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From Alexey Goncharuk <alexey.goncha...@gmail.com>
Subject Re: How to free up space on disc after removing entries from IgniteCache with enabled PDS?
Date Thu, 03 Oct 2019 08:52:50 GMT
Anton, do you think we should file a single ticket for this or should we go
with an IEP? As of now, the change does not look big enough for an IEP for
me.

чт, 3 окт. 2019 г. в 11:18, Anton Vinogradov <av@apache.org>:

> Alexey,
>
> Sounds good to me.
>
> On Thu, Oct 3, 2019 at 10:51 AM Alexey Goncharuk <
> alexey.goncharuk@gmail.com>
> wrote:
>
> > Anton,
> >
> > Switching a partition to and from the SHRINKING state will require
> > intricate synchronizations in order to properly determine the start
> > position for historical rebalance without PME.
> >
> > I would still go with an offline-node approach, but instead of cleaning
> the
> > persistence, we can do effective defragmentation when the node is offline
> > because we are sure that there is no concurrent load. After the
> > defragmentation completes, we bring the node back to the cluster and
> > historical rebalance will kick in automatically. It will still require
> > manual node restarts, but since the data is not removed, there are no
> > additional risks. Also, this will be an excellent solution for those who
> > can afford downtime and execute the defragment command on all nodes in
> the
> > cluster simultaneously - this will be the fastest way possible.
> >
> > --AG
> >
> > пн, 30 сент. 2019 г. в 09:29, Anton Vinogradov <av@apache.org>:
> >
> > > Alexei,
> > > >> stopping fragmented node and removing partition data, then starting
> it
> > > again
> > >
> > > That's exactly what we're doing to solve the fragmentation issue.
> > > The problem here is that we have to perform N/B restart-rebalance
> > > operations (N - cluster size, B - backups count) and it takes a lot of
> > time
> > > with risks to lose the data.
> > >
> > > On Fri, Sep 27, 2019 at 5:49 PM Alexei Scherbakov <
> > > alexey.scherbakoff@gmail.com> wrote:
> > >
> > > > Probably this should be allowed to do using public API, actually this
> > is
> > > > same as manual rebalancing.
> > > >
> > > > пт, 27 сент. 2019 г. в 17:40, Alexei Scherbakov <
> > > > alexey.scherbakoff@gmail.com>:
> > > >
> > > > > The poor man's solution for the problem would be stopping
> fragmented
> > > node
> > > > > and removing partition data, then starting it again allowing full
> > state
> > > > > transfer already without deletes.
> > > > > Rinse and repeat for all owners.
> > > > >
> > > > > Anton Vinogradov, would this work for you as workaround ?
> > > > >
> > > > > чт, 19 сент. 2019 г. в 13:03, Anton Vinogradov <av@apache.org>:
> > > > >
> > > > >> Alexey,
> > > > >>
> > > > >> Let's combine your and Ivan's proposals.
> > > > >>
> > > > >> >> vacuum command, which acquires exclusive table lock,
so no
> > > concurrent
> > > > >> activities on the table are possible.
> > > > >> and
> > > > >> >> Could the problem be solved by stopping a node which
needs to
> be
> > > > >> defragmented, clearing persistence files and restarting the node?
> > > > >> >> After rebalancing the node will receive all data back
without
> > > > >> fragmentation.
> > > > >>
> > > > >> How about to have special partition state SHRINKING?
> > > > >> This state should mean that partition unavailable for reads and
> > > updates
> > > > >> but
> > > > >> should keep it's update-counters and should not be marked as
lost,
> > > > renting
> > > > >> or evicted.
> > > > >> At this state we able to iterate over the partition and apply
it's
> > > > entries
> > > > >> to another file in a compact way.
> > > > >> Indices should be updated during the copy-on-shrink procedure
or
> at
> > > the
> > > > >> shrink completion.
> > > > >> Once shrank file is ready we should replace the original partition
> > > file
> > > > >> with it and mark it as MOVING which will start the historical
> > > rebalance.
> > > > >> Shrinking should be performed during the low activity periods,
but
> > > even
> > > > in
> > > > >> case we found that activity was high and historical rebalance
is
> not
> > > > >> suitable we may just remove the file and use regular rebalance
to
> > > > restore
> > > > >> the partition (this will also lead to shrink).
> > > > >>
> > > > >> BTW, seems, we able to implement partition shrink in a cheap
way.
> > > > >> We may just use rebalancing code to apply fat partition's entries
> to
> > > the
> > > > >> new file.
> > > > >> So, 3 stages here: local rebalance, indices update and global
> > > historical
> > > > >> rebalance.
> > > > >>
> > > > >> On Thu, Sep 19, 2019 at 11:43 AM Alexey Goncharuk <
> > > > >> alexey.goncharuk@gmail.com> wrote:
> > > > >>
> > > > >> > Anton,
> > > > >> >
> > > > >> >
> > > > >> > > >>  The solution which Anton suggested does not
look easy
> > because
> > > it
> > > > >> will
> > > > >> > > most likely significantly hurt performance
> > > > >> > > Mostly agree here, but what drop do we expect? What
price do
> we
> > > > ready
> > > > >> to
> > > > >> > > pay?
> > > > >> > > Not sure, but seems some vendors ready to pay, for
example, 5%
> > > drop
> > > > >> for
> > > > >> > > this.
> > > > >> >
> > > > >> > 5% may be a big drop for some use-cases, so I think we should
> look
> > > at
> > > > >> how
> > > > >> > to improve performance, not how to make it worse.
> > > > >> >
> > > > >> >
> > > > >> > >
> > > > >> > > >> it is hard to maintain a data structure to
choose "page
> from
> > > > >> free-list
> > > > >> > > with enough space closest to the beginning of the file".
> > > > >> > > We can just split each free-list bucket to the couple
and use
> > > first
> > > > >> for
> > > > >> > > pages in the first half of the file and the second
for the
> last.
> > > > >> > > Only two buckets required here since, during the file
shrink,
> > > first
> > > > >> > > bucket's window will be shrank too.
> > > > >> > > Seems, this give us the same price on put, just use
the first
> > > bucket
> > > > >> in
> > > > >> > > case it's not empty.
> > > > >> > > Remove price (with merge) will be increased, of course.
> > > > >> > >
> > > > >> > > The compromise solution is to have priority put (to
the first
> > path
> > > > of
> > > > >> the
> > > > >> > > file), with keeping removal as is, and schedulable
per-page
> > > > migration
> > > > >> for
> > > > >> > > the rest of the data during the low activity period.
> > > > >> > >
> > > > >> > Free lists are large and slow by themselves, it is expensive
to
> > > > >> checkpoint
> > > > >> > and read them on start, so as a long-term solution I would
look
> > into
> > > > >> > removing them. Moreover, not sure if adding yet another
> background
> > > > >> process
> > > > >> > will improve the codebase reliability and simplicity.
> > > > >> >
> > > > >> > If we want to go the hard path, I would look at free page
> tracking
> > > > >> bitmap -
> > > > >> > a special bitmask page, where each page in an adjacent block
is
> > > marked
> > > > >> as 0
> > > > >> > if it has free space more than a certain configurable threshold
> > > (say,
> > > > >> 80%)
> > > > >> > - free, and 1 if less (full). Some vendors have successfully
> > > > implemented
> > > > >> > this approach, which looks much more promising, but harder
to
> > > > implement.
> > > > >> >
> > > > >> > --AG
> > > > >> >
> > > > >>
> > > > >
> > > > >
> > > > > --
> > > > >
> > > > > Best regards,
> > > > > Alexei Scherbakov
> > > > >
> > > >
> > > >
> > > > --
> > > >
> > > > Best regards,
> > > > Alexei Scherbakov
> > > >
> > >
> >
>

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