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Non-l2arc pool reads shouldn't be l2arc misses

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The current l2_misses accounting behavior treats all reads to pools 
without a configured l2arc as an l2arc miss, IFF there is at least 
one other pool on the system which does have an l2arc configured.

This makes it extremely hard to tune for an improved l2arc hit/miss 
ratio because this ratio will be modulated by reads from pools which 
do not (and should not) have l2arc devices; its upper limit will 
depend on the ratio of reads from l2arc'd pools and non-l2arc'd pools.

This PR prevents ARC reads affecting l2arc stats (n.b. l2_misses is 
the only relevant one) where the target spa doesn't have an l2arc.

Includes new test - l2arc_l2miss_pos.ksh

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #10921
This commit is contained in:
Adam D. Moss
2020-10-20 11:39:52 -07:00
committed by GitHub
parent 241c62bdd7
commit 666aa69f32
6 changed files with 119 additions and 9 deletions
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module/zfs/arc.c
+21 -8
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@@ -6305,7 +6305,11 @@ top:
metadata, misses);
}
if (vd != NULL && l2arc_ndev != 0 && !(l2arc_norw && devw)) {
/* Check if the spa even has l2 configured */
const boolean_t spa_has_l2 = l2arc_ndev != 0 &&
spa->spa_l2cache.sav_count > 0;
if (vd != NULL && spa_has_l2 && !(l2arc_norw && devw)) {
/*
* Read from the L2ARC if the following are true:
* 1. The L2ARC vdev was previously cached.
@@ -6406,15 +6410,24 @@ top:
} else {
if (vd != NULL)
spa_config_exit(spa, SCL_L2ARC, vd);
/*
* Skip ARC stat bump for block pointers with
* embedded data. The data are read from the blkptr
* itself via decode_embedded_bp_compressed().
* Only a spa with l2 should contribute to l2
* miss stats. (Including the case of having a
* faulted cache device - that's also a miss.)
*/
if (l2arc_ndev != 0 && !embedded_bp) {
DTRACE_PROBE1(l2arc__miss,
arc_buf_hdr_t *, hdr);
ARCSTAT_BUMP(arcstat_l2_misses);
if (spa_has_l2) {
/*
* Skip ARC stat bump for block pointers with
* embedded data. The data are read from the
* blkptr itself via
* decode_embedded_bp_compressed().
*/
if (!embedded_bp) {
DTRACE_PROBE1(l2arc__miss,
arc_buf_hdr_t *, hdr);
ARCSTAT_BUMP(arcstat_l2_misses);
}
}
}