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From Alexei Scherbakov <alexey.scherbak...@gmail.com>
Subject Re: Asynchronous registration of binary metadata
Date Wed, 14 Aug 2019 16:37:08 GMT
Denis Mekhanikov,

1. Yes, only on OS failures. In such case data will be received from alive
nodes later.
2. Yes, for walmode=FSYNC writes to metastore will be slow. But such mode
should not be used if you have more than two nodes in grid because it has
huge impact on performance.

ср, 14 авг. 2019 г. в 14:29, Denis Mekhanikov <dmekhanikov@gmail.com>:

> Folks,
>
> Thanks for showing interest in this issue!
>
> Alexey,
>
> > I think removing fsync could help to mitigate performance issues with
> current implementation
>
> Is my understanding correct, that if we remove fsync, then discovery won’t
> be blocked, and data will be flushed to disk in background, and loss of
> information will be possible only on OS failure? It sounds like an
> acceptable workaround to me.
>
> Will moving metadata to metastore actually resolve this issue? Please
> correct me if I’m wrong, but we will still need to write the information to
> WAL before releasing the discovery thread. If WAL mode is FSYNC, then the
> issue will still be there. Or is it planned to abandon the discovery-based
> protocol at all?
>
> Evgeniy, Ivan,
>
> In my particular case the data wasn’t too big. It was a slow virtualised
> disk with encryption, that made operations slow. Given that there are 200
> nodes in a cluster, where every node writes slowly, and this process is
> sequential, one piece of metadata is registered extremely slowly.
>
> Ivan, answering to your other questions:
>
> > 2. Do we need a persistent metadata for in-memory caches? Or is it so
> accidentally?
>
> It should be checked, if it’s safe to stop writing marshaller mappings to
> disk without loosing any guarantees.
> But anyway, I would like to have a property, that would control this. If
> metadata registration is slow, then initial cluster warmup may take a
> while. So, if we preserve metadata on disk, then we will need to warm it up
> only once, and further restarts won’t be affected.
>
> > Do we really need a fast fix here?
>
> I would like a fix, that could be implemented now, since the activity with
> moving metadata to metastore doesn’t sound like a quick one. Having a
> temporary solution would be nice.
>
> Denis
>
> > On 14 Aug 2019, at 11:53, Павлухин Иван <vololo100@gmail.com>
wrote:
> >
> > Denis,
> >
> > Several clarifying questions:
> > 1. Do you have an idea why metadata registration takes so long? So
> > poor disks? So many data to write? A contention with disk writes by
> > other subsystems?
> > 2. Do we need a persistent metadata for in-memory caches? Or is it so
> > accidentally?
> >
> > Generally, I think that it is possible to move metadata saving
> > operations out of discovery thread without loosing required
> > consistency/integrity.
> >
> > As Alex mentioned using metastore looks like a better solution. Do we
> > really need a fast fix here? (Are we talking about fast fix?)
> >
> > ср, 14 авг. 2019 г. в 11:45, Zhenya Stanilovsky
> <arzamas123@mail.ru.invalid>:
> >>
> >> Alexey, but in this case customer need to be informed, that whole (for
> example 1 node) cluster crash (power off) could lead to partial data
> unavailability.
> >> And may be further index corruption.
> >> 1. Why your meta takes a substantial size? may be context leaking ?
> >> 2. Could meta be compressed ?
> >>
> >>
> >>> Среда, 14 августа 2019, 11:22 +03:00 от Alexei Scherbakov
<
> alexey.scherbakoff@gmail.com>:
> >>>
> >>> Denis Mekhanikov,
> >>>
> >>> Currently metadata are fsync'ed on write. This might be the case of
> >>> slow-downs in case of metadata burst writes.
> >>> I think removing fsync could help to mitigate performance issues with
> >>> current implementation until proper solution will be implemented:
> moving
> >>> metadata to metastore.
> >>>
> >>>
> >>> вт, 13 авг. 2019 г. в 17:09, Denis Mekhanikov < dmekhanikov@gmail.com
> >:
> >>>
> >>>> I would also like to mention, that marshaller mappings are written to
> disk
> >>>> even if persistence is disabled.
> >>>> So, this issue affects purely in-memory clusters as well.
> >>>>
> >>>> Denis
> >>>>
> >>>>> On 13 Aug 2019, at 17:06, Denis Mekhanikov < dmekhanikov@gmail.com
>
> >>>> wrote:
> >>>>>
> >>>>> Hi!
> >>>>>
> >>>>> When persistence is enabled, binary metadata is written to disk
upon
> >>>> registration. Currently it happens in the discovery thread, which
> makes
> >>>> processing of related messages very slow.
> >>>>> There are cases, when a lot of nodes and slow disks can make every
> >>>> binary type be registered for several minutes. Plus it blocks
> processing of
> >>>> other messages.
> >>>>>
> >>>>> I propose starting a separate thread that will be responsible for
> >>>> writing binary metadata to disk. So, binary type registration will be
> >>>> considered finished before information about it will is written to
> disks on
> >>>> all nodes.
> >>>>>
> >>>>> The main concern here is data consistency in cases when a node
> >>>> acknowledges type registration and then fails before writing the
> metadata
> >>>> to disk.
> >>>>> I see two parts of this issue:
> >>>>> Nodes will have different metadata after restarting.
> >>>>> If we write some data into a persisted cache and shut down nodes
> faster
> >>>> than a new binary type is written to disk, then after a restart we
> won’t
> >>>> have a binary type to work with.
> >>>>>
> >>>>> The first case is similar to a situation, when one node fails, and
> after
> >>>> that a new type is registered in the cluster. This issue is resolved
> by the
> >>>> discovery data exchange. All nodes receive information about all
> binary
> >>>> types in the initial discovery messages sent by other nodes. So, once
> you
> >>>> restart a node, it will receive information, that it failed to finish
> >>>> writing to disk, from other nodes.
> >>>>> If all nodes shut down before finishing writing the metadata to
disk,
> >>>> then after a restart the type will be considered unregistered, so
> another
> >>>> registration will be required.
> >>>>>
> >>>>> The second case is a bit more complicated. But it can be resolved
by
> >>>> making the discovery threads on every node create a future, that will
> be
> >>>> completed when writing to disk is finished. So, every node will have
> such
> >>>> future, that will reflect the current state of persisting the
> metadata to
> >>>> disk.
> >>>>> After that, if some operation needs this binary type, it will need
to
> >>>> wait on that future until flushing to disk is finished.
> >>>>> This way discovery threads won’t be blocked, but other threads,
that
> >>>> actually need this type, will be.
> >>>>>
> >>>>> Please let me know what you think about that.
> >>>>>
> >>>>> Denis
> >>>>
> >>>>
> >>>
> >>> --
> >>>
> >>> Best regards,
> >>> Alexei Scherbakov
> >>
> >>
> >> --
> >> Zhenya Stanilovsky
> >
> >
> >
> > --
> > Best regards,
> > Ivan Pavlukhin
>
>

-- 

Best regards,
Alexei Scherbakov

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