Re: [PATCH v2 1/6] xlog: store prev vclock in xlog header

[Konstantin Osipov <kostja@tarantool.org>]

* Vladimir Davydov <vdavydov.dev@gmail.com> [18/07/05 14:01]:
> On Thu, Jul 05, 2018 at 09:52:41AM +0300, Konstantin Osipov wrote:
> > * Vladimir Davydov <vdavydov.dev@gmail.com> [18/06/29 19:49]:
> > > This patch adds a new key to xlog header, PrevVclock, which contains the
> > > vclock of the previous xlog file in the xlog directory. It is set by
> > > xdir_create_xlog() to the last vclock in xdir::index. The new key is
> > > only present in XLOG files (it doesn't make sense for SNAP or VYLOG
> > > anyway). It will be used to make the check for xlog gaps more thorough.
> > 
> > What about the following diff:
> > 
> > ---
> > diff --git a/src/box/xlog.c b/src/box/xlog.c
> > index 59459b25d..a60f2ffa8 100644
> > --- a/src/box/xlog.c
> > +++ b/src/box/xlog.c
> > @@ -126,12 +126,10 @@ xlog_meta_format(const struct xlog_meta *meta, char *buf, int size)
> >  		VCLOCK_KEY ": %s\n",
> >  		meta->filetype, v13, PACKAGE_VERSION, instance_uuid, vstr);
> >  	free(vstr);
> > -	if (meta->has_prev_vclock) {
> > -		vstr = vclock_to_string(&meta->prev_vclock);
> > -		SNPRINT(total, snprintf, buf, size,
> > -			PREV_VCLOCK_KEY ": %s\n", vstr);
> > -		free(vstr);
> > -	}
> > +	vstr = vclock_to_string(&meta->prev_vclock);
> > +	SNPRINT(total, snprintf, buf, size,
> > +		PREV_VCLOCK_KEY ": %s\n", vstr);
> > +	free(vstr);
> >  	SNPRINT(total, snprintf, buf, size, "\n");
> >  	assert(total > 0);
> >  	return total;
> > @@ -263,7 +261,6 @@ xlog_meta_parse(struct xlog_meta *meta, const char **data,
> >  			 */
> >  			if (parse_vclock(val, val_end, &meta->prev_vclock) != 0)
> >  				return -1;
> > -			meta->has_prev_vclock = true;
> 
> We need to know if PrevVclock is available so that we can skip the check
> when recovering from an xlog generated by a previous version.

In theory we could use Version key for this.

> 
> >  		} else if (memcmp(key, VERSION_KEY, key_end - key) == 0) {
> >  			/* Ignore Version: for now */
> >  		} else {
> > @@ -926,12 +923,10 @@ xdir_create_xlog(struct xdir *dir, struct xlog *xlog,
> >  	 * For WAL dir: store vclock of the previous xlog file
> >  	 * to check for gaps on recovery.
> >  	 */
> > -	if (dir->type == XLOG && !vclockset_empty(&dir->index)) {
> > +	if (!vclockset_empty(&dir->index)) {
> >  		vclock_copy(&meta.prev_vclock, vclockset_last(&dir->index));
> > -		meta.has_prev_vclock = true;
> >  	} else {
> >  		vclock_create(&meta.prev_vclock);
> > -		meta.has_prev_vclock = false;
> >  	}
> 
> Storing vclock in snap and vylog files doesn't make any sense IMHO.
> I'd prefer to check dir->type explicitly.
> 
> If you dislike the extra variable, we can probably introduce a helper to
> check if vclock is set, something like
> 
> 	static inline bool
> 	vclock_is_set(const struct vclock *vclock)
> 	{
> 		return vclock->signature >= 0;
> 	}

This is better IMHO.


-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov