2023-05-08 12:55:33 +08:00
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=============
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zoned-storage
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=============
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Zoned Block Devices (ZBDs) divide the LBA space into block regions called zones
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that are larger than the LBA size. They can only allow sequential writes, which
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can reduce write amplification in SSDs, and potentially lead to higher
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throughput and increased capacity. More details about ZBDs can be found at:
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https://zonedstorage.io/docs/introduction/zoned-storage
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1. Block layer APIs for zoned storage
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-------------------------------------
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QEMU block layer supports three zoned storage models:
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- BLK_Z_HM: The host-managed zoned model only allows sequential writes access
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to zones. It supports ZBD-specific I/O commands that can be used by a host to
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manage the zones of a device.
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- BLK_Z_HA: The host-aware zoned model allows random write operations in
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zones, making it backward compatible with regular block devices.
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- BLK_Z_NONE: The non-zoned model has no zones support. It includes both
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regular and drive-managed ZBD devices. ZBD-specific I/O commands are not
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supported.
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The block device information resides inside BlockDriverState. QEMU uses
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BlockLimits struct(BlockDriverState::bl) that is continuously accessed by the
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block layer while processing I/O requests. A BlockBackend has a root pointer to
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a BlockDriverState graph(for example, raw format on top of file-posix). The
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zoned storage information can be propagated from the leaf BlockDriverState all
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the way up to the BlockBackend. If the zoned storage model in file-posix is
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set to BLK_Z_HM, then block drivers will declare support for zoned host device.
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The block layer APIs support commands needed for zoned storage devices,
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including report zones, four zone operations, and zone append.
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2. Emulating zoned storage controllers
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--------------------------------------
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When the BlockBackend's BlockLimits model reports a zoned storage device, users
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like the virtio-blk emulation or the qemu-io-cmds.c utility can use block layer
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APIs for zoned storage emulation or testing.
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For example, to test zone_report on a null_blk device using qemu-io is::
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$ path/to/qemu-io --image-opts -n driver=host_device,filename=/dev/nullb0 -c "zrp offset nr_zones"
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2023-05-08 13:19:16 +08:00
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To expose the host's zoned block device through virtio-blk, the command line
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can be (includes the -device parameter)::
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-blockdev node-name=drive0,driver=host_device,filename=/dev/nullb0,cache.direct=on \
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-device virtio-blk-pci,drive=drive0
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Or only use the -drive parameter::
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-driver driver=host_device,file=/dev/nullb0,if=virtio,cache.direct=on
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Additionally, QEMU has several ways of supporting zoned storage, including:
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(1) Using virtio-scsi: --device scsi-block allows for the passing through of
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SCSI ZBC devices, enabling the attachment of ZBC or ZAC HDDs to QEMU.
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(2) PCI device pass-through: While NVMe ZNS emulation is available for testing
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purposes, it cannot yet pass through a zoned device from the host. To pass on
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the NVMe ZNS device to the guest, use VFIO PCI pass the entire NVMe PCI adapter
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through to the guest. Likewise, an HDD HBA can be passed on to QEMU all HDDs
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attached to the HBA.
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