view Documentation/block/barrier.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
line source
1 I/O Barriers
2 ============
3 Tejun Heo <htejun@gmail.com>, July 22 2005
5 I/O barrier requests are used to guarantee ordering around the barrier
6 requests. Unless you're crazy enough to use disk drives for
7 implementing synchronization constructs (wow, sounds interesting...),
8 the ordering is meaningful only for write requests for things like
9 journal checkpoints. All requests queued before a barrier request
10 must be finished (made it to the physical medium) before the barrier
11 request is started, and all requests queued after the barrier request
12 must be started only after the barrier request is finished (again,
13 made it to the physical medium).
15 In other words, I/O barrier requests have the following two properties.
17 1. Request ordering
19 Requests cannot pass the barrier request. Preceding requests are
20 processed before the barrier and following requests after.
22 Depending on what features a drive supports, this can be done in one
23 of the following three ways.
25 i. For devices which have queue depth greater than 1 (TCQ devices) and
26 support ordered tags, block layer can just issue the barrier as an
27 ordered request and the lower level driver, controller and drive
28 itself are responsible for making sure that the ordering contraint is
29 met. Most modern SCSI controllers/drives should support this.
31 NOTE: SCSI ordered tag isn't currently used due to limitation in the
32 SCSI midlayer, see the following random notes section.
34 ii. For devices which have queue depth greater than 1 but don't
35 support ordered tags, block layer ensures that the requests preceding
36 a barrier request finishes before issuing the barrier request. Also,
37 it defers requests following the barrier until the barrier request is
38 finished. Older SCSI controllers/drives and SATA drives fall in this
39 category.
41 iii. Devices which have queue depth of 1. This is a degenerate case
42 of ii. Just keeping issue order suffices. Ancient SCSI
43 controllers/drives and IDE drives are in this category.
45 2. Forced flushing to physcial medium
47 Again, if you're not gonna do synchronization with disk drives (dang,
48 it sounds even more appealing now!), the reason you use I/O barriers
49 is mainly to protect filesystem integrity when power failure or some
50 other events abruptly stop the drive from operating and possibly make
51 the drive lose data in its cache. So, I/O barriers need to guarantee
52 that requests actually get written to non-volatile medium in order.
54 There are four cases,
56 i. No write-back cache. Keeping requests ordered is enough.
58 ii. Write-back cache but no flush operation. There's no way to
59 gurantee physical-medium commit order. This kind of devices can't to
60 I/O barriers.
62 iii. Write-back cache and flush operation but no FUA (forced unit
63 access). We need two cache flushes - before and after the barrier
64 request.
66 iv. Write-back cache, flush operation and FUA. We still need one
67 flush to make sure requests preceding a barrier are written to medium,
68 but post-barrier flush can be avoided by using FUA write on the
69 barrier itself.
72 How to support barrier requests in drivers
73 ------------------------------------------
75 All barrier handling is done inside block layer proper. All low level
76 drivers have to are implementing its prepare_flush_fn and using one
77 the following two functions to indicate what barrier type it supports
78 and how to prepare flush requests. Note that the term 'ordered' is
79 used to indicate the whole sequence of performing barrier requests
80 including draining and flushing.
82 typedef void (prepare_flush_fn)(request_queue_t *q, struct request *rq);
84 int blk_queue_ordered(request_queue_t *q, unsigned ordered,
85 prepare_flush_fn *prepare_flush_fn,
86 unsigned gfp_mask);
88 int blk_queue_ordered_locked(request_queue_t *q, unsigned ordered,
89 prepare_flush_fn *prepare_flush_fn,
90 unsigned gfp_mask);
92 The only difference between the two functions is whether or not the
93 caller is holding q->queue_lock on entry. The latter expects the
94 caller is holding the lock.
96 @q : the queue in question
97 @ordered : the ordered mode the driver/device supports
98 @prepare_flush_fn : this function should prepare @rq such that it
99 flushes cache to physical medium when executed
100 @gfp_mask : gfp_mask used when allocating data structures
101 for ordered processing
103 For example, SCSI disk driver's prepare_flush_fn looks like the
104 following.
106 static void sd_prepare_flush(request_queue_t *q, struct request *rq)
107 {
108 memset(rq->cmd, 0, sizeof(rq->cmd));
109 rq->flags |= REQ_BLOCK_PC;
110 rq->timeout = SD_TIMEOUT;
111 rq->cmd[0] = SYNCHRONIZE_CACHE;
112 }
114 The following seven ordered modes are supported. The following table
115 shows which mode should be used depending on what features a
116 device/driver supports. In the leftmost column of table,
117 QUEUE_ORDERED_ prefix is omitted from the mode names to save space.
119 The table is followed by description of each mode. Note that in the
120 descriptions of QUEUE_ORDERED_DRAIN*, '=>' is used whereas '->' is
121 used for QUEUE_ORDERED_TAG* descriptions. '=>' indicates that the
122 preceding step must be complete before proceeding to the next step.
123 '->' indicates that the next step can start as soon as the previous
124 step is issued.
126 write-back cache ordered tag flush FUA
127 -----------------------------------------------------------------------
128 NONE yes/no N/A no N/A
129 DRAIN no no N/A N/A
130 DRAIN_FLUSH yes no yes no
131 DRAIN_FUA yes no yes yes
132 TAG no yes N/A N/A
133 TAG_FLUSH yes yes yes no
134 TAG_FUA yes yes yes yes
138 I/O barriers are not needed and/or supported.
140 Sequence: N/A
143 Requests are ordered by draining the request queue and cache
144 flushing isn't needed.
146 Sequence: drain => barrier
149 Requests are ordered by draining the request queue and both
150 pre-barrier and post-barrier cache flushings are needed.
152 Sequence: drain => preflush => barrier => postflush
155 Requests are ordered by draining the request queue and
156 pre-barrier cache flushing is needed. By using FUA on barrier
157 request, post-barrier flushing can be skipped.
159 Sequence: drain => preflush => barrier
162 Requests are ordered by ordered tag and cache flushing isn't
163 needed.
165 Sequence: barrier
168 Requests are ordered by ordered tag and both pre-barrier and
169 post-barrier cache flushings are needed.
171 Sequence: preflush -> barrier -> postflush
174 Requests are ordered by ordered tag and pre-barrier cache
175 flushing is needed. By using FUA on barrier request,
176 post-barrier flushing can be skipped.
178 Sequence: preflush -> barrier
181 Random notes/caveats
182 --------------------
184 * SCSI layer currently can't use TAG ordering even if the drive,
185 controller and driver support it. The problem is that SCSI midlayer
186 request dispatch function is not atomic. It releases queue lock and
187 switch to SCSI host lock during issue and it's possible and likely to
188 happen in time that requests change their relative positions. Once
189 this problem is solved, TAG ordering can be enabled.
191 * Currently, no matter which ordered mode is used, there can be only
192 one barrier request in progress. All I/O barriers are held off by
193 block layer until the previous I/O barrier is complete. This doesn't
194 make any difference for DRAIN ordered devices, but, for TAG ordered
195 devices with very high command latency, passing multiple I/O barriers
196 to low level *might* be helpful if they are very frequent. Well, this
197 certainly is a non-issue. I'm writing this just to make clear that no
198 two I/O barrier is ever passed to low-level driver.
200 * Completion order. Requests in ordered sequence are issued in order
201 but not required to finish in order. Barrier implementation can
202 handle out-of-order completion of ordered sequence. IOW, the requests
203 MUST be processed in order but the hardware/software completion paths
204 are allowed to reorder completion notifications - eg. current SCSI
205 midlayer doesn't preserve completion order during error handling.
207 * Requeueing order. Low-level drivers are free to requeue any request
208 after they removed it from the request queue with
209 blkdev_dequeue_request(). As barrier sequence should be kept in order
210 when requeued, generic elevator code takes care of putting requests in
211 order around barrier. See blk_ordered_req_seq() and
212 ELEVATOR_INSERT_REQUEUE handling in __elv_add_request() for details.
214 Note that block drivers must not requeue preceding requests while
215 completing latter requests in an ordered sequence. Currently, no
216 error checking is done against this.
218 * Error handling. Currently, block layer will report error to upper
219 layer if any of requests in an ordered sequence fails. Unfortunately,
220 this doesn't seem to be enough. Look at the following request flow.
223 [0] [1] [2] [3] [pre] [barrier] [post] < [4] [5] [6] ... >
224 still in elevator
226 Let's say request [2], [3] are write requests to update file system
227 metadata (journal or whatever) and [barrier] is used to mark that
228 those updates are valid. Consider the following sequence.
230 i. Requests [0] ~ [post] leaves the request queue and enters
231 low-level driver.
232 ii. After a while, unfortunately, something goes wrong and the
233 drive fails [2]. Note that any of [0], [1] and [3] could have
234 completed by this time, but [pre] couldn't have been finished
235 as the drive must process it in order and it failed before
236 processing that command.
237 iii. Error handling kicks in and determines that the error is
238 unrecoverable and fails [2], and resumes operation.
239 iv. [pre] [barrier] [post] gets processed.
240 v. *BOOM* power fails
242 The problem here is that the barrier request is *supposed* to indicate
243 that filesystem update requests [2] and [3] made it safely to the
244 physical medium and, if the machine crashes after the barrier is
245 written, filesystem recovery code can depend on that. Sadly, that
246 isn't true in this case anymore. IOW, the success of a I/O barrier
247 should also be dependent on success of some of the preceding requests,
248 where only upper layer (filesystem) knows what 'some' is.
250 This can be solved by implementing a way to tell the block layer which
251 requests affect the success of the following barrier request and
252 making lower lever drivers to resume operation on error only after
253 block layer tells it to do so.
255 As the probability of this happening is very low and the drive should
256 be faulty, implementing the fix is probably an overkill. But, still,
257 it's there.
259 * In previous drafts of barrier implementation, there was fallback
260 mechanism such that, if FUA or ordered TAG fails, less fancy ordered
261 mode can be selected and the failed barrier request is retried
262 automatically. The rationale for this feature was that as FUA is
263 pretty new in ATA world and ordered tag was never used widely, there
264 could be devices which report to support those features but choke when
265 actually given such requests.
267 This was removed for two reasons 1. it's an overkill 2. it's
268 impossible to implement properly when TAG ordering is used as low
269 level drivers resume after an error automatically. If it's ever
270 needed adding it back and modifying low level drivers accordingly
271 shouldn't be difficult.