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From Vladimir Ozerov <voze...@gridgain.com>
Subject Re: Page eviction for in-memory mode with enabled MVCC
Date Thu, 13 Dec 2018 19:14:33 GMT
No, I mean that we should think about what kind of guarantees it possible.
My proposal was to prevent evictions of locked entries. This way we can say
users: "if you want true REPEATABLE_READ when evictions are enabled, then
make sure to lock entries on every access". This effectively means that all
SELECT's should be replaced with "SELECT FOR UPDATE".

On Thu, Dec 13, 2018 at 10:09 PM Roman Kondakov <kondakov87@mail.ru.invalid>
wrote:

> Vladimir,
>
> correct me please if i misunderstood your thought. So, if eviction is
> not about a consistency at all, we may evict keys in any way because
> broken repeatable read semantics is not the biggest problem here. But we
> should add some notes about it to user documentation. Right?
>
>
> --
> Kind Regards
> Roman Kondakov
>
> On 13.12.2018 17:45, Vladimir Ozerov wrote:
> > Roman,
> >
> > I would start with the fact that eviction can never be consistent unless
> it
> > utilizes atomic broadcast protocol, which is not the case for Ignite. In
> > Ignite entries on node are evicted independently.
> >
> > So you may easily get into situation like this:
> > 1) Start a cache with 1 backup and FULL_SYNC mode
> > 2) Put a key to primary node
> > 3) Stop primary
> > 4) Try reading from new primary and get null because key was evicted
> > concurrently
> >
> > Or:
> > 1) Start a transaction in PESSIMISTIC/READ_COMMITTED mode
> > 2) Read a key, get value
> > 3) Read the same key again, get null
> >
> > So in reality the choice is not between consistent and inconsistent
> > behavior, but rather about degree of inconsistency. Any solution is
> > possible as long as we can explain it to the user. E.g. "do not evict a
> key
> > if it is either write-locked".
> >
> >
> > On Thu, Dec 13, 2018 at 5:19 PM Vladimir Ozerov <vozerov@gridgain.com>
> > wrote:
> >
> >> Andrey,
> >>
> >> We will not be able to cache the whole data set locally, as it
> potentially
> >> lead to OOME. We will have this only as an option and only for non-SQL
> >> updates. Thus, similar semantics is not possible.
> >>
> >> On Thu, Dec 13, 2018 at 4:56 PM Andrey Mashenkov <
> >> andrey.mashenkov@gmail.com> wrote:
> >>
> >>> Roman,
> >>>
> >>> We have a ticket to improve repeatable_read mode [1] via caching
> entries
> >>> locally.
> >>> This should make mvcc transaction repeatable_read semantic similar to
> >>> non-mvcc Txs
> >>> and allow us to implement eviction in correct way.
> >>>
> >>> Another way is to introduce mvcc shared (read) entry locks and evict
> only
> >>> entries if no one hold any lock on it,
> >>> but this looks tricky and error prone as your first one as it may lead
> to
> >>> eviction policy unexpected behavior,
> >>> e.g some versions can be visible while others - no (evicted).
> >>>
> >>> [1] https://issues.apache.org/jira/browse/IGNITE-7371
> >>>
> >>> On Thu, Dec 13, 2018 at 4:34 PM Ilya Kasnacheev <
> >>> ilya.kasnacheev@gmail.com>
> >>> wrote:
> >>>
> >>>> Hello!
> >>>>
> >>>> Is it possible to 'touch' entries read by MVCC transactions to ensure
> >>> that
> >>>> they are considered recent and therefore are almost never targeted by
> >>>> eviction?
> >>>>
> >>>> This is 1) with benefits.
> >>>>
> >>>> Regards,
> >>>> --
> >>>> Ilya Kasnacheev
> >>>>
> >>>>
> >>>> чт, 13 дек. 2018 г. в 16:22, Roman Kondakov
> <kondakov87@mail.ru.invalid
> >>>> :
> >>>>
> >>>>> Hi igniters,
> >>>>>
> >>>>> I need your advice with the following issue. When in-memory cache
> >>>>> reaches it's memory limit, some data may be purged to avoid OOM
> error.
> >>>>> This process is described in [1]. For MVCC caches this eviction
may
> >>>>> break repeatable read semantics. For example, if transaction reads
> key
> >>>>> before eviction, this key is visible for it. But if key is evicted
> >>> some
> >>>>> time later, this key become invisible to anyone, including our
> >>>>> transaction, which means broken repeatable read semantics.
> >>>>>
> >>>>> Now we see the following solutions of this problem:
> >>>>>
> >>>>> 1. Ignore broken repeatable read semantics. If cache is set to allow
> >>>>> data eviction, it may lose it's data. This means that there is no
> >>>>> valuable information stored in cache and occasional repeatable read
> >>>>> violations can be tolerated.
> >>>>>
> >>>>> 2. Prohibit eviction of MVCC caches at all. For example, stop writing
> >>> to
> >>>>> caches and throw an appropriate exception in the case when there
is
> no
> >>>>> free space in page memory. Before this exception Ignite should do
> it's
> >>>>> best to avoid this situation, for example, evict all non-mvcc caches
> >>> and
> >>>>> run full vacuum to free as much space as possible.
> >>>>>
> >>>>> First approach is bad because it leads to cache consistency
> violation.
> >>>>> Second approach is bad because it's behavior may be unexpected to
> user
> >>>>> if he has set an eviction policy for cache, but instead of eviction
> >>>>> Ignite trying to avoid it as much as possible.
> >>>>>
> >>>>> IMO first approach looks better - it is much simpler to implement
and
> >>>>> met user expectations in all points except possible repeatable read
> >>>>> violations.
> >>>>>
> >>>>> What do you think?
> >>>>>
> >>>>>
> >>>>> [1] https://apacheignite.readme.io/docs/evictions
> >>>>>
> >>>>> --
> >>>>> Kind Regards
> >>>>> Roman Kondakov
> >>>>>
> >>>>>
> >>>
> >>> --
> >>> Best regards,
> >>> Andrey V. Mashenkov
> >>>
>

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