dev/nbd0
# Stop the NBD device (stop exporting /dev/nbd0). nbd_stop_disk /dev/nbd0
# Now Nvme0n1 is configured with a GPT partition table, and
# the first partition will be automatically exposed as
# Nvme0n1p1 in SPDK applications.

iSCSI bdev

The SPDK iSCSI bdev driver depends on libiscsi and hence is not enabled by default. In order to use it, build SPDK with an extra --with-iscsi-initiator configure option.

The following command creates an iSCSI0 bdev from a single LUN exposed at given iSCSI URL with as the reported initiator IQN. bdev_iscsi_create -b iSCSI0 -i --url iscsi://

The URL is in the following format: iscsi://[<username>[%<password>]@]<host>[:<port>]/<target-iqn>/<lun>

Linux AIO bdev

The SPDK AIO bdev driver provides SPDK block layer access to Linux kernel block devices or a file on a Linux filesystem via Linux AIO. Note that O_DIRECT is used and thus bypasses the Linux page cache. This mode is probably as close to a typical kernel based target as a user space target can get without using a user-space driver. To create AIO bdev RPC command bdev_aio_create should be used.

Example commands bdev_aio_create /dev/sda aio0

This command will create aio0 device from /dev/sda. bdev_aio_create /tmp/file file 4096

This command will create file device with block size 4096 from /tmp/file.

To delete an aio bdev use the bdev_aio_delete command. bdev_aio_delete aio0

OCF Virtual bdev

OCF virtual bdev module is based on Open CAS Framework - a high performance block storage caching meta-library. To enable the module, configure SPDK using --with-ocf flag. OCF bdev can be used to enable caching for any underlying bdev.

Below is an example command for creating OCF bdev: bdev_ocf_create Cache1 wt Malloc0 Nvme0n1

This command will create new OCF bdev Cache1 having bdev Malloc0 as caching-device and Nvme0n1 as core-device and initial cache mode Write-Through. Malloc0 will be used as cache for Nvme0n1, so data written to Cache1 will be present on Nvme0n1 eventually. By default, OCF will be configured with cache line size equal 4KiB and non-volatile metadata will be disabled.

To remove Cache1: bdev_ocf_delete Cache1

During removal OCF-cache will be stopped and all cached data will be written to the core device.

Note that OCF has a per-device RAM requirement of about 56000 + cache device size * 58 / cache line size (in bytes). To get more information on OCF please visit OCF documentation.

Malloc bdev

Malloc bdevs are ramdisks. Because of its nature they are volatile. They are created from hugepage memory given to SPDK application.


The SPDK null bdev driver is a dummy block I/O target that discards all writes and returns undefined data for reads. It is useful for benchmarking the rest of the bdev I/O stack with minimal block device overhead and for testing configurations that can't easily be created with the Malloc bdev. To create Null bdev RPC command bdev_null_create should be used.

Example command bdev_null_create Null0 8589934592 4096

This command will create an 8 petabyte Null0 device with block size 4096.

To delete a null bdev use the bdev_null_delete command. bdev_null_delete Null0

NVMe bdev

There are two ways to create block device based on NVMe device in SPDK. First way is to connect local PCIe drive and second one is to connect NVMe-oF device. In both cases user should use bdev_nvme_attach_controller RPC command to achieve that.

Example commands bdev_nvme_attach_controller -b NVMe1 -t PCIe -a 0000:01:00.0

This command will create NVMe bdev of physical device in the system. bdev_nvme_attach_controller -b Nvme0 -t RDMA -a -f IPv4 -s 4420 -n

This command will create NVMe bdev of NVMe-oF resource.

To remove an NVMe controller use the bdev_nvme_detach_controller command. bdev_nvme_detach_controller Nvme0

This command will remove NVMe bdev named Nvme0.

NVMe bdev character device

This feature is considered as experimental.

Example commands bdev_nvme_cuse_register -n Nvme0 -p spdk/nvme0

This command will register /dev/spdk/nvme0 character device associated with Nvme0 controller. If there are namespaces created on Nvme0 controller, for each namespace device /dev/spdk/nvme0nX is created.

Cuse devices are removed from system, when NVMe controller is detached or unregistered with command: bdev_nvme_cuse_unregister -n Nvme0

Logical volumes

The Logical Volumes library is a flexible storage space management system. It allows creating and managing virtual block devices with variable size on top of other bdevs. The SPDK Logical Volume library is built on top of Blobstore Programmer's Guide. For detailed description please refer to Logical volume.

Logical volume store

Before creating any logical volumes (lvols), an lvol store has to be created first on selected block device. Lvol store is lvols vessel responsible for managing underlying bdev space assignment to lvol bdevs and storing metadata. To create lvol store user should use using bdev_lvol_create_lvstore RPC command.

Example command bdev_lvol_create_lvstore Malloc2 lvs -c 4096

This will create lvol store named lvs with cluster size 4096, build on top of Malloc2 bdev. In response user will be provided with uuid which is unique lvol store identifier.

User can get list of available lvol stores using bdev_lvol_get_lvstores RPC command (no parameters available).

Example response

"uuid": "330a6ab2-f468-11e7-983e-001e67edf35d",
"base_bdev": "Malloc2",
"free_clusters": 8190,
"cluster_size": 8192,
"total_data_clusters": 8190,
"block_size": 4096,
"name": "lvs"

To delete lvol store user should use bdev_lvol_delete_lvstore RPC command.

Example commands bdev_lvol_delete_lvstore -u 330a6ab2-f468-11e7-983e-001e67edf35d bdev_lvol_delete_lvstore -l lvs


To create lvols on existing lvol store user should use bdev_lvol_create RPC command. Each created lvol will be represented by new bdev.

Example commands bdev_lvol_create lvol1 25 -l lvs bdev_lvol_create lvol2 25 -u 330a6ab2-f468-11e7-983e-001e67edf35d


RAID virtual bdev module provides functionality to combine any SPDK bdevs into one RAID bdev. Currently SPDK supports only RAID 0. RAID functionality does not store on-disk metadata on the member disks, so user must recreate the RAID volume when restarting application. User may specify member disks to create RAID volume event if they do not exists yet - as the member disks are registered at a later time, the RAID module will claim them and will surface the RAID volume after all of the member disks are available. It is allowed to use disks of different sizes - the smallest disk size will be the amount of space used on each member disk.

Example commands bdev_raid_create -n Raid0 -z 64 -r 0 -b "lvol0 lvol1 lvol2 lvol3" bdev_raid_get_bdevs bdev_raid_delete Raid0


The SPDK Passthru virtual block device module serves as an example of how to write a virtual block device module. It implements the required functionality of a vbdev module and demonstrates some other basic features such as the use of per I/O context.

Example commands bdev_passthru_create -b aio -p pt bdev_passthru_delete pt


The SPDK pmem bdev driver uses pmemblk pool as the target for block I/O operations. For details on Pmem memory please refer to PMDK documentation on website. First, user needs to configure SPDK to include PMDK support:

configure --with-pmdk

To create pmemblk pool for use with SPDK user should use bdev_pmem_create_pool RPC command.

Example command bdev_pmem_create_pool /path/to/pmem_pool 25 4096

To get information on created pmem pool file user can use bdev_pmem_get_pool_info RPC command.

Example command bdev_pmem_get_pool_info /path/to/pmem_pool

To remove pmem pool file user can use bdev_pmem_delete_pool RPC command.

Example command bdev_pmem_delete_pool /path/to/pmem_pool

To create bdev based on pmemblk pool file user should use bdev_pmem_create RPC command.

Example command bdev_pmem_create /path/to/pmem_pool -n pmem

To remove a block device representation use the bdev_pmem_delete command. bdev_pmem_delete pmem

Virtio Block

The Virtio-Block driver allows creating SPDK bdevs from Virtio-Block devices.

The following command creates a Virtio-Block device named VirtioBlk0 from a vhost-user socket /tmp/vhost.0 exposed directly by SPDK vhost Target. Optional vq-count and vq-size params specify number of request queues and queue depth to be used. bdev_virtio_attach_controller --dev-type blk --trtype user --traddr /tmp/vhost.0 --vq-count 2 --vq-size 512 VirtioBlk0

The driver can be also used inside QEMU-based VMs. The following command creates a Virtio Block device named VirtioBlk0 from a Virtio PCI device at address 0000:00:01.0. The entire configuration will be read automatically from PCI Configuration Space. It will reflect all parameters passed to QEMU's vhost-user-scsi-pci device. bdev_virtio_attach_controller --dev-type blk --trtype pci --traddr 0000:01:00.0 VirtioBlk1

Virtio-Block devices can be removed with the following command bdev_virtio_detach_controller VirtioBlk0

Virtio SCSI

The Virtio-SCSI driver allows creating SPDK block devices from Virtio-SCSI LUNs.

Virtio-SCSI bdevs are created the same way as Virtio-Block ones. bdev_virtio_attach_controller --dev-type scsi --trtype user --traddr /tmp/vhost.0 --vq-count 2 --vq-size 512 VirtioScsi0 bdev_virtio_attach_controller --dev-type scsi --trtype pci --traddr 0000:01:00.0 VirtioScsi0

Each Virtio-SCSI device may export up to 64 block devices named VirtioScsi0t0 ~ VirtioScsi0t63, one LUN (LUN0) per SCSI device. The above 2 commands will output names of all exposed bdevs.

Virtio-SCSI devices can be removed with the following command bdev_virtio_detach_controller VirtioScsi0

Removing a Virtio-SCSI device will destroy all its bdevs.