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SMR Hard Disk Emulation with scsi_debug

The scsi_debug kernel module can be used to create emulated ZBC SCSI disks that use memory backing to store data, which is written to sectors. Because this method uses memory as a backing store, the creation of large disks requires a host with a large amount of DRAM.

note

This method stores sector data using volatile memory. This means that the data written to the emulated device will not survive the device's destruction and the data written to this emulated device will not survive a host reboot.

Creating an Emulated ZBC Disk​

scsi_debug ZBC disks can be created using modprobe with arguments. The following is an example that creates a host managed ZBC disk with 16GiB capacity, 64MiB zones, and 32 conventional zones.

# modprobe scsi_debug \
	max_luns=1 \
	sector_size=4096 \
	dev_size_mb=16384 \
	zbc=managed \
	zone_size_mb=64 \
	zone_nr_conv=32

After the disk has been created, it can be examined by using the lsscsi command.

# lsscsi -g
...
[11:0:0:0]   zbc     Linux    scsi_debug       0190  /dev/sdj   /dev/sg9
...

The vendor field of the disk is set to "scsi_debug". The kernel messages also show the process whereby this disk came online.

# dmesg
...
scsi_debug:sdebug_driver_probe: scsi_debug: trim poll_queues to 0. poll_q/nr_hw = (0/1)
scsi host11: scsi_debug: version 0190 [20200710]
                 dev_size_mb=16384, opts=0x0, submit_queues=1, statistics=0
scsi 11:0:0:0: Direct-Access-ZBC Linux    scsi_debug       0190 PQ: 0 ANSI: 7
sd 11:0:0:0: Power-on or device reset occurred
sd 11:0:0:0: Attached scsi generic sg9 type 20
sd 11:0:0:0: [sdj] Host-managed zoned block device
sd 11:0:0:0: [sdj] 4194304 4096-byte logical blocks: (17.2 GB/16.0 GiB)
sd 11:0:0:0: [sdj] Write Protect is off
sd 11:0:0:0: [sdj] Mode Sense: 5b 00 10 08
sd 11:0:0:0: [sdj] Write cache: enabled, read cache: enabled, supports DPO and FUA
sd 11:0:0:0: [sdj] Optimal transfer size 4194304 bytes
sd 11:0:0:0: [sdj] 256 zones of 16384 logical blocks
sd 11:0:0:0: [sdj] Attached SCSI disk

Verifying The Emulated Disk​

The zone configuration of the emulated disk can be inspected by using libzbc, sg3utils and util-linux tools.

Using zbc_report_zones​

Use zbc_report_zones to verify the zone configuration of the newly-created emulated ZBC disk:

# zbc_report_zones /dev/sg9
Device /dev/sg9:
    Vendor ID: Linux scsi_debug 0190
    SCSI ZBC device interface, Host-managed zone model
    33554432 512-bytes sectors
    4194304 logical blocks of 4096 B
    4194304 physical blocks of 4096 B
    17.180 GB capacity
    Read commands are unrestricted
    4096 KiB max R/W size
    Maximum number of open sequential write required zones: 8
    256 zones from 0, reporting option 0x00
256 / 256 zones:
Zone 00000: type 0x1 (Conventional), cond 0x0 (Not-write-pointer), sector 0, 131072 sectors
Zone 00001: type 0x1 (Conventional), cond 0x0 (Not-write-pointer), sector 131072, 131072 sectors
Zone 00002: type 0x1 (Conventional), cond 0x0 (Not-write-pointer), sector 262144, 131072 sectors
...
Zone 00031: type 0x1 (Conventional), cond 0x0 (Not-write-pointer), sector 4063232, 131072 sectors
Zone 00032: type 0x2 (Sequential-write-required), cond 0x1 (Empty), reset recommended 0, non_seq 0, sector 4194304, 131072 sectors, wp 4194304
Zone 00033: type 0x2 (Sequential-write-required), cond 0x1 (Empty), reset recommended 0, non_seq 0, sector 4325376, 131072 sectors, wp 4325376
...
Zone 00254: type 0x2 (Sequential-write-required), cond 0x1 (Empty), reset recommended 0, non_seq 0, sector 33292288, 131072 sectors, wp 33292288
Zone 00255: type 0x2 (Sequential-write-required), cond 0x1 (Empty), reset recommended 0, non_seq 0, sector 33423360, 131072 sectors, wp 33423360

Using blkzone​

Use blkzone to verify the zone configuration of the newly-created emulated ZBC disk. This displays the same information that is returned by "zbc_report_zones", but it is displayed in a different format:

# blkzone report /dev/sdj
  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: 0x000040000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 0(nw) [type: 1(CONVENTIONAL)]
...
  start: 0x0003e0000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 0(nw) [type: 1(CONVENTIONAL)]
  start: 0x000400000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]
  start: 0x000420000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]
...
  start: 0x001fc0000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]
  start: 0x001fe0000, len 0x020000, cap 0x020000, wptr 0x000000 reset:0 non-seq:0, zcond: 1(em) [type: 2(SEQ_WRITE_REQUIRED)]