Block Device User Guide

Target Audience

This user guide is intended for software developers who have knowledge of block storage, storage drivers, issuing JSON-RPC commands and storage services such as RAID, compression, crypto, and others.


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.

Configuring Block Device Modules

Block devices can be configured using JSON RPCs. A complete list of available RPC commands with detailed information can be found on the Block Device Abstraction Layer page.

Common Block Device Configuration Examples

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_register_cluster and bdev_rbd_create should be used.

SPDK provides two ways of creating a RBD bdev. One is to create a new Rados cluster object for each RBD bdev. Another is to share the same Rados cluster object for multiple RBD bdevs. Each Rados cluster object creates a small number of io_context_pool and messenger threads. Ceph commands ceph config help librados_thread_count and ceph config help ms_async_op_threads could help to check these threads information. Besides, you can specify the number of threads by updating ceph.conf file or using Ceph config commands. For more information, please refer to Ceph configuration One set of threads may not be enough to maximize performance with a large number of RBD bdevs, but one set of threads per RBD bdev may add too much context switching. Therefore, performance tuning on the number of RBD bdevs per cluster object and thread may be required.

Example command bdev_rbd_register_cluster rbd_cluster

This command will register a cluster named rbd_cluster. Optional --config-file and --key-file params are specified for the cluster.

To remove a registered cluster use the bdev_rbd_unregister_cluster command. bdev_rbd_unregister_cluster rbd_cluster

To create RBD bdev with a registered cluster. bdev_rbd_create rbd foo 512 -c rbd_cluster

This command will create a bdev that represents the 'foo' image from a pool called 'rbd'. When specifying -c for bdev_rbd_create, RBD bdevs will share the same rados cluster with one connection of Ceph in librbd module. Instead it will create a new rados cluster with one cluster connection for every bdev without specifying -c.

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 compression bdev module leverages the Acceleration Framework to carry out the actual compression and decompression. The acceleration framework can be configured to use ISA-L software optimized compression or the DPDK Compressdev module for hardware acceleration. To configure the Compressdev module please see the compressdev_scan_accel_module documentation here

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.

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 SPDK Accel Framework to provide all cryptographic functionality. One of the accel modules, dpdk_cryptodev is implemented with the DPDK CryptoDev API, it provides support for many different software only cryptographic modules as well hardware assisted support for the Intel QAT board and NVIDIA crypto enabled NICs.

For reads, the buffer provided to the crypto block device 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.

Below is information about accel modules which support crypto operations:

dpdk_cryptodev accel module

Supports the following ciphers:

  • 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, RTE_CRYPTO_CIPHER_AES128_XTS (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.

SW accel module

Supports the following ciphers:

  • AES_XTS cipher with 128 or 256 bit keys implemented with ISA-L_crypto

General workflow

  • Set desired accel module to perform crypto operations, that can be done with accel_assign_opc RPC command
  • Create a named crypto key using accel_crypto_key_create RPC command. The key will use the assigned accel module. Set of parameters and supported ciphers may be different in each accel module.
  • Create virtual crypto block device providing the base block device name and the crypto key name using bdev_crypto_create RPC command


Example command which uses dpdk_cryptodev accel module

# start SPDK application with `--wait-for-rpc` parameter dpdk_cryptodev_scan_accel_module dpdk_cryptodev_set_driver crypto_aesni_mb accel_assign_opc -o encrypt -m dpdk_cryptodev accel_assign_opc -o decrypt -m dpdk_cryptodev framework_start_init accel_crypto_key_create -c AES_CBC -k 01234567891234560123456789123456 -n key_aesni_cbc_1 bdev_crypto_create NVMe1n1 CryNvmeA -n key_aesni_cbc_1

These commands will create a crypto vbdev called 'CryNvmeA' on top of the NVMe bdev 'NVMe1n1' and will use a key named key_aesni_cbc_1. The key will work with the accel module which has been assigned for encrypt operations, in this example it will be the dpdk_cryptodev.

Crypto key format

Please make sure the keys are provided in hexlified format. This means string passed to must be twice as long than the key length in binary form.

Example command accel_crypto_key_create -c AES_XTS -k2 7859243a027411e581e0c40a35c8228f -k d16a2f3a9e9f5b32daefacd7f5984f4578add84425be4a0baa489b9de8884b09 -n sample_key

This command will create a key called sample_key, the AES key 'd16a2f3a9e9f5b32daefacd7f5984f4578add84425be4a0baa489b9de8884b09' and the XTS key '7859243a027411e581e0c40a35c8228f'. In other words, the compound AES_XTS key to be used is 'd16a2f3a9e9f5b32daefacd7f5984f4578add84425be4a0baa489b9de8884b097859243a027411e581e0c40a35c8228f'

Delete the virtual crypto block device

To remove the vbdev use the bdev_crypto_delete command. bdev_crypto_delete CryNvmeA

dpdk_cryptodev mlx5_pci driver configuration

The mlx5_pci driver works with crypto enabled Nvidia NICs and requires special configuration of DPDK environment to enable crypto function. It can be done via SPDK event library by configuring env_context member of spdk_app_opts structure or by passing corresponding CLI arguments in the following form: --allow=BDF,class=crypto,wcs_file=/full/path/to/wrapped/credentials, e.g. --allow=0000:01:00.0,class=crypto,wcs_file=/path/credentials.txt.

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 6527994e-2c5a-4eec-9613-8f5944074e8b. 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 1:6527994e-2c5a-4eec-9613-8f5944074e8b /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. More details can be found in the 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.

Example command for creating malloc bdev: bdev_malloc_create -b Malloc0 64 512

Example command for removing malloc bdev: bdev_malloc_delete Malloc0


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.

The SPDK NVMe bdev driver provides the multipath feature. Please refer to NVMe Multipath for details.

NVMe bdev character device

This feature is considered as experimental. You must configure with –with-nvme-cuse option to enable this RPC.

Example commands bdev_nvme_cuse_register -n Nvme3

This command will register a character device under /dev/spdk associated with Nvme3 controller. If there are namespaces created on Nvme3 controller, a namespace character device is also created for each namespace.

For example, the first controller registered will have a character device path of /dev/spdk/nvmeX, where X is replaced with a unique integer to differentiate it from other controllers. Note that this 'nvmeX' name here has no correlation to the name associated with the controller in SPDK. Namespace character devices will have a path of /dev/spdk/nvmeXnY, where Y is the namespace ID.

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


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


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 split block device module takes an underlying block device and splits it into several smaller equal-sized virtual block devices. This serves as an example to create more vbdevs on a given base bdev for user testing.

Example commands

To create four split bdevs with base bdev_b0 use the bdev_split_create command. Each split bdev will be one fourth the size of the base bdev. bdev_split_create bdev_b0 4

The split_size_mb(-s) parameter restricts the size of each split bdev. The total size of all split bdevs must not exceed the base bdev size. bdev_split_create bdev_b0 4 -s 128

To remove the split bdevs, use the bdev_split_delete command with the base bdev name. bdev_split_delete bdev_b0


The uring bdev module issues I/O to kernel block devices using the io_uring Linux kernel API. This module requires liburing. For more information on io_uring refer to kernel IO_uring

The user needs to configure SPDK to include io_uring support:

configure --with-uring

Support for zoned devices is enabled by default in uring bdev. It can be explicitly disabled as follows:

configure --with-uring --without-uring-zns

To create a uring bdev with given filename, bdev name and block size use the bdev_uring_create RPC. bdev_uring_create /path/to/device bdev_u0 512

To remove a uring bdev use the bdev_uring_delete RPC. bdev_uring_delete bdev_u0


The xnvme bdev module issues I/O to the underlying NVMe devices through various I/O mechanisms such as libaio, io_uring, Asynchronous IOCTL using io_uring passthrough, POSIX aio, emulated aio etc.

This module requires xNVMe library. For more information on xNVMe refer to xNVMe

The user needs to configure SPDK to include xNVMe support:

configure --with-xnvme

To create a xnvme bdev with given filename, bdev name and I/O mechanism use the bdev_xnvme_create RPC. bdev_xnvme_create /dev/ng0n1 bdev_ng0n1 io_uring_cmd

To remove a xnvme bdev use the bdev_xnvme_delete RPC. bdev_xnvme_delete bdev_ng0n1

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.

DAOS bdev

DAOS bdev creates SPDK block device on top of DAOS DFS, the name of the bdev defines the file name in DFS namespace. Note that DAOS container has to be POSIX type, e.g.: daos cont create --pool=test-pool --label=test-cont --type=POSIX

To build SPDK with daos support, daos-devel package has to be installed, please see the setup guide. To enable the module, configure SPDK using --with-daos flag.

Running daos_agent service on the target machine is required for the SPDK DAOS bdev communication with a DAOS cluster.

The implementation uses the independent pool and container connections per device's channel for the best IO throughput, therefore, running a target application with multiple cores (`-m [0-7], for example) is highly advisable.

Example command for creating daos bdev: bdev_daos_create daosdev0 test-pool test-cont 64 4096

Example command for removing daos bdev: bdev_daos_delete daosdev0

To resize a bdev use the bdev_daos_resize command. bdev_daos_resize daosdev0 8192

This command will resize the daosdev0 bdev to 8192 MiB.