Zoned Block Device Emulation with zloop

The zoned loop block device driver (zloop) was added in Linux 6.16. It creates a host-managed zoned block device that uses one regular file per zone as backing storage:

A conventional zone is backed by a file whose size always equals the zone size.
A sequential write required zone is backed by a file whose size is the amount of data written to the zone, that is, the file size is the zone write pointer position. Resetting a zone truncates the backing file to zero, and finishing the zone extends the size of the backing file to its maximum size (the zone size).

Because data is stored in files rather than in system memory, zloop can emulate devices that have a capacity much larger than the size of the host memory. Since the backing files of sequential zones only grow as data is written, the device capacity can also be larger than the free space of the backing file system. Writing more data than the file system can results in write errors.

The zloop driver implements the complete zone state machine: zones can be empty, implicitly open, explicitly open, closed or full. No user space tools are needed to create or delete zloop devices.

Prerequisites

The kernel must be built with zoned block device support (CONFIG_BLK_DEV_ZONED) and the zoned loop driver (CONFIG_BLK_DEV_ZONED_LOOP), either built in or as the zloop module. Once the driver is loaded, devices are managed through the /dev/zloop-control character device.

# modprobe zloop
# ls -l /dev/zloop-control
crw-------. 1 root root 10, 123 Jan  6 19:18 /dev/zloop-control

Reading /dev/zloop-control lists the commands and options supported by the running kernel. Newer kernels accept more options than shown here.

# cat /dev/zloop-control
add id=%d,capacity_mb=%u,zone_size_mb=%u,zone_capacity_mb=%u,conv_zones=%u,base_dir=%s,nr_queues=%u,queue_depth=%u,buffered_io
remove id=%d

The options that control the zone configuration are shown in the table below.

Option	Value	Description
id	number	Device number, the X in /dev/zloopX (default: first free number)
capacity_mb	capacity in MiB	Total device capacity; use a multiple of the zone size (default: 16384)
zone_size_mb	zone size in MiB	The size of each zone; must be a power of two (default: 256)
zone_capacity_mb	zone capacity in MiB	The capacity of sequential zones (default: zone size)
conv_zones	number	Number of conventional zones, starting at sector 0 (default: 8)
base_dir	path	Directory containing one subdirectory of zone files per device (default: /var/local/zloop)

Creating a zloop Zoned Block Device

The directory that holds the zone files must exist before the device is added. It is named after the device ID, inside the base directory. The following example creates /dev/zloop0, a 2 GiB device with 64 MiB zones and the default 8 conventional zones:

# modprobe zloop
# mkdir -p /var/local/zloop/0
# echo "add id=0,capacity_mb=2048,zone_size_mb=64" > /dev/zloop-control

The zone files are created in /var/local/zloop/0. Conventional zone files (cnv-) are allocated at the full zone size, while sequential zone files (seq-) are empty until data is written:

# ls -l /var/local/zloop/0
total 0
-rw-------. 1 root root 67108864 Jan  6 22:23 cnv-000000
-rw-------. 1 root root 67108864 Jan  6 22:23 cnv-000001
...
-rw-------. 1 root root 67108864 Jan  6 22:23 cnv-000007
-rw-------. 1 root root        0 Jan  6 22:23 seq-000008
-rw-------. 1 root root        0 Jan  6 22:23 seq-000009
...

The new device can be used like any other zoned block device:

# lsblk -z /dev/zloop0
NAME   ZONED        ZONE-SZ ZONE-NR ZONE-AMAX ZONE-OMAX ZONE-APP ZONE-WGRAN
zloop0 host-managed     64M      32         0         0       1M         4K
# blkzone report /dev/zloop0
  start: 0x000000000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 0(nw) [type: 1(CONVENTIONAL)]
  start: 0x000020000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 0(nw) [type: 1(CONVENTIONAL)]
...
  start: 0x0000e0000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 0(nw) [type: 1(CONVENTIONAL)]
  start: 0x000100000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]
  start: 0x000120000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]
...

Removing and Deleting a zloop Zoned Block Device

An unused zloop device is removed by writing a remove command with the device ID:

# echo "remove id=0" > /dev/zloop-control

Removing the device does not delete its zone files. Running the same add command again, for example after a reboot, restores the device with the data already written when it was removed. The configuration must match the one used when the device was first created; otherwise the add command fails. The state of zones is preserved for empty and full zones. However, like a real device, zones that were open when the device was removed are changed to the closed state.

To delete the device permanently, remove it and then delete its zone files:

# echo "remove id=0" > /dev/zloop-control
# rm -r /var/local/zloop/0

Prerequisites​

Creating a zloop Zoned Block Device​

Removing and Deleting a zloop Zoned Block Device​

Prerequisites

Creating a zloop Zoned Block Device

Removing and Deleting a zloop Zoned Block Device