120 lines
3.8 KiB
ReStructuredText
120 lines
3.8 KiB
ReStructuredText
===============================================
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Block layer support for Persistent Reservations
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===============================================
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The Linux kernel supports a user space interface for simplified
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Persistent Reservations which map to block devices that support
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these (like SCSI). Persistent Reservations allow restricting
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access to block devices to specific initiators in a shared storage
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setup.
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This document gives a general overview of the support ioctl commands.
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For a more detailed reference please refer to the SCSI Primary
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Commands standard, specifically the section on Reservations and the
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"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands.
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All implementations are expected to ensure the reservations survive
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a power loss and cover all connections in a multi path environment.
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These behaviors are optional in SPC but will be automatically applied
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by Linux.
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The following types of reservations are supported:
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--------------------------------------------------
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- PR_WRITE_EXCLUSIVE
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Only the initiator that owns the reservation can write to the
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device. Any initiator can read from the device.
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- PR_EXCLUSIVE_ACCESS
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Only the initiator that owns the reservation can access the
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device.
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- PR_WRITE_EXCLUSIVE_REG_ONLY
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Only initiators with a registered key can write to the device,
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Any initiator can read from the device.
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- PR_EXCLUSIVE_ACCESS_REG_ONLY
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Only initiators with a registered key can access the device.
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- PR_WRITE_EXCLUSIVE_ALL_REGS
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Only initiators with a registered key can write to the device,
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Any initiator can read from the device.
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All initiators with a registered key are considered reservation
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holders.
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Please reference the SPC spec on the meaning of a reservation
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holder if you want to use this type.
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- PR_EXCLUSIVE_ACCESS_ALL_REGS
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Only initiators with a registered key can access the device.
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All initiators with a registered key are considered reservation
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holders.
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Please reference the SPC spec on the meaning of a reservation
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holder if you want to use this type.
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The following ioctl are supported:
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----------------------------------
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1. IOC_PR_REGISTER
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^^^^^^^^^^^^^^^^^^
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This ioctl command registers a new reservation if the new_key argument
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is non-null. If no existing reservation exists old_key must be zero,
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if an existing reservation should be replaced old_key must contain
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the old reservation key.
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If the new_key argument is 0 it unregisters the existing reservation passed
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in old_key.
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2. IOC_PR_RESERVE
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^^^^^^^^^^^^^^^^^
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This ioctl command reserves the device and thus restricts access for other
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devices based on the type argument. The key argument must be the existing
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reservation key for the device as acquired by the IOC_PR_REGISTER,
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IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands.
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3. IOC_PR_RELEASE
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^^^^^^^^^^^^^^^^^
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This ioctl command releases the reservation specified by key and flags
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and thus removes any access restriction implied by it.
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4. IOC_PR_PREEMPT
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^^^^^^^^^^^^^^^^^
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This ioctl command releases the existing reservation referred to by
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old_key and replaces it with a new reservation of type for the
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reservation key new_key.
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5. IOC_PR_PREEMPT_ABORT
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^^^^^^^^^^^^^^^^^^^^^^^
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This ioctl command works like IOC_PR_PREEMPT except that it also aborts
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any outstanding command sent over a connection identified by old_key.
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6. IOC_PR_CLEAR
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^^^^^^^^^^^^^^^
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This ioctl command unregisters both key and any other reservation key
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registered with the device and drops any existing reservation.
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Flags
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-----
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All the ioctls have a flag field. Currently only one flag is supported:
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- PR_FL_IGNORE_KEY
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Ignore the existing reservation key. This is commonly supported for
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IOC_PR_REGISTER, and some implementation may support the flag for
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IOC_PR_RESERVE.
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For all unknown flags the kernel will return -EOPNOTSUPP.
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