/share/man/man4/da.4

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  1. .\" Copyright (c) 1996
  2. .\" Julian Elischer <julian@FreeBSD.org>. All rights reserved.
  3. .\"
  4. .\" Redistribution and use in source and binary forms, with or without
  5. .\" modification, are permitted provided that the following conditions
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  26. .\" $FreeBSD$
  27. .\"
  28. .Dd February 8, 2012
  29. .Dt DA 4
  30. .Os
  31. .Sh NAME
  32. .Nm da
  33. .Nd SCSI Direct Access device driver
  34. .Sh SYNOPSIS
  35. .Cd device da
  36. .Sh DESCRIPTION
  37. The
  38. .Nm
  39. driver provides support for all
  40. .Tn SCSI
  41. devices of the direct access class that are attached to the system
  42. through a supported
  43. .Tn SCSI
  44. Host Adapter.
  45. The direct access class includes disk, magneto-optical,
  46. and solid-state devices.
  47. .Pp
  48. A
  49. .Tn SCSI
  50. Host
  51. adapter must also be separately configured into the system
  52. before a
  53. .Tn SCSI
  54. direct access device can be configured.
  55. .Sh CACHE EFFECTS
  56. Many direct access devices are equipped with read and/or write caches.
  57. Parameters affecting the device's cache are stored in mode page 8,
  58. the caching control page.
  59. Mode pages can be examined and modified via the
  60. .Xr camcontrol 8
  61. utility.
  62. .Pp
  63. The read cache is used to store data from device-initiated read ahead
  64. operations as well as frequently used data.
  65. The read cache is transparent
  66. to the user and can be enabled without any adverse effect.
  67. Most devices
  68. with a read cache come from the factory with it enabled.
  69. The read cache can be disabled by setting the
  70. .Tn RCD
  71. (Read Cache Disable) bit in the caching control mode page.
  72. .Pp
  73. The write cache can greatly decrease the latency of write operations
  74. and allows the device to reorganize writes to increase efficiency and
  75. performance.
  76. This performance gain comes at a price.
  77. Should the device
  78. lose power while its cache contains uncommitted write operations, these
  79. writes will be lost.
  80. The effect of a loss of write transactions on
  81. a file system is non-deterministic and can cause corruption.
  82. Most
  83. devices age write transactions to limit vulnerability to a few transactions
  84. recently reported as complete, but it is none-the-less recommended that
  85. systems with write cache enabled devices reside on an Uninterruptible
  86. Power Supply (UPS).
  87. The
  88. .Nm
  89. device driver ensures that the cache and media are synchronized upon
  90. final close of the device or an unexpected shutdown (panic) event.
  91. This ensures that it is safe to disconnect power once the operating system
  92. has reported that it has halted.
  93. The write cache can be enabled by setting the
  94. .Tn WCE
  95. (Write Cache Enable) bit in the caching control mode page.
  96. .Sh TAGGED QUEUING
  97. The
  98. .Nm
  99. device driver will take full advantage of the SCSI feature known as tagged
  100. queueing.
  101. Tagged queueing allows the device to process multiple transactions
  102. concurrently, often re-ordering them to reduce the number and length of
  103. seeks.
  104. To ensure that transactions to distant portions of the media,
  105. which may be deferred indefinitely by servicing requests nearer the current
  106. head position, are completed in a timely fashion, an ordered tagged
  107. transaction is sent every 15 seconds during continuous device operation.
  108. .Sh BAD BLOCK RECOVERY
  109. Direct Access devices have the capability of mapping out portions of
  110. defective media.
  111. Media recovery parameters are located in mode page 1,
  112. the Read-Write Error Recovery mode page.
  113. The most important media
  114. remapping features are 'Auto Write Reallocation' and 'Auto Read
  115. Reallocation' which can be enabled via the AWRE and ARRE bits,
  116. respectively, of the Read-Write Error Recovery page.
  117. Many devices do not ship from the factory with these feature enabled.
  118. Mode pages can be examined and modified
  119. via the
  120. .Xr camcontrol 8
  121. utility.
  122. .Sh KERNEL CONFIGURATION
  123. It is only necessary to explicitly configure one
  124. .Nm
  125. device; data structures are dynamically allocated as disks are found
  126. on the
  127. .Tn SCSI
  128. bus.
  129. .Sh SYSCTL VARIABLES
  130. The following variables are available as both
  131. .Xr sysctl 8
  132. variables and
  133. .Xr loader 8
  134. tunables:
  135. .Bl -tag -width 12
  136. .It kern.cam.da.retry_count
  137. .Pp
  138. This variable determines how many times the
  139. .Nm
  140. driver will retry a READ or WRITE command.
  141. This does not affect the number of retries used during probe time or for
  142. the
  143. .Nm
  144. driver dump routine.
  145. This value currently defaults to 4.
  146. .It kern.cam.da.default_timeout
  147. .Pp
  148. This variable determines how long the
  149. .Nm
  150. driver will wait before timing out an outstanding command.
  151. The units for this value are seconds, and the default is currently 60
  152. seconds.
  153. .It kern.cam.da.%d.minimum_cmd_size
  154. .Pp
  155. This variable determines what the minimum READ/WRITE CDB size is for a
  156. given
  157. .Nm
  158. unit.
  159. (The %d above denotes the unit number of the
  160. .Nm
  161. driver instance, e.g.\& 1, 2, 4, 8, etc.)
  162. Valid minimum command size values are 6, 10, 12 and 16 bytes.
  163. The default is 6 bytes.
  164. .Pp
  165. The
  166. .Nm
  167. driver issues a CAM Path Inquiry CCB at probe time to determine whether the
  168. protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
  169. 6 byte commands.
  170. If it does not, the
  171. .Nm
  172. driver will default to using at least 10 byte CDBs.
  173. If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
  174. .Nm
  175. driver will then increase the default CDB size for the device to 10 bytes and
  176. retry the command.
  177. CDB size is always
  178. chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
  179. command size, and the LBA and length of the READ or WRITE in question.
  180. (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
  181. .El
  182. .Sh NOTES
  183. If a device becomes invalidated (media is removed, device becomes unresponsive)
  184. the disklabel and information held within the kernel about the device will
  185. be invalidated.
  186. To avoid corruption of a newly inserted piece of media or
  187. a replacement device, all accesses to the device will be discarded until
  188. the last file descriptor referencing the old device is closed.
  189. During this period, all new open attempts will be rejected.
  190. .Sh FILES
  191. .Bl -tag -width ".Pa /dev/da*" -compact
  192. .It Pa /dev/da*
  193. SCSI disk device nodes
  194. .El
  195. .Sh DIAGNOSTICS
  196. None.
  197. .Sh SEE ALSO
  198. .Xr ada 4 ,
  199. .Xr cam 4 ,
  200. .Xr geom 4 ,
  201. .Xr bsdlabel 8 ,
  202. .Xr fdisk 8
  203. .Sh HISTORY
  204. The
  205. .Nm
  206. driver was written for the
  207. .Tn CAM
  208. .Tn SCSI
  209. subsystem by
  210. .An Justin T. Gibbs .
  211. Many ideas were gleaned from the
  212. .Nm sd
  213. device driver written and ported from
  214. .Tn Mach
  215. 2.5
  216. by
  217. .An Julian Elischer .