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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.
  We probably need a new mechanism similar to dmu_tx_create_assigned to assign
  ZIL transactions.
  (Ab)using TXG_WAITED does not seem to be sufficient.

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>
This commit is contained in:
Andriy Gapon 2017-08-08 11:24:13 +00:00
parent 23dbc36649
commit 9fdc44732a
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/vendor-sys/illumos/dist/; revision=322242
1 changed files with 18 additions and 1 deletions
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19
uts/common/fs/zfs/zil.c
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@ -974,7 +974,24 @@ 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);