4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
22 * Copyright 2009 Sun Microsystems, Inc. All rights reserved.
23 * Use is subject to license terms.
27 * Copyright (c) 2013 by Delphix. All rights reserved.
30 #include <sys/zfs_context.h>
31 #include <sys/dnode.h>
32 #include <sys/dmu_objset.h>
33 #include <sys/dmu_zfetch.h>
36 #include <sys/kstat.h>
39 * I'm against tune-ables, but these should probably exist as tweakable globals
40 * until we can get this working the way we want it to.
43 int zfs_prefetch_disable
= 0;
45 /* max # of streams per zfetch */
46 unsigned int zfetch_max_streams
= 8;
47 /* min time before stream reclaim */
48 unsigned int zfetch_min_sec_reap
= 2;
49 /* max number of blocks to fetch at a time */
50 unsigned int zfetch_block_cap
= 256;
51 /* number of bytes in a array_read at which we stop prefetching (1Mb) */
52 unsigned long zfetch_array_rd_sz
= 1024 * 1024;
54 /* forward decls for static routines */
55 static boolean_t
dmu_zfetch_colinear(zfetch_t
*, zstream_t
*);
56 static void dmu_zfetch_dofetch(zfetch_t
*, zstream_t
*);
57 static uint64_t dmu_zfetch_fetch(dnode_t
*, uint64_t, uint64_t);
58 static uint64_t dmu_zfetch_fetchsz(dnode_t
*, uint64_t, uint64_t);
59 static boolean_t
dmu_zfetch_find(zfetch_t
*, zstream_t
*, int);
60 static int dmu_zfetch_stream_insert(zfetch_t
*, zstream_t
*);
61 static zstream_t
*dmu_zfetch_stream_reclaim(zfetch_t
*);
62 static void dmu_zfetch_stream_remove(zfetch_t
*, zstream_t
*);
63 static int dmu_zfetch_streams_equal(zstream_t
*, zstream_t
*);
65 typedef struct zfetch_stats
{
66 kstat_named_t zfetchstat_hits
;
67 kstat_named_t zfetchstat_misses
;
68 kstat_named_t zfetchstat_colinear_hits
;
69 kstat_named_t zfetchstat_colinear_misses
;
70 kstat_named_t zfetchstat_stride_hits
;
71 kstat_named_t zfetchstat_stride_misses
;
72 kstat_named_t zfetchstat_reclaim_successes
;
73 kstat_named_t zfetchstat_reclaim_failures
;
74 kstat_named_t zfetchstat_stream_resets
;
75 kstat_named_t zfetchstat_stream_noresets
;
76 kstat_named_t zfetchstat_bogus_streams
;
79 static zfetch_stats_t zfetch_stats
= {
80 { "hits", KSTAT_DATA_UINT64
},
81 { "misses", KSTAT_DATA_UINT64
},
82 { "colinear_hits", KSTAT_DATA_UINT64
},
83 { "colinear_misses", KSTAT_DATA_UINT64
},
84 { "stride_hits", KSTAT_DATA_UINT64
},
85 { "stride_misses", KSTAT_DATA_UINT64
},
86 { "reclaim_successes", KSTAT_DATA_UINT64
},
87 { "reclaim_failures", KSTAT_DATA_UINT64
},
88 { "streams_resets", KSTAT_DATA_UINT64
},
89 { "streams_noresets", KSTAT_DATA_UINT64
},
90 { "bogus_streams", KSTAT_DATA_UINT64
},
93 #define ZFETCHSTAT_INCR(stat, val) \
94 atomic_add_64(&zfetch_stats.stat.value.ui64, (val));
96 #define ZFETCHSTAT_BUMP(stat) ZFETCHSTAT_INCR(stat, 1);
101 * Given a zfetch structure and a zstream structure, determine whether the
102 * blocks to be read are part of a co-linear pair of existing prefetch
103 * streams. If a set is found, coalesce the streams, removing one, and
104 * configure the prefetch so it looks for a strided access pattern.
106 * In other words: if we find two sequential access streams that are
107 * the same length and distance N appart, and this read is N from the
108 * last stream, then we are probably in a strided access pattern. So
109 * combine the two sequential streams into a single strided stream.
111 * Returns whether co-linear streams were found.
114 dmu_zfetch_colinear(zfetch_t
*zf
, zstream_t
*zh
)
119 if (! rw_tryenter(&zf
->zf_rwlock
, RW_WRITER
))
123 rw_exit(&zf
->zf_rwlock
);
127 for (z_walk
= list_head(&zf
->zf_stream
); z_walk
;
128 z_walk
= list_next(&zf
->zf_stream
, z_walk
)) {
129 for (z_comp
= list_next(&zf
->zf_stream
, z_walk
); z_comp
;
130 z_comp
= list_next(&zf
->zf_stream
, z_comp
)) {
133 if (z_walk
->zst_len
!= z_walk
->zst_stride
||
134 z_comp
->zst_len
!= z_comp
->zst_stride
) {
138 diff
= z_comp
->zst_offset
- z_walk
->zst_offset
;
139 if (z_comp
->zst_offset
+ diff
== zh
->zst_offset
) {
140 z_walk
->zst_offset
= zh
->zst_offset
;
141 z_walk
->zst_direction
= diff
< 0 ?
142 ZFETCH_BACKWARD
: ZFETCH_FORWARD
;
144 diff
* z_walk
->zst_direction
;
145 z_walk
->zst_ph_offset
=
146 zh
->zst_offset
+ z_walk
->zst_stride
;
147 dmu_zfetch_stream_remove(zf
, z_comp
);
148 mutex_destroy(&z_comp
->zst_lock
);
149 kmem_free(z_comp
, sizeof (zstream_t
));
151 dmu_zfetch_dofetch(zf
, z_walk
);
153 rw_exit(&zf
->zf_rwlock
);
157 diff
= z_walk
->zst_offset
- z_comp
->zst_offset
;
158 if (z_walk
->zst_offset
+ diff
== zh
->zst_offset
) {
159 z_walk
->zst_offset
= zh
->zst_offset
;
160 z_walk
->zst_direction
= diff
< 0 ?
161 ZFETCH_BACKWARD
: ZFETCH_FORWARD
;
163 diff
* z_walk
->zst_direction
;
164 z_walk
->zst_ph_offset
=
165 zh
->zst_offset
+ z_walk
->zst_stride
;
166 dmu_zfetch_stream_remove(zf
, z_comp
);
167 mutex_destroy(&z_comp
->zst_lock
);
168 kmem_free(z_comp
, sizeof (zstream_t
));
170 dmu_zfetch_dofetch(zf
, z_walk
);
172 rw_exit(&zf
->zf_rwlock
);
178 rw_exit(&zf
->zf_rwlock
);
183 * Given a zstream_t, determine the bounds of the prefetch. Then call the
184 * routine that actually prefetches the individual blocks.
187 dmu_zfetch_dofetch(zfetch_t
*zf
, zstream_t
*zs
)
189 uint64_t prefetch_tail
;
190 uint64_t prefetch_limit
;
191 uint64_t prefetch_ofst
;
192 uint64_t prefetch_len
;
193 uint64_t blocks_fetched
;
195 zs
->zst_stride
= MAX((int64_t)zs
->zst_stride
, zs
->zst_len
);
196 zs
->zst_cap
= MIN(zfetch_block_cap
, 2 * zs
->zst_cap
);
198 prefetch_tail
= MAX((int64_t)zs
->zst_ph_offset
,
199 (int64_t)(zs
->zst_offset
+ zs
->zst_stride
));
201 * XXX: use a faster division method?
203 prefetch_limit
= zs
->zst_offset
+ zs
->zst_len
+
204 (zs
->zst_cap
* zs
->zst_stride
) / zs
->zst_len
;
206 while (prefetch_tail
< prefetch_limit
) {
207 prefetch_ofst
= zs
->zst_offset
+ zs
->zst_direction
*
208 (prefetch_tail
- zs
->zst_offset
);
210 prefetch_len
= zs
->zst_len
;
213 * Don't prefetch beyond the end of the file, if working
216 if ((zs
->zst_direction
== ZFETCH_BACKWARD
) &&
217 (prefetch_ofst
> prefetch_tail
)) {
218 prefetch_len
+= prefetch_ofst
;
222 /* don't prefetch more than we're supposed to */
223 if (prefetch_len
> zs
->zst_len
)
226 blocks_fetched
= dmu_zfetch_fetch(zf
->zf_dnode
,
227 prefetch_ofst
, zs
->zst_len
);
229 prefetch_tail
+= zs
->zst_stride
;
230 /* stop if we've run out of stuff to prefetch */
231 if (blocks_fetched
< zs
->zst_len
)
234 zs
->zst_ph_offset
= prefetch_tail
;
235 zs
->zst_last
= ddi_get_lbolt();
242 zfetch_ksp
= kstat_create("zfs", 0, "zfetchstats", "misc",
243 KSTAT_TYPE_NAMED
, sizeof (zfetch_stats
) / sizeof (kstat_named_t
),
246 if (zfetch_ksp
!= NULL
) {
247 zfetch_ksp
->ks_data
= &zfetch_stats
;
248 kstat_install(zfetch_ksp
);
255 if (zfetch_ksp
!= NULL
) {
256 kstat_delete(zfetch_ksp
);
262 * This takes a pointer to a zfetch structure and a dnode. It performs the
263 * necessary setup for the zfetch structure, grokking data from the
267 dmu_zfetch_init(zfetch_t
*zf
, dnode_t
*dno
)
274 zf
->zf_stream_cnt
= 0;
275 zf
->zf_alloc_fail
= 0;
277 list_create(&zf
->zf_stream
, sizeof (zstream_t
),
278 offsetof(zstream_t
, zst_node
));
280 rw_init(&zf
->zf_rwlock
, NULL
, RW_DEFAULT
, NULL
);
284 * This function computes the actual size, in blocks, that can be prefetched,
288 dmu_zfetch_fetch(dnode_t
*dn
, uint64_t blkid
, uint64_t nblks
)
293 fetchsz
= dmu_zfetch_fetchsz(dn
, blkid
, nblks
);
295 for (i
= 0; i
< fetchsz
; i
++) {
296 dbuf_prefetch(dn
, blkid
+ i
, ZIO_PRIORITY_ASYNC_READ
);
303 * this function returns the number of blocks that would be prefetched, based
304 * upon the supplied dnode, blockid, and nblks. This is used so that we can
305 * update streams in place, and then prefetch with their old value after the
306 * fact. This way, we can delay the prefetch, but subsequent accesses to the
307 * stream won't result in the same data being prefetched multiple times.
310 dmu_zfetch_fetchsz(dnode_t
*dn
, uint64_t blkid
, uint64_t nblks
)
314 if (blkid
> dn
->dn_maxblkid
) {
318 /* compute fetch size */
319 if (blkid
+ nblks
+ 1 > dn
->dn_maxblkid
) {
320 fetchsz
= (dn
->dn_maxblkid
- blkid
) + 1;
321 ASSERT(blkid
+ fetchsz
- 1 <= dn
->dn_maxblkid
);
331 * given a zfetch and a zstream structure, see if there is an associated zstream
332 * for this block read. If so, it starts a prefetch for the stream it
333 * located and returns true, otherwise it returns false
336 dmu_zfetch_find(zfetch_t
*zf
, zstream_t
*zh
, int prefetched
)
340 int reset
= !prefetched
;
347 * XXX: This locking strategy is a bit coarse; however, it's impact has
348 * yet to be tested. If this turns out to be an issue, it can be
349 * modified in a number of different ways.
352 rw_enter(&zf
->zf_rwlock
, RW_READER
);
355 for (zs
= list_head(&zf
->zf_stream
); zs
;
356 zs
= list_next(&zf
->zf_stream
, zs
)) {
359 * XXX - should this be an assert?
361 if (zs
->zst_len
== 0) {
363 ZFETCHSTAT_BUMP(zfetchstat_bogus_streams
);
368 * We hit this case when we are in a strided prefetch stream:
369 * we will read "len" blocks before "striding".
371 if (zh
->zst_offset
>= zs
->zst_offset
&&
372 zh
->zst_offset
< zs
->zst_offset
+ zs
->zst_len
) {
374 /* already fetched */
375 ZFETCHSTAT_BUMP(zfetchstat_stride_hits
);
379 ZFETCHSTAT_BUMP(zfetchstat_stride_misses
);
384 * This is the forward sequential read case: we increment
385 * len by one each time we hit here, so we will enter this
386 * case on every read.
388 if (zh
->zst_offset
== zs
->zst_offset
+ zs
->zst_len
) {
390 reset
= !prefetched
&& zs
->zst_len
> 1;
392 mutex_enter(&zs
->zst_lock
);
394 if (zh
->zst_offset
!= zs
->zst_offset
+ zs
->zst_len
) {
395 mutex_exit(&zs
->zst_lock
);
398 zs
->zst_len
+= zh
->zst_len
;
399 diff
= zs
->zst_len
- zfetch_block_cap
;
401 zs
->zst_offset
+= diff
;
402 zs
->zst_len
= zs
->zst_len
> diff
?
403 zs
->zst_len
- diff
: 0;
405 zs
->zst_direction
= ZFETCH_FORWARD
;
410 * Same as above, but reading backwards through the file.
412 } else if (zh
->zst_offset
== zs
->zst_offset
- zh
->zst_len
) {
413 /* backwards sequential access */
415 reset
= !prefetched
&& zs
->zst_len
> 1;
417 mutex_enter(&zs
->zst_lock
);
419 if (zh
->zst_offset
!= zs
->zst_offset
- zh
->zst_len
) {
420 mutex_exit(&zs
->zst_lock
);
424 zs
->zst_offset
= zs
->zst_offset
> zh
->zst_len
?
425 zs
->zst_offset
- zh
->zst_len
: 0;
426 zs
->zst_ph_offset
= zs
->zst_ph_offset
> zh
->zst_len
?
427 zs
->zst_ph_offset
- zh
->zst_len
: 0;
428 zs
->zst_len
+= zh
->zst_len
;
430 diff
= zs
->zst_len
- zfetch_block_cap
;
432 zs
->zst_ph_offset
= zs
->zst_ph_offset
> diff
?
433 zs
->zst_ph_offset
- diff
: 0;
434 zs
->zst_len
= zs
->zst_len
> diff
?
435 zs
->zst_len
- diff
: zs
->zst_len
;
437 zs
->zst_direction
= ZFETCH_BACKWARD
;
441 } else if ((zh
->zst_offset
- zs
->zst_offset
- zs
->zst_stride
<
442 zs
->zst_len
) && (zs
->zst_len
!= zs
->zst_stride
)) {
443 /* strided forward access */
445 mutex_enter(&zs
->zst_lock
);
447 if ((zh
->zst_offset
- zs
->zst_offset
- zs
->zst_stride
>=
448 zs
->zst_len
) || (zs
->zst_len
== zs
->zst_stride
)) {
449 mutex_exit(&zs
->zst_lock
);
453 zs
->zst_offset
+= zs
->zst_stride
;
454 zs
->zst_direction
= ZFETCH_FORWARD
;
458 } else if ((zh
->zst_offset
- zs
->zst_offset
+ zs
->zst_stride
<
459 zs
->zst_len
) && (zs
->zst_len
!= zs
->zst_stride
)) {
460 /* strided reverse access */
462 mutex_enter(&zs
->zst_lock
);
464 if ((zh
->zst_offset
- zs
->zst_offset
+ zs
->zst_stride
>=
465 zs
->zst_len
) || (zs
->zst_len
== zs
->zst_stride
)) {
466 mutex_exit(&zs
->zst_lock
);
470 zs
->zst_offset
= zs
->zst_offset
> zs
->zst_stride
?
471 zs
->zst_offset
- zs
->zst_stride
: 0;
472 zs
->zst_ph_offset
= (zs
->zst_ph_offset
>
473 (2 * zs
->zst_stride
)) ?
474 (zs
->zst_ph_offset
- (2 * zs
->zst_stride
)) : 0;
475 zs
->zst_direction
= ZFETCH_BACKWARD
;
483 zstream_t
*remove
= zs
;
485 ZFETCHSTAT_BUMP(zfetchstat_stream_resets
);
487 mutex_exit(&zs
->zst_lock
);
488 rw_exit(&zf
->zf_rwlock
);
489 rw_enter(&zf
->zf_rwlock
, RW_WRITER
);
491 * Relocate the stream, in case someone removes
492 * it while we were acquiring the WRITER lock.
494 for (zs
= list_head(&zf
->zf_stream
); zs
;
495 zs
= list_next(&zf
->zf_stream
, zs
)) {
497 dmu_zfetch_stream_remove(zf
, zs
);
498 mutex_destroy(&zs
->zst_lock
);
499 kmem_free(zs
, sizeof (zstream_t
));
504 ZFETCHSTAT_BUMP(zfetchstat_stream_noresets
);
506 dmu_zfetch_dofetch(zf
, zs
);
507 mutex_exit(&zs
->zst_lock
);
511 rw_exit(&zf
->zf_rwlock
);
516 * Clean-up state associated with a zfetch structure. This frees allocated
517 * structure members, empties the zf_stream tree, and generally makes things
518 * nice. This doesn't free the zfetch_t itself, that's left to the caller.
521 dmu_zfetch_rele(zfetch_t
*zf
)
526 ASSERT(!RW_LOCK_HELD(&zf
->zf_rwlock
));
528 for (zs
= list_head(&zf
->zf_stream
); zs
; zs
= zs_next
) {
529 zs_next
= list_next(&zf
->zf_stream
, zs
);
531 list_remove(&zf
->zf_stream
, zs
);
532 mutex_destroy(&zs
->zst_lock
);
533 kmem_free(zs
, sizeof (zstream_t
));
535 list_destroy(&zf
->zf_stream
);
536 rw_destroy(&zf
->zf_rwlock
);
542 * Given a zfetch and zstream structure, insert the zstream structure into the
543 * AVL tree contained within the zfetch structure. Peform the appropriate
544 * book-keeping. It is possible that another thread has inserted a stream which
545 * matches one that we are about to insert, so we must be sure to check for this
546 * case. If one is found, return failure, and let the caller cleanup the
550 dmu_zfetch_stream_insert(zfetch_t
*zf
, zstream_t
*zs
)
555 ASSERT(RW_WRITE_HELD(&zf
->zf_rwlock
));
557 for (zs_walk
= list_head(&zf
->zf_stream
); zs_walk
; zs_walk
= zs_next
) {
558 zs_next
= list_next(&zf
->zf_stream
, zs_walk
);
560 if (dmu_zfetch_streams_equal(zs_walk
, zs
)) {
565 list_insert_head(&zf
->zf_stream
, zs
);
572 * Walk the list of zstreams in the given zfetch, find an old one (by time), and
573 * reclaim it for use by the caller.
576 dmu_zfetch_stream_reclaim(zfetch_t
*zf
)
580 if (! rw_tryenter(&zf
->zf_rwlock
, RW_WRITER
))
583 for (zs
= list_head(&zf
->zf_stream
); zs
;
584 zs
= list_next(&zf
->zf_stream
, zs
)) {
586 if (((ddi_get_lbolt() - zs
->zst_last
)/hz
) > zfetch_min_sec_reap
)
591 dmu_zfetch_stream_remove(zf
, zs
);
592 mutex_destroy(&zs
->zst_lock
);
593 bzero(zs
, sizeof (zstream_t
));
597 rw_exit(&zf
->zf_rwlock
);
603 * Given a zfetch and zstream structure, remove the zstream structure from its
604 * container in the zfetch structure. Perform the appropriate book-keeping.
607 dmu_zfetch_stream_remove(zfetch_t
*zf
, zstream_t
*zs
)
609 ASSERT(RW_WRITE_HELD(&zf
->zf_rwlock
));
611 list_remove(&zf
->zf_stream
, zs
);
616 dmu_zfetch_streams_equal(zstream_t
*zs1
, zstream_t
*zs2
)
618 if (zs1
->zst_offset
!= zs2
->zst_offset
)
621 if (zs1
->zst_len
!= zs2
->zst_len
)
624 if (zs1
->zst_stride
!= zs2
->zst_stride
)
627 if (zs1
->zst_ph_offset
!= zs2
->zst_ph_offset
)
630 if (zs1
->zst_cap
!= zs2
->zst_cap
)
633 if (zs1
->zst_direction
!= zs2
->zst_direction
)
640 * This is the prefetch entry point. It calls all of the other dmu_zfetch
641 * routines to create, delete, find, or operate upon prefetch streams.
644 dmu_zfetch(zfetch_t
*zf
, uint64_t offset
, uint64_t size
, int prefetched
)
647 zstream_t
*newstream
;
650 unsigned int blkshft
;
653 if (zfs_prefetch_disable
)
656 /* files that aren't ln2 blocksz are only one block -- nothing to do */
657 if (!zf
->zf_dnode
->dn_datablkshift
)
660 /* convert offset and size, into blockid and nblocks */
661 blkshft
= zf
->zf_dnode
->dn_datablkshift
;
662 blksz
= (1 << blkshft
);
664 bzero(&zst
, sizeof (zstream_t
));
665 zst
.zst_offset
= offset
>> blkshft
;
666 zst
.zst_len
= (P2ROUNDUP(offset
+ size
, blksz
) -
667 P2ALIGN(offset
, blksz
)) >> blkshft
;
669 fetched
= dmu_zfetch_find(zf
, &zst
, prefetched
);
671 ZFETCHSTAT_BUMP(zfetchstat_hits
);
673 ZFETCHSTAT_BUMP(zfetchstat_misses
);
674 if ((fetched
= dmu_zfetch_colinear(zf
, &zst
))) {
675 ZFETCHSTAT_BUMP(zfetchstat_colinear_hits
);
677 ZFETCHSTAT_BUMP(zfetchstat_colinear_misses
);
682 newstream
= dmu_zfetch_stream_reclaim(zf
);
685 * we still couldn't find a stream, drop the lock, and allocate
686 * one if possible. Otherwise, give up and go home.
689 ZFETCHSTAT_BUMP(zfetchstat_reclaim_successes
);
692 uint32_t max_streams
;
693 uint32_t cur_streams
;
695 ZFETCHSTAT_BUMP(zfetchstat_reclaim_failures
);
696 cur_streams
= zf
->zf_stream_cnt
;
697 maxblocks
= zf
->zf_dnode
->dn_maxblkid
;
699 max_streams
= MIN(zfetch_max_streams
,
700 (maxblocks
/ zfetch_block_cap
));
701 if (max_streams
== 0) {
705 if (cur_streams
>= max_streams
) {
709 kmem_zalloc(sizeof (zstream_t
), KM_PUSHPAGE
);
712 newstream
->zst_offset
= zst
.zst_offset
;
713 newstream
->zst_len
= zst
.zst_len
;
714 newstream
->zst_stride
= zst
.zst_len
;
715 newstream
->zst_ph_offset
= zst
.zst_len
+ zst
.zst_offset
;
716 newstream
->zst_cap
= zst
.zst_len
;
717 newstream
->zst_direction
= ZFETCH_FORWARD
;
718 newstream
->zst_last
= ddi_get_lbolt();
720 mutex_init(&newstream
->zst_lock
, NULL
, MUTEX_DEFAULT
, NULL
);
722 rw_enter(&zf
->zf_rwlock
, RW_WRITER
);
723 inserted
= dmu_zfetch_stream_insert(zf
, newstream
);
724 rw_exit(&zf
->zf_rwlock
);
727 mutex_destroy(&newstream
->zst_lock
);
728 kmem_free(newstream
, sizeof (zstream_t
));
733 #if defined(_KERNEL) && defined(HAVE_SPL)
734 module_param(zfs_prefetch_disable
, int, 0644);
735 MODULE_PARM_DESC(zfs_prefetch_disable
, "Disable all ZFS prefetching");
737 module_param(zfetch_max_streams
, uint
, 0644);
738 MODULE_PARM_DESC(zfetch_max_streams
, "Max number of streams per zfetch");
740 module_param(zfetch_min_sec_reap
, uint
, 0644);
741 MODULE_PARM_DESC(zfetch_min_sec_reap
, "Min time before stream reclaim");
743 module_param(zfetch_block_cap
, uint
, 0644);
744 MODULE_PARM_DESC(zfetch_block_cap
, "Max number of blocks to fetch at a time");
746 module_param(zfetch_array_rd_sz
, ulong
, 0644);
747 MODULE_PARM_DESC(zfetch_array_rd_sz
, "Number of bytes in a array_read");