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sys_attrs_io(5)						      sys_attrs_io(5)



NAME

  sys_attrs_io - io subsystem attributes

DESCRIPTION

  This reference page lists and	describes attributes for the I/O  (io) kernel
  subsystem. Refer to the sys_attrs(5) reference page for an introduction to
  the topic of kernel subsystem	attributes.



  basic_dma_window_size
      Amount of	space in megabytes to allocate for the 32-bit scatter-gather
      DMA window on PCI-based AlphaServers.  This parameter is set by the
      TruCluster software.

      Default value: 0 (MB)

      Do not modify the	value of this attribute	unless instructed to do	so by
      support personnel	or by patch kit	documentation.



  bdevsw_size
      Number of	entry dispatch points supported	by the in-memory table for
      the bdev (block device driver) switch. This table	is used	to access
      block device drivers, and	must be	large enough to	accommodate the
      drivers registered in the	bdevsw table in	the /usr/sys/system_name
      /conf.c file, plus any additional	device drivers that will be dynami-
      cally loaded.

      Default value: 200

      Minimum value: 0

      Maximum value: INT_MAX

      If the number of third-party block device	drivers	that will be dynami-
      cally loaded to the system plus the number of block device drivers
      already registered in the	system_name /conf.c file exceeds 200, you may
      want to increase the value of bdevsw_size.



  booted_media
      A	query-only attribute that specifies the	media from which the system
      was booted.



  cam_ccb_increment
      Number of	buffers	either added to	or removed from	the buffer pool	free
      list. Buffers are	allocated as they are needed in	order to handle
      immediate	demands; however, they are deallocated carefully to prevent
      spikes.

      Default value: 50	buffers

      Do not modify the	default	setting	for this attribute unless instructed
      to do so by support personnel or by patch	kit instructions.



  cam_ccb_low_water
      A	threshold value	that determines	when more buffers are allocated	from
      the kernel. More buffers are allocated from the kernel when the number
      of buffers in the	pool free list falls below this	value.	CAM reserves
      the number of buffers specified by the cam_ccb_low_water attribute  to
      ensure that the kernel always has	enough memory to shut down runaway
      processes.

      Default value: 100 (buffers)

      Do not modify the	default	setting	for this attribute unless instructed
      to do so by support personnel or by patch	kit instructions.



  cam_ccb_pool_size
      The base number of Common	Access Method (CAM) control blocks (ccbs)
      allocated	to the buffer pool free	list. One ccb is needed	for each out-
      standing I/O request.  The system	will automatically increase the
      number of	ccbs, as necessary for the I/O load, but will not decrease
      allocation of ccbs below cam_ccb_pool_size.

      Default value: 200 (ccbs)

      Minimum value: 100

      Maximum value: 1000

      If the I/O load on your system is	consistently very high,	increasing
      the number of ccbs that are always available in memory (rather than
      starting with a lower number and letting the system dynamically
      increase it when necessary) may improve performance. However, by
      increasing cam_ccb_pool_size, you	prevent	the system from	using the
      additional memory	for other purposes at times when the number of I/O
      requests is lower	than the number	of ccbs. Any performance improvement
      that results from	a cam_ccb_pool_size increase can occur only if your
      system's memory size is large enough to support the dedication of	addi-
      tional memory resources to I/O.

      Decreasing cam_ccb_pool_size to a	value below the	default	is very
      likely to	cause I/O problems and is strongly discouraged.



  cdevsw_size
      Number of	entry dispatch points supported	by the in-memory table for
      the cdev (character device driver) switch. This table is used to access
      character	device drivers,	and must be large enough to accommodate	the
      drivers registered in the	cdevsw table in	the /usr/sys/system_name
      /conf.c file, plus any additional	character device drivers that will be
      dynamically loaded.

      Default value: 200 (entry	dispatch points)

      Minimum value: 0

      Maximum value: INT_MAX

      If the number of third-party character device drivers that will be
      dynamically loaded to the	system plus the	number of character device
      drivers already registered in the	system_name /conf.c file exceeds 200,
      you may want to increase the value of cdevsw_size.



  device_switch_inited
      An on/off	switch for internal use	only. Do not modify this attribute
      unless instructed	to do so by support personnel or by patch kit
      instructions.



  device_switch_print
      An on/off	switch for internal use	only. Do not modify this attribute
      unless instructed	to do so by support personnel or by patch kit
      instructions.



  device_switch_stale
      An on/off	switch for internal use	only. Do not modify this attribute
      unless instructed	to do so by support personnel or by patch kit
      instructions.



  dma_dac_capable
      A	value used by developers when debugging	subsystem source code.

      Do not modify the	default	value of this attribute	unless instructed to
      do so by support personnel or by patch kit documentation.



  dma_sg_map_64bit
      Amount of	space in megabytes to allocate for the 64-bit scatter-gather
      DMA window on PCI-based AlphaServers.

      Default value: 0 (MB)

      Do not modify this attribute unless instructed to	do so by support per-
      sonnel or	by patch kit documentation.



  dma_sg_map_unload_zero
      A	value that controls the	zeroing	of scatter-gather mapping entries in
      the PCI adapter on AS8x00	Alphaservers.

      Default value: 2

      Do not modify this attribute unless instructed to	do so by support per-
      sonnel or	by patch kit documentation.



  force_envmon
      A	value used by developers for debugging subsystem source	code.

      Do not modify this attribute unless instructed to	do so by support per-
      sonnel or	by patch kit documentation.



  max_iosize_read
      The size,	in bytes, of an	I/O read request used by nondriver kernel
      code if an ioctl() call fails or returns 0 (zero). (When nondriver
      kernel code needs	to know	the maximum size of an I/O read	request	for a
      block-I/O	device,	it issues an ioctl call	to the driver.)

      Default value: 65,536 (bytes)

      Change the default value of this attribute only if a third-party device
      driver does not use the ioctl call and has a maximum size	that is	less
      than the default value (65536 bytes, or 64 kilobytes) in the syscon-
      figtab file .  (The person writing the third-party device	driver must
      provide information, either in documentation or an installation script,
      on how to	change the entry in the	sysconfigtab file to the correct
      value.)



  max_iosize_write
      The size,	in bytes, of an	I/O write request used by nondriver kernel
      code if an ioctl() call fails or returns 0 (zero). (When nondriver ker-
      nel code needs to	know the maximum size of an I/O	write request for a
      block-I/O	device,	it issues an ioctl() call to the driver.)

      Default value: 65,536 (bytes)

      Change the default value of this attribute only if a third-party device
      driver does not use the ioctl call and has a maximum size	that is	less
      than the default value  (64 KB) in the sysconfigtab file.	 (The person
      writing the third-party device driver must provide information, either
      in documentation or an installation script, on how to change the entry
      in the sysconfigtab file to the correct value.)

  no_scatter_gather
      Default value: 0

      Do not modify the	default	setting	of this	attribute unless instructed
      to do so by support personnel or by patch	kit documentation.



  NPort_Max_IOs
      Maximum number of	NPort I/Os. This attribute is visible only if the
      Emulex Fibre Channel Host	Adapter	is installed on	the system. Refer to
      the emx(7) reference page	for information	on setting this	attribute.

      Default value: 256.

      To turn I/O limiting off,	set the	value of NPort_Max_IOs to equal
      0xFFFFFFFF.

      Do not attempt to	set the	value of NPort_Max_IOs to less than the	value
      of NPort_Min_IOs.



  NPort_Min_IOs
      Minumum number of	NPort I/Os. This attribute is visible only if the
      Emulex Fibre Channel Host	Adapter	is installed on	the system. Refer to
      the emx(7) reference page	for information	on setting this	attribute.

      Default value: 20.

      The value	of NPort_Min_IOs must always be	less than the value of
      NPort_Max_IOs.



  NPort_IORamp_Delay
      The NPort	I/O ramp time delay in seconds.	This attribute is visible
      only if the Emulex Fibre Channel Host Adapter is installed on the	sys-
      tem. Refer to the	emx(7) reference page for information on setting this
      attribute.

      Default value: 1.



  NPort_IORamp_Percent
      The NPort	I/O ramp percentage (integer). This attribute is visible only
      if the Emulex Fibre Channel Host Adapter is installed on the system.
      Refer to the emx(7) reference page for information on setting this
      attribute.

      Default value: 10



  root_dev
      A	query-only attribute used for subsystem	debugging.



  root_hardware_id
      A	query-only attribute used for subsystem	debugging.



  root_mounted_rw
      An attribute used	only internally.

      Do not modify the	default	setting	of this	attribute unless instructed
      to do so by support personnel or by patch	kit documentation.

SEE ALSO

  sys_attrs(5)