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Don't preload at or after final dirty txg #9216

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Don't preload at or after final dirty txg #9216

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Don't preload at or after final dirty txg
pcd1193182 Aug 26, 2019
e84d035
9185 metaslab_verify_weight_and_frag() shouldn't cause side-effects
sdimitro Aug 23, 2019
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2 changes: 1 addition & 1 deletion include/sys/metaslab.h
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Original file line number Diff line number Diff line change
Expand Up @@ -64,7 +64,7 @@ uint64_t metaslab_allocated_space(metaslab_t *);

void metaslab_sync(metaslab_t *, uint64_t);
void metaslab_sync_done(metaslab_t *, uint64_t);
void metaslab_sync_reassess(metaslab_group_t *);
void metaslab_sync_reassess(metaslab_group_t *, uint64_t);
uint64_t metaslab_largest_allocatable(metaslab_t *);

/*
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23 changes: 21 additions & 2 deletions module/zfs/metaslab.c
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Expand Up @@ -1820,6 +1820,20 @@ metaslab_verify_weight_and_frag(metaslab_t *msp)
if (!spa_writeable(msp->ms_group->mg_vd->vdev_spa))
return;

/*
* This is a verification function and thus should not
* introduce any side-effects/mutations on the system.
* Unfortunately, calling metaslab_set_fragmentation()
* through metaslab_weight() later in this codepath
* can dirty an otherwise undirty metaslab and set
* ms_condense_wanted if the metaslab's spacemap hasn't
* been upgraded (e.g. after enabling SPACEMAP_HISTOGRAMS
* in a old pool). To avoid mutating system state, we skip
* those metaslabs.
*/
if (msp->ms_sm->sm_dbuf->db_size != sizeof (space_map_phys_t))
return;
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The comment explains this well, but this seems like this is exposing a poorly-designed corner of the code. Could we do something like pass a nodirty flag to metaslab_weight(), or have a metaslab_weight_impl(), which just recalculates the weight and doesn't check about condensing? That way, if metaslab_weight() decides to mutate the metaslab due to some other condition, the relevant code will be close by, and we won't have to add another check here.

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realized this is actually part of #9209. copied my comment over there. This commit is good to go.


/* some extra verification for in-core tree if you can */
if (msp->ms_loaded) {
range_tree_stat_verify(msp->ms_allocatable);
Expand Down Expand Up @@ -3987,7 +4001,7 @@ metaslab_sync_done(metaslab_t *msp, uint64_t txg)
}

void
metaslab_sync_reassess(metaslab_group_t *mg)
metaslab_sync_reassess(metaslab_group_t *mg, uint64_t txg)
{
spa_t *spa = mg->mg_class->mc_spa;

Expand All @@ -4001,8 +4015,13 @@ metaslab_sync_reassess(metaslab_group_t *mg)
* is no longer active since we dirty metaslabs as we remove a
* a device, thus potentially making the metaslab group eligible
* for preloading.
*
* We also forbid preloading if the pool is at or beyond the final
* dirty txg. If we don't, and the metaslab is dirtied after it is
* loaded, we will be violating the "final dirty txg" invariant, which
* will cause an assertion failure in metaslab_sync.
*/
if (mg->mg_activation_count > 0) {
if (mg->mg_activation_count > 0 && spa_final_dirty_txg(spa) < txg) {
metaslab_group_preload(mg);
}
spa_config_exit(spa, SCL_ALLOC, FTAG);
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2 changes: 1 addition & 1 deletion module/zfs/vdev.c
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Expand Up @@ -3263,7 +3263,7 @@ vdev_sync_done(vdev_t *vd, uint64_t txg)
metaslab_sync_done(msp, txg);

if (reassess)
metaslab_sync_reassess(vd->vdev_mg);
metaslab_sync_reassess(vd->vdev_mg, txg);
}

void
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