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Avoid extra memory copies during mirror scrub. #13593

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Avoid extra memory copies during mirror scrub. #13593

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17 changes: 9 additions & 8 deletions module/zfs/vdev_mirror.c
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Original file line number Diff line number Diff line change
Expand Up @@ -120,6 +120,7 @@ typedef struct mirror_map {
boolean_t mm_resilvering;
boolean_t mm_rebuilding;
boolean_t mm_root;
boolean_t mm_got_data;
mirror_child_t mm_child[];
} mirror_map_t;

Expand Down Expand Up @@ -451,17 +452,17 @@ vdev_mirror_scrub_done(zio_t *zio)
mirror_child_t *mc = zio->io_private;

if (zio->io_error == 0) {
zio_t *pio;
zio_link_t *zl = NULL;

mutex_enter(&zio->io_lock);
while ((pio = zio_walk_parents(zio, &zl)) != NULL) {
mutex_enter(&pio->io_lock);
ASSERT3U(zio->io_size, >=, pio->io_size);
zio_t *pio = zio_unique_parent(zio);
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The original algorithm walked all parents to do the copy... while it seems reasonable that there should generally be only a single parent here, do you have a reason to believe that is an invariant? Why did the original algorithm assume the possibility of multiple parents?

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I have no idea what the original author was thinking about. zio_unique_parent() actually verifies there are not other parents, and the assertion didn't fire on my tests. My best guess is that it was some paranoia, same as unneeded locking.

mirror_map_t *mm = pio->io_vsd;
mutex_enter(&pio->io_lock);
if (!mm->mm_got_data) {
mm->mm_got_data = B_TRUE;
mutex_exit(&pio->io_lock);
ASSERT3U(zio->io_size, ==, pio->io_size);
abd_copy(pio->io_abd, zio->io_abd, pio->io_size);
} else {
mutex_exit(&pio->io_lock);
}
mutex_exit(&zio->io_lock);
}

abd_free(zio->io_abd);
Expand Down