Re: [tarantool-patches] [PATCH v2 1/5] Do not promote wal vclock for failed writes

["Георгий Кириченко" <georgy@tarantool.org>]

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On Monday, January 28, 2019 2:20:18 PM MSK Vladimir Davydov wrote:
> On Tue, Jan 22, 2019 at 01:31:09PM +0300, Georgy Kirichenko wrote:
> > Increase replica lsn only if row was successfully written to disk. This
> > prevents wal from lsn gaps in case of IO errors and enforces wal
> > consistency.
> > 
> > Needs for #980
> > ---
> > 
> >  src/box/wal.c                     | 19 ++++++---
> >  test/xlog/errinj.result           |  1 -
> >  test/xlog/panic_on_lsn_gap.result | 65 +++++++++++++++----------------
> >  3 files changed, 45 insertions(+), 40 deletions(-)
> > 
> > diff --git a/src/box/wal.c b/src/box/wal.c
> > index 3b50d3629..a55b544aa 100644
> > --- a/src/box/wal.c
> > +++ b/src/box/wal.c
> > @@ -901,16 +901,16 @@ wal_writer_begin_rollback(struct wal_writer *writer)
> > 
> >  }
> >  
> >  static void
> > 
> > -wal_assign_lsn(struct wal_writer *writer, struct xrow_header **row,
> > +wal_assign_lsn(struct vclock *vclock, struct xrow_header **row,
> > 
> >  	       struct xrow_header **end)
> >  
> >  {
> >  
> >  	/** Assign LSN to all local rows. */
> >  	for ( ; row < end; row++) {
> >  	
> >  		if ((*row)->replica_id == 0) {
> > 
> > -			(*row)->lsn = vclock_inc(&writer->vclock, instance_id);
> > +			(*row)->lsn = vclock_inc(vclock, instance_id);
> > 
> >  			(*row)->replica_id = instance_id;
> >  		
> >  		} else {
> > 
> > -			vclock_follow_xrow(&writer->vclock, *row);
> > +			vclock_follow_xrow(vclock, *row);
> > 
> >  		}
> >  	
> >  	}
> >  
> >  }
> > 
> > @@ -922,6 +922,11 @@ wal_write_to_disk(struct cmsg *msg)
> > 
> >  	struct wal_msg *wal_msg = (struct wal_msg *) msg;
> >  	struct error *error;
> > 
> > +	/* Local vclock copy. */
> > +	struct vclock vclock;
> > +	vclock_create(&vclock);
> > +	vclock_copy(&vclock, &writer->vclock);
> > +
> > 
> >  	struct errinj *inj = errinj(ERRINJ_WAL_DELAY, ERRINJ_BOOL);
> >  	while (inj != NULL && inj->bparam)
> >  	
> >  		usleep(10);
> > 
> > @@ -974,14 +979,15 @@ wal_write_to_disk(struct cmsg *msg)
> > 
> >  	struct journal_entry *entry;
> >  	struct stailq_entry *last_committed = NULL;
> >  	stailq_foreach_entry(entry, &wal_msg->commit, fifo) {
> > 
> > -		wal_assign_lsn(writer, entry->rows, entry->rows + entry-
>n_rows);
> > -		entry->res = vclock_sum(&writer->vclock);
> > +		wal_assign_lsn(&vclock, entry->rows, entry->rows + entry-
>n_rows);
> > +		entry->res = vclock_sum(&vclock);
> > 
> >  		rc = xlog_write_entry(l, entry);
> >  		if (rc < 0)
> >  		
> >  			goto done;
> >  		
> >  		if (rc > 0) {
> >  		
> >  			writer->checkpoint_wal_size += rc;
> >  			last_committed = &entry->fifo;
> > 
> > +			vclock_copy(&writer->vclock, &vclock);
> 
> I don't like that you copy a vclock after applying each entry.
> Currently, it should be pretty cheap, but in future, when we make
> vclock store any number of ids, this might get pretty heavy.
> Can we minimize the number of memcpys somehow, ideally do it only
> on the rollback path?
In that case we should preserve vclock for rollback but it can be done only 
with vclock_copy to.  vclock_copy is used only for whole batch with all 
entries.
When we introduce unlimited vclock we should introduce vclock_diff also and 
then use them.
> 
> >  		}
> >  		/* rc == 0: the write is buffered in xlog_tx */
> >  	
> >  	}
> > 
> > @@ -991,6 +997,7 @@ wal_write_to_disk(struct cmsg *msg)
> > 
> >  	writer->checkpoint_wal_size += rc;
> >  	last_committed = stailq_last(&wal_msg->commit);
> > 
> > +	vclock_copy(&writer->vclock, &vclock);
> > 
> >  	/*
> >  	
> >  	 * Notify TX if the checkpoint threshold has been exceeded.
> > 
> > @@ -1185,7 +1192,7 @@ wal_write_in_wal_mode_none(struct journal *journal,
> > 
> >  			   struct journal_entry *entry)
> >  
> >  {
> >  
> >  	struct wal_writer *writer = (struct wal_writer *) journal;
> > 
> > -	wal_assign_lsn(writer, entry->rows, entry->rows + entry->n_rows);
> > +	wal_assign_lsn(&writer->vclock, entry->rows, entry->rows +
> > entry->n_rows);> 
> >  	int64_t old_lsn = vclock_get(&replicaset.vclock, instance_id);
> >  	int64_t new_lsn = vclock_get(&writer->vclock, instance_id);
> >  	if (new_lsn > old_lsn) {
> > 
> > diff --git a/test/xlog/errinj.result b/test/xlog/errinj.result
> > index 390404b47..7f15bef35 100644
> > --- a/test/xlog/errinj.result
> > +++ b/test/xlog/errinj.result
> > @@ -43,7 +43,6 @@ require('fio').glob(name .. "/*.xlog")
> > 
> >  ---
> >  - - xlog/00000000000000000000.xlog
> >  
> >    - xlog/00000000000000000001.xlog
> > 
> > -  - xlog/00000000000000000002.xlog
> > 
> >  ...
> >  test_run:cmd('restart server default with cleanup=1')
> >  -- gh-881 iproto request with wal IO error
> > 
> > diff --git a/test/xlog/panic_on_lsn_gap.result
> > b/test/xlog/panic_on_lsn_gap.result index 4dd1291f8..8054baab4 100644
> > --- a/test/xlog/panic_on_lsn_gap.result
> > +++ b/test/xlog/panic_on_lsn_gap.result
> > @@ -105,7 +105,7 @@ test_run:cmd("restart server panic")
> > 
> >  --
> >  box.info.vclock
> >  ---
> > 
> > -- {1: 11}
> > +- {1: 1}
> 
> After this patch the comments contradict the expected result of this
> test. Please fix.


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