linux/Documentation/block/pr.rst
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   1===============================================
   2Block layer support for Persistent Reservations
   3===============================================
   4
   5The Linux kernel supports a user space interface for simplified
   6Persistent Reservations which map to block devices that support
   7these (like SCSI). Persistent Reservations allow restricting
   8access to block devices to specific initiators in a shared storage
   9setup.
  10
  11This document gives a general overview of the support ioctl commands.
  12For a more detailed reference please refer to the SCSI Primary
  13Commands standard, specifically the section on Reservations and the
  14"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands.
  15
  16All implementations are expected to ensure the reservations survive
  17a power loss and cover all connections in a multi path environment.
  18These behaviors are optional in SPC but will be automatically applied
  19by Linux.
  20
  21
  22The following types of reservations are supported:
  23--------------------------------------------------
  24
  25 - PR_WRITE_EXCLUSIVE
  26        Only the initiator that owns the reservation can write to the
  27        device.  Any initiator can read from the device.
  28
  29 - PR_EXCLUSIVE_ACCESS
  30        Only the initiator that owns the reservation can access the
  31        device.
  32
  33 - PR_WRITE_EXCLUSIVE_REG_ONLY
  34        Only initiators with a registered key can write to the device,
  35        Any initiator can read from the device.
  36
  37 - PR_EXCLUSIVE_ACCESS_REG_ONLY
  38        Only initiators with a registered key can access the device.
  39
  40 - PR_WRITE_EXCLUSIVE_ALL_REGS
  41
  42        Only initiators with a registered key can write to the device,
  43        Any initiator can read from the device.
  44        All initiators with a registered key are considered reservation
  45        holders.
  46        Please reference the SPC spec on the meaning of a reservation
  47        holder if you want to use this type.
  48
  49 - PR_EXCLUSIVE_ACCESS_ALL_REGS
  50        Only initiators with a registered key can access the device.
  51        All initiators with a registered key are considered reservation
  52        holders.
  53        Please reference the SPC spec on the meaning of a reservation
  54        holder if you want to use this type.
  55
  56
  57The following ioctl are supported:
  58----------------------------------
  59
  601. IOC_PR_REGISTER
  61^^^^^^^^^^^^^^^^^^
  62
  63This ioctl command registers a new reservation if the new_key argument
  64is non-null.  If no existing reservation exists old_key must be zero,
  65if an existing reservation should be replaced old_key must contain
  66the old reservation key.
  67
  68If the new_key argument is 0 it unregisters the existing reservation passed
  69in old_key.
  70
  71
  722. IOC_PR_RESERVE
  73^^^^^^^^^^^^^^^^^
  74
  75This ioctl command reserves the device and thus restricts access for other
  76devices based on the type argument.  The key argument must be the existing
  77reservation key for the device as acquired by the IOC_PR_REGISTER,
  78IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands.
  79
  80
  813. IOC_PR_RELEASE
  82^^^^^^^^^^^^^^^^^
  83
  84This ioctl command releases the reservation specified by key and flags
  85and thus removes any access restriction implied by it.
  86
  87
  884. IOC_PR_PREEMPT
  89^^^^^^^^^^^^^^^^^
  90
  91This ioctl command releases the existing reservation referred to by
  92old_key and replaces it with a new reservation of type for the
  93reservation key new_key.
  94
  95
  965. IOC_PR_PREEMPT_ABORT
  97^^^^^^^^^^^^^^^^^^^^^^^
  98
  99This ioctl command works like IOC_PR_PREEMPT except that it also aborts
 100any outstanding command sent over a connection identified by old_key.
 101
 1026. IOC_PR_CLEAR
 103^^^^^^^^^^^^^^^
 104
 105This ioctl command unregisters both key and any other reservation key
 106registered with the device and drops any existing reservation.
 107
 108
 109Flags
 110-----
 111
 112All the ioctls have a flag field.  Currently only one flag is supported:
 113
 114 - PR_FL_IGNORE_KEY
 115        Ignore the existing reservation key.  This is commonly supported for
 116        IOC_PR_REGISTER, and some implementation may support the flag for
 117        IOC_PR_RESERVE.
 118
 119For all unknown flags the kernel will return -EOPNOTSUPP.
 120