Reservation

Introduction

After the director set up a job on the SD it will then issue a “use” command to tell the SD what storage device it wants to use. The SD then checks the list of devices the director sent and tries to reserve a suitable device. In the end the SD either tells the director which device to use or that there is no device available for the job.

There is a so-called reservation context that is passed to every function that has a swimlane in this diagram. This context is basically a bunch of flags that are set and cleared to change the behaviour of the rest of the code.

For example CanReserveDevice() from the second diagram sets the flag low_use_drive which is then evaluated in UseDeviceCmd().

Diagram Conventions

Each swimlane denotes a separate function.

@startuml
#aqua:reservation context is read or set;
detach
#limegreen:Decision based on configuration directive;
detach
#tomato:Control flow broken;
detach
(M)
note right
  goto or jump-mark
end note
detach
:Read read data from the network<
detach
:Send data to the network>
detach
:Call another function|
detach
:Remote procedure call/

@enduml

Diagram legend

UseDeviceCmd

This function reads the list of storages and devices the director is willing to use for this job. Afterwards several different methods of finding a device to reseve are used. If no device could be reserved the function waits for up to a minute or until ReleaseDeviceCond() is called and then tries again.

@startuml
'BEGIN macros
!definelong RESERVE_DEVICE()
:ReserveDevice()|
!enddefinelong

!definelong AUTOCHANGER()
partition autochanger {
  while (with each device in autochanger device)
    #limegreen:if (device autoselect?) then (yes)
      RESERVE_DEVICE()
      if (have_device?) then (yes)
        #tomato:return;
        detach
      endif
    endif
  endwhile
}
!enddefinelong

!definelong PLAIN_DEVICE()
partition plain_device {
RESERVE_DEVICE()
note right
  try to reserve the named device
end note
if (have_device?) then (yes)
  #tomato:return;
  detach
endif
#limegreen:if (DeviceReserveByMediatype) then(yes)
  while (each device with matching media_type)
    RESERVE_DEVICE()
    if (have_device?) then (yes)
      #tomato:return;
      detach
    endif
  endwhile
endif
}
!enddefinelong

!definelong MOUNTED_VOLUMES()
partition mounted_volumes {
if (append?) then(yes)
  while(with each mounted volume)
    :ask director if volume is ok/
    if (volume ok?) then(yes)
      :find device for volume;
      RESERVE_DEVICE()
      if (have_device?) then (yes)
        #tomato:return;
        detach
      endif
    endif
  endwhile
endif
}
!enddefinelong
'END macros
|UseDeviceCmd|
start
partition read_data_from_director {
  repeat
    :storage specification<
    repeat
      :device specification<
    repeat while (more devices?)
  repeat while (more storages?)
}

  repeat
  #aqua:clear suitable_device]
  #aqua:clear have_volume]
  #aqua:clear VolumeName]
  #aqua:clear any_drive]
  #limegreen:if (PreferMountedVols) then (no)
      #aqua:clear low_use_drive]
      #aqua:clear PreferMountedVols]
      #aqua:clear exact_match]
      #aqua:set autochanger_only]
      |FindSuitableDeviceForJob|
      while (with every device)
        |SearchResForDevice|
        AUTOCHANGER()
        |FindSuitableDeviceForJob|
      endwhile
      |UseDeviceCmd|
      if (have_device) then (yes)
        stop
      else (no)
        #aqua:if (low_use_drive set?) then (yes)
          #aqua:set try_low_use_drive]
          |FindSuitableDeviceForJob|
          while (with every device)
            |SearchResForDevice|
            AUTOCHANGER()
            |FindSuitableDeviceForJob|
          endwhile
          |UseDeviceCmd|
          if (have_device) then (yes)
            stop
          endif
          #aqua:clear try_low_use_drive]
        endif
        #aqua:clear autochanger_only]
        |FindSuitableDeviceForJob|
        while (with every device)
          |SearchResForDevice|
          AUTOCHANGER()
          PLAIN_DEVICE()
          |FindSuitableDeviceForJob|
        endwhile
        |UseDeviceCmd|
        if (have_device) then (yes)
          stop
        endif
      endif
    else (yes)
    endif
    #aqua:set PreferMountedVols]
    #aqua:set exact_match]
    #aqua:clear autochanger_only]
    |FindSuitableDeviceForJob|
    MOUNTED_VOLUMES()
        while (with every device)
          |SearchResForDevice|
          AUTOCHANGER()
          PLAIN_DEVICE()
          |FindSuitableDeviceForJob|
        endwhile
    |UseDeviceCmd|
    if (have_device) then (yes)
      stop
    else (no)
      #aqua:clear exact_match]
      |FindSuitableDeviceForJob|
    MOUNTED_VOLUMES()
        while (with every device)
          |SearchResForDevice|
          AUTOCHANGER()
          PLAIN_DEVICE()
          |FindSuitableDeviceForJob|
        endwhile
      |UseDeviceCmd|
      if (have_device) then (yes)
        stop
      else (no)
        #aqua:set any_drive]
        |FindSuitableDeviceForJob|
    MOUNTED_VOLUMES()
        while (with every device)
          |SearchResForDevice|
          AUTOCHANGER()
          PLAIN_DEVICE()
          |FindSuitableDeviceForJob|
        endwhile
        |UseDeviceCmd|
        if (have_device) then (yes)
          stop
        else (no)
          if (attempt 3+?) then (yes)
            :wait 30 seconds;
          else (no)
            #aqua:if (suitable_device set?) then (yes)
              :WaitForDevice()|
              note right
                This will acquire a mutex to queue up
                multiple jobs waiting for a device.
                Then it waits up to 60 seconds for some
                other thread to call ReleaseDeviceCond()
              end note
            else (no)
              (F)
              detach
            endif
          endif
        endif
      endif
    endif
  repeat while (repeat forever)
  detach
partition failed_to_reserve {
  (F)
  :no device message>
  stop
}
@enduml

Control flow of UseDeviceCmd()

ReserveDevice

Here we see wether the media type matches and actually try to open the device. The actual reservation is delegated to ReserveDeviceForRead() or ReserveDeviceForAppend(). While the first one is more or less trivial, the latter one is really complicated.

@startuml
|ReserveDevice|
start
#limegreen:if (device matches requested media_type?) then (yes)
  if (device initialized?) then (no)
    :InitDev()|
  endif
  if (device exists and is open?) then (yes)
    #aqua:set suitable_device]
    :initialize DCR;
    if (append?) then (no)
      :ReserveDeviceForRead()|
    else (yes)
      |ReserveDeviceForAppend|
      if (CanRead()) then (no)
        if (IsDeviceUnmounted()) then (no)
          |CanReserveDrive|
          if (IsMaxJobsOk()) then (yes)
            #aqua:if (any_drive set?) then (no)
              #aqua:if (try_low_use_drive set?) then (yes)
                #aqua:if (low_use_drive is current drive?) then (yes)
                  (S)
                  detach
                endif
              endif
              #aqua:if (PreferMountedVols set?) then (no)
                if (isBusy()) then (yes)
                  #aqua:if (dev.num_writers + NumReserved() < num_writers) then (yes)
                    #aqua:set num_writers := dev.numwriters + NumReserved()]
                    #aqua:set low_use_drive := dev]
                  endif
                  (X)
                  detach
                endif
              else (yes)
                if (IsTape() but no volume) then (yes)
                  (X)
                  detach
                endif
              endif
              #aqua:if(exact_match set? and have_volume set?) then (yes)
                #aqua:if(VolumeName matches mounted volume) then (no)
                  (X)
                  detach
                endif
                if (Can_i_use_volume()) then (no)
                  (X)
                  detach
                endif
              endif
              :line 1080;

              :line 1152;
            endif
            #aqua:if (autochanger_only) then (yes)
              if (IsBusy()) then (no)
                if (volume in drive) then (no)
                  (S)
                  detach
                endif
              endif
            endif
            if (num_writers == 0) then (yes)
              if (NumReserved()) then (yes)
                if (IsPoolOk()) then (yes)
                  (S)
                  detach
                else (no)
                  (X)
                  detach
                endif
              elseif (CanAppend()) then (yes)
                if (IsPoolOk()) then (no)
                  :UnloadAutoChanger()|
                endif
                (S)
                detach
              endif
            endif
            if (num_writers > 0 || CanAppend()) then (yes)
              if (IsPoolOk()) then (yes)
                (S)
                #tomato:return success;
              else (no)
                (X)
                detach
              endif
            else (no)
              #tomato:cancel job (M_FATAL);
              detach
            endif
          else (no)
            (X)
            #tomato:return failure;
          endif
          |ReserveDeviceForAppend|
          if (success?) then (yes)
            :reserve device;
            :set have_device]
          endif
        endif
      endif
      |ReserveDevice|
      if (have_device?) then (yes)
        #aqua:if (have_volume set?) then (yes)
          if (reserve_volume()) then (no)
            (F)
            detach
          endif
        else (no)
          if (DirFindNextAppendableVolume()) then (yes)
            #aqua:set have_volume]
            #aqua:set VolumeName]
          else (no)
            #aqua:clear have_volume]
            #aqua:clear VolumeName]
            if (FoundInUse()) then (yes)
              #aqua:if(PreferMountedVols set?) then (no)
                #aqua:set PreferMountedVols]
                if (dcr has volume?) then (yes)
                  :UnreserveDevice()|
                endif
                (F)
                detach
              endif
            endif
            if (num_writers != 0) then (yes)
                if (dcr has volume?) then (yes)
                  :UnreserveDevice()|
                endif
                (F)
                detach
            endif
          endif
        endif
      else (no)
        (F)
        detach
      endif
    endif
if (have_device?) then (yes)
  :OK device message>
else (no)
  (F)
  #aqua:clear have_volume]
  #aqua:clear VolumeName]
endif
  endif
endif

@enduml

Control flow of ReserveDevice()