1============= 2zoned-storage 3============= 4 5Zoned Block Devices (ZBDs) divide the LBA space into block regions called zones 6that are larger than the LBA size. They can only allow sequential writes, which 7can reduce write amplification in SSDs, and potentially lead to higher 8throughput and increased capacity. More details about ZBDs can be found at: 9 10https://zonedstorage.io/docs/introduction/zoned-storage 11 121. Block layer APIs for zoned storage 13------------------------------------- 14QEMU block layer supports three zoned storage models: 15- BLK_Z_HM: The host-managed zoned model only allows sequential writes access 16to zones. It supports ZBD-specific I/O commands that can be used by a host to 17manage the zones of a device. 18- BLK_Z_HA: The host-aware zoned model allows random write operations in 19zones, making it backward compatible with regular block devices. 20- BLK_Z_NONE: The non-zoned model has no zones support. It includes both 21regular and drive-managed ZBD devices. ZBD-specific I/O commands are not 22supported. 23 24The block device information resides inside BlockDriverState. QEMU uses 25BlockLimits struct(BlockDriverState::bl) that is continuously accessed by the 26block layer while processing I/O requests. A BlockBackend has a root pointer to 27a BlockDriverState graph(for example, raw format on top of file-posix). The 28zoned storage information can be propagated from the leaf BlockDriverState all 29the way up to the BlockBackend. If the zoned storage model in file-posix is 30set to BLK_Z_HM, then block drivers will declare support for zoned host device. 31 32The block layer APIs support commands needed for zoned storage devices, 33including report zones, four zone operations, and zone append. 34 352. Emulating zoned storage controllers 36-------------------------------------- 37When the BlockBackend's BlockLimits model reports a zoned storage device, users 38like the virtio-blk emulation or the qemu-io-cmds.c utility can use block layer 39APIs for zoned storage emulation or testing. 40 41For example, to test zone_report on a null_blk device using qemu-io is:: 42 43 $ path/to/qemu-io --image-opts -n driver=host_device,filename=/dev/nullb0 -c "zrp offset nr_zones" 44