OpenZFS 8373 - TXG_WAIT in ZIL commit path

Authored by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8373
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7f04961
Closes #6403

diff --git a/module/zfs/zil.c b/module/zfs/zil.c
index 6a1f190f5e6c20ae61a87f935162d61a86adccd5..6b0346893680433c3e1b1619a94bc4b0c0f0e4c1 100644 (file)
--- a/module/zfs/zil.c
+++ b/module/zfs/zil.c
@@ -1009,7 +1009,24 @@ zil_lwb_write_start(zilog_t *zilog, lwb_t *lwb)
         * to clean up in the event of allocation failure or I/O failure.
         */
        tx = dmu_tx_create(zilog->zl_os);
-       VERIFY(dmu_tx_assign(tx, TXG_WAIT) == 0);
+
+       /*
+        * Since we are not going to create any new dirty data and we can even
+        * help with clearing the existing dirty data, we should not be subject
+        * to the dirty data based delays.
+        * We (ab)use TXG_WAITED to bypass the delay mechanism.
+        * One side effect from using TXG_WAITED is that dmu_tx_assign() can
+        * fail if the pool is suspended.  Those are dramatic circumstances,
+        * so we return NULL to signal that the normal ZIL processing is not
+        * possible and txg_wait_synced() should be used to ensure that the data
+        * is on disk.
+        */
+       error = dmu_tx_assign(tx, TXG_WAITED);
+       if (error != 0) {
+               ASSERT3S(error, ==, EIO);
+               dmu_tx_abort(tx);
+               return (NULL);
+       }
        dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
        txg = dmu_tx_get_txg(tx);