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MFV r322242: 8373 TXG_WAIT in ZIL commit path

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illumos/illumos-gate@d28671a3b0
d28671a3b0

https://www.illumos.org/issues/8373
  The code that writes ZIL blocks uses dmu_tx_assign(TXG_WAIT) to assign
  a transaction to a transaction group.  That seems to be logically
  incorrect as writing of the ZIL block does not introduce any new dirty
  data.  Also, when there is a lot of dirty data, the call can introduce
  significant delays into the ZIL commit path, thus affecting all
  synchronous writes. Additionally, ARC throttling may affect the ZIL
  writing.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>

MFC after:	2 weeks
This commit is contained in:
Andriy Gapon 2017-08-08 11:26:03 +00:00
commit 984d43cca5
1 changed files with 18 additions and 1 deletions
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19
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c
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@ -985,7 +985,24 @@ zil_lwb_write_start(zilog_t *zilog, lwb_t *lwb, boolean_t last)
* 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);