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From Dmitry Pavlov <dpavlov....@gmail.com>
Subject Re: Reconsider default WAL mode: we need something between LOG_ONLY and FSYNC
Date Wed, 21 Mar 2018 15:11:06 GMT
Hi, I think option 1 is better. As Val said any mode that allows corruption
does not make much sense.

What Ivan mentioned here as drop, in relation to old mode DEFAULT (FSYNC
now), is still significant perfromance boost.

Sincerely,
Dmitriy Pavlov

ср, 21 мар. 2018 г. в 17:56, Ivan Rakov <ivan.glukos@gmail.com>:

> I've attached benchmark results to the JIRA ticket.
> We observe ~7% drop in "fair" LOG_ONLY_SAFE mode, independent of WAL
> compaction enabled flag. It's pretty significant drop: WAL compaction
> itself gives only ~3% drop.
>
> I see two options here:
> 1) Change LOG_ONLY behavior. That implies that we'll be ready to release
> AI 2.5 with 7% drop.
> 2) Introduce LOG_ONLY_SAFE, make it default, add release note to AI 2.5
> that we added power loss durability in default mode, but user may
> fallback to previous LOG_ONLY in order to retain performance.
>
> Thoughts?
>
> Best Regards,
> Ivan Rakov
>
> On 20.03.2018 16:00, Ivan Rakov wrote:
> > Val,
> >
> >> If a storage is in
> >> corrupted state, does it mean that it needs to be completely removed and
> >> cluster needs to be restarted without data?
> >
> > Yes, there's a chance that in LOG_ONLY all local data will be lost,
> > but only in *power loss**/ OS crash* case.
> > kill -9, JVM crash, death of critical system thread and all other
> > cases that usually take place are variations of *process crash*. All
> > WAL modes (except NONE, of course) ensure corruption-safety in case of
> > process crash.
> >
> >> If so, I'm not sure any mode
> >> that allows corruption makes much sense to me.
> > It depends on performance impact of enforcing power-loss corruption
> > safety. Price of full protection from power loss is high - FSYNC is
> > way slower (2-10 times) than other WAL modes. The question is whether
> > ensuring weaker guarantees (corruption can't happen, but loss of last
> > updates can) will affect performance as badly as strong guarantees.
> > I'll share benchmark results soon.
> >
> > Best Regards,
> > Ivan Rakov
> >
> > On 20.03.2018 5:09, Valentin Kulichenko wrote:
> >> Guys,
> >>
> >> What do we understand under "data corruption" here? If a storage is in
> >> corrupted state, does it mean that it needs to be completely removed and
> >> cluster needs to be restarted without data? If so, I'm not sure any mode
> >> that allows corruption makes much sense to me. How am I supposed to
> >> use a
> >> database, if virtually any failure can end with complete loss of data?
> >>
> >> In any case, this definitely should not be a default behavior. If
> >> user ever
> >> switches to corruption-unsafe mode, there should be a clear warning
> >> about
> >> this.
> >>
> >> -Val
> >>
> >> On Fri, Mar 16, 2018 at 1:06 AM, Ivan Rakov <ivan.glukos@gmail.com>
> >> wrote:
> >>
> >>> Ticket to track changes:
> >>> https://issues.apache.org/jira/browse/IGNITE-7754
> >>>
> >>> Best Regards,
> >>> Ivan Rakov
> >>>
> >>>
> >>> On 16.03.2018 10:58, Dmitriy Setrakyan wrote:
> >>>
> >>>> On Fri, Mar 16, 2018 at 12:55 AM, Ivan Rakov <ivan.glukos@gmail.com>
> >>>> wrote:
> >>>>
> >>>> Vladimir,
> >>>>> Unlike BACKGROUND, LOG_ONLY provides strict write guarantees
> >>>>> unless power
> >>>>> loss has happened.
> >>>>> Seems like we need to measure performance difference to decide
> >>>>> whether do
> >>>>> we need separate WAL mode. If it will be invisible, we'll just fix
> >>>>> these
> >>>>> bugs without introducing new mode; if it will be perceptible, we'll
> >>>>> continue the discussion about introducing LOG_ONLY_SAFE.
> >>>>> Makes sense?
> >>>>>
> >>>>> Yes, this sounds like the right approach.
> >>>>
> >
> >
>
>

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