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From Dong Lin <lindon...@gmail.com>
Subject Re: [DISCUSS] KIP-263: Allow broker to skip sanity check of inactive segments on broker startup
Date Sat, 21 Jul 2018 06:45:25 GMT
Here is some information related to the KIP.

Previously we thought we can ignore the sanity check by default to speedup
broker startup and there is no need for extra configuration. However based
on the code and some previous experiment result it is not clear whether it
will work. When LogManager.cleanupLogs() is called by the background
thread, the thread will go over all segments and read the last entry of
timeindex file of each segment to see whether the segment should be deleted
based on time. This may cause spike in the disk usage and reduce broker
startup time.

More test is needed to validate whether this is an issue. If this is an
issue, we have validated that the issue can be solved by using a few extra
configs. More information will be posted later when we get the result.



On Wed, Jun 27, 2018 at 10:59 AM, Dong Lin <lindong28@gmail.com> wrote:

> Thanks for the reply Jason and Dhruvil.
>
> Yeah we don't need config for the sanity check and thus we don't need a
> KIP. I think we are on the same page of just skipping the sanity check of
> segments before the recovery offset. I will close the KIP and submit a
> patch for this.
>
> On Wed, Jun 27, 2018 at 10:09 AM, Dhruvil Shah <dhruvil@confluent.io>
> wrote:
>
>> +1 to what Jason said. We need a better long-term strategy for dealing
>> with
>> corrupted log and index data, but the sanity checks we have do not
>> guarantee much in this regard.
>>
>> For now, we could do away with these index sanity checks in my opinion. We
>> could handle the missing index case at startup. I think we could have
>> missing index files only when users are upgrading from a version that did
>> not have a particular type of index to a version that does, or if the
>> operator physically deleted these files. Because these are rare scenarios,
>> having to recreate a missing index should typically not affect normal
>> startup time.
>>
>> - Dhruvil
>>
>> On Wed, Jun 27, 2018 at 8:47 AM Jason Gustafson <jason@confluent.io>
>> wrote:
>>
>> > Hey Dong,
>> >
>> >
>> > So the main concern with the above approach is that, if for any reason
>> the
>> > > index files of inactive segment is deleted or corrupted, the broker
>> will
>> > > halt if there is only one log directory. This is different from the
>> > > existing behavior where the broker will rebuild the index for this
>> > inactive
>> > > segment before it can accept any request from consumer. Though we
>> don't
>> > > have provide guarantee for segments already flushed to disk, this
>> still
>> > > seems like a change in behavior for user. Maybe we don't have to worry
>> > > about this if we decide it is very rare, e.g. it happens only when
>> there
>> > is
>> > > disk error or when there is human error.
>> >
>> >
>> > I think we should probably still handle the case when an index file is
>> > missing during startup? But considering how weak the sanity check is, it
>> > seems fine to skip it.  Also, could we just make this change without a
>> KIP?
>> > Adding a config to enable a wimpy sanity check seems unnecessary.
>> >
>> > One scenario that does come up with users is actual segment corruption,
>> > which is only detected by consumers that are validating CRCs. To fix
>> it, we
>> > have to manually delete the segments and force re-replication. It would
>> be
>> > helpful to have a config to enable deep checking on startup for
>> particular
>> > topics or partitions. This could also just be a separate tool though
>> > ("kafka-fsck" or something).
>> >
>> > Thinking longer term, I think we need a more systematic approach to
>> dealing
>> > with corruption, not just in index files, but in the segments as well.
>> It
>> > might be nice, for example, if the consumer had a way to hint the broker
>> > that a particular offset is corrupt. The leader might then demote
>> itself,
>> > for example, and try to repair. Lots to think through though.
>> >
>> > -Jason
>> >
>> >
>> >
>> >
>> > On Wed, Jun 27, 2018 at 12:29 AM, Dong Lin <lindong28@gmail.com> wrote:
>> >
>> > > So the main concern with the above approach is that, if for any reason
>> > the
>> > > index files of inactive segment is deleted or corrupted, the broker
>> will
>> > > halt if there is only one log directory. This is different from the
>> > > existing behavior where the broker will rebuild the index for this
>> > inactive
>> > > segment before it can accept any request from consumer. Though we
>> don't
>> > > have provide guarantee for segments already flushed to disk, this
>> still
>> > > seems like a change in behavior for user. Maybe we don't have to worry
>> > > about this if we decide it is very rare, e.g. it happens only when
>> there
>> > is
>> > > disk error or when there is human error.
>> > >
>> > >
>> > >
>> > > On Wed, Jun 27, 2018 at 12:04 AM, Dong Lin <lindong28@gmail.com>
>> wrote:
>> > >
>> > > > Hey Jason,
>> > > >
>> > > > Thanks for the comment!
>> > > >
>> > > > Your comment reminded me to read through Jay's comments and my reply
>> > > > again. It seems that I probably have not captured idea of Jay's
>> comment
>> > > > that says sanity check is not part of any formal guarantee we
>> provide.
>> > I
>> > > > probably should have thought about this comment more. Let me reply
>> to
>> > > both
>> > > > yours and Jay's comment and see if I can understand you better.
>> > > >
>> > > > Here are some clarifications:
>> > > > - KIP does not intend to optimize recovery. It aims to optimize the
>> the
>> > > > sanity check when there is clean shutdown.
>> > > > - Sanity check only read the last entry of the index rather than the
>> > full
>> > > > index
>> > > > - We have already done data driven investigation though it is not
>> done
>> > > > using hprof or strace. The resulting rolling bounce time is
>> acceptable
>> > > now.
>> > > > If it appears to be an issue e.g. after more data then we may need
>> to
>> > > > revisit this with more data driven investigation
>> > > >
>> > > > I agree with the following comments:
>> > > > - We should optimize the default behavior instead of adding a new
>> > config.
>> > > > - sanity check of the segments before recovery offset is not part
of
>> > any
>> > > > formal guarantee and thus we probably can just skip it.
>> > > >
>> > > > So we are all leaning towards skipping the sanity check of all
>> segments
>> > > > before the recovery offset. This solution would be pretty
>> > straightforward
>> > > > to understand and implement. And I am sure it will give us all the
>> > > benefits
>> > > > that this KIP intends to achieve. Here is only one question to
>> double
>> > > check:
>> > > >
>> > > > If consumer fetches from an inactive segment, broker will just use
>> the
>> > > > index of that inactive segment. If anything goes wrong, e.g. the
>> index
>> > > file
>> > > > is corrupted or the index file does not exist, then the broker will
>> > just
>> > > > consider it as IOException, mark the disk and the partitions on the
>> > disk
>> > > > offline and respond KafkaStorageException to consumer. Does this
>> sound
>> > > OK?
>> > > > One alternative solution is to let broker rebuild index. But this
>> > > > alternative solution is inconsistent with the idea that "sanity
>> check
>> > is
>> > > not
>> > > > part of any formal guarantee" and it may tie up all request handler
>> > > > thread for rebuilding the indexed.
>> > > >
>> > > >
>> > > > If this solution sounds right, I will update the KIP accordingly.
>> > > >
>> > > > Thanks,
>> > > > Dong
>> > > >
>> > > > On Tue, Jun 26, 2018 at 3:23 PM, Jason Gustafson <
>> jason@confluent.io>
>> > > > wrote:
>> > > >
>> > > >> Hey Dong,
>> > > >>
>> > > >> Sorry for being slow to catch up to this.
>> > > >>
>> > > >> I think the benefit of the sanity check seems a little dubious
in
>> the
>> > > >> first
>> > > >> place. We detect garbage at the end of the index file, but that's
>> > about
>> > > >> it.
>> > > >> Is there any reason to think that corruption is more likely to
>> occur
>> > > there
>> > > >> or any other reason to think this check is still beneficial for
>> > flushed
>> > > >> data? I assume we did the check because we presumed it was cheap,
>> but
>> > > >> perhaps the cost is adding up as the number of partitions grows.
>> How
>> > > much
>> > > >> does startup time improve if we skip the sanity check for data
>> earlier
>> > > >> than
>> > > >> the recovery point? Does the lazy loading itself give some
>> additional
>> > > >> benefit beyond skipping the sanity check? As Jay mentions above,
>> the
>> > > >> sanity
>> > > >> checks seem strictly speaking optional. We don't bother checking
>> the
>> > > >> segments themselves for example.
>> > > >>
>> > > >> Thanks,
>> > > >> Jason
>> > > >>
>> > > >>
>> > > >>
>> > > >>
>> > > >> It probably still makes sense for segments beyond the recovery
>> point
>> > > >>
>> > > >> On Wed, Mar 21, 2018 at 9:59 PM, Dong Lin <lindong28@gmail.com>
>> > wrote:
>> > > >>
>> > > >> > Hey Jay,
>> > > >> >
>> > > >> > Yeah our existing sanity check only read the last entry in
the
>> index
>> > > >> files.
>> > > >> > I must have miscommunicated if I previously said it was reading
>> the
>> > > full
>> > > >> > index. Broker appears to be spending a lot of time just to
read
>> the
>> > > last
>> > > >> > entry of index files for every log segment. This is probably
>> because
>> > > OS
>> > > >> > will load a chunk of data that is much larger than the entry
>> itself
>> > > from
>> > > >> > disk to page cache. This KIP tries to make this part of operation
>> > > lazy.
>> > > >> I
>> > > >> > guess you are suggesting that we should just make the lazy
>> loading
>> > the
>> > > >> > default behavior?
>> > > >> >
>> > > >> > Yes we currently require manual intervention if the log file
is
>> > > >> corrupted,
>> > > >> > i.e. if two messages with the same offset are appended to
the
>> disk
>> > > >> > (KAFKA-6488). The sanity check on broker startup is a bit
>> different
>> > > >> since
>> > > >> > it deals with the corruption of index files (e.g. offset
index,
>> time
>> > > >> index
>> > > >> > and snapshot files) instead of the log data. In this case
if
>> index
>> > > files
>> > > >> > are corrupted broker will automatically recover it by rebuilding
>> the
>> > > >> index
>> > > >> > files using data in the log files, without requiring manual
>> > > >> intervention.
>> > > >> > Thus the design question is whether this should be done before
>> > broker
>> > > >> can
>> > > >> > become leader for any partitions -- there is tradeoff between
>> broker
>> > > >> > startup time and risk of delaying user requests if broker
need to
>> > > >> rebuild
>> > > >> > index files when it is already leader. I prefer lazy loading
to
>> > reduce
>> > > >> > broker startup time. Not sure what are the feedback from
the
>> > community
>> > > >> on
>> > > >> > this issue.
>> > > >> >
>> > > >> > Thanks,
>> > > >> > Dong
>> > > >> >
>> > > >> >
>> > > >> > On Wed, Mar 21, 2018 at 7:36 AM, Jay Kreps <jay@confluent.io>
>> > wrote:
>> > > >> >
>> > > >> > > Hey Dong,
>> > > >> > >
>> > > >> > > Makes total sense. What I'm saying is I don't think
that the
>> > sanity
>> > > >> check
>> > > >> > > is part of any formal guarantee we provide. It is true
that
>> > > >> corruption of
>> > > >> > > data flushed to disk will be a potential problem, but
I don't
>> > think
>> > > >> the
>> > > >> > > sanity check solves that it just has a couple heuristics
to
>> help
>> > > >> detect
>> > > >> > > certain possible instances of it, right? In general
I think our
>> > > >> > assumption
>> > > >> > > has been that flushed data doesn't disappear or get
corrupted
>> and
>> > if
>> > > >> it
>> > > >> > > does you need to manually intervene. I don't think people
want
>> to
>> > > >> > configure
>> > > >> > > things at this level so what I was suggesting was understanding
>> > why
>> > > >> the
>> > > >> > > sanity check is slow and trying to avoid that rather
than
>> making
>> > it
>> > > >> > > configurable. I think you mentioned it was reading the
full
>> index
>> > > into
>> > > >> > > memory. Based on the performance you describe this could
be
>> true,
>> > > but
>> > > >> it
>> > > >> > > definitely should not be reading anything but the last
entry in
>> > the
>> > > >> index
>> > > >> > > so that would be a bug. That read also happens in sanityCheck()
>> > only
>> > > >> in
>> > > >> > the
>> > > >> > > time-based index right? In the offset index we do the
same read
>> > but
>> > > it
>> > > >> > > happens in initialization. If that read is the slow
thing it
>> might
>> > > >> make
>> > > >> > > sense to try to remove it or make it lazy in both cases.
If it
>> is
>> > > some
>> > > >> > > other part of the code then (e.g. the size check) then
that
>> may be
>> > > >> able
>> > > >> > to
>> > > >> > > be avoided entirely (I think by the time we sanity check
we
>> > already
>> > > >> know
>> > > >> > > the file size from the mapping...). That was what I
meant by
>> doing
>> > > >> some
>> > > >> > > data driven analysis. Maybe a quick run with hprof would
help
>> > > >> determine
>> > > >> > the
>> > > >> > > root cause of why sanityCheck is slow?
>> > > >> > >
>> > > >> > > -Jay
>> > > >> > >
>> > > >> > > On Tue, Mar 20, 2018 at 12:13 AM Dong Lin <lindong28@gmail.com
>> >
>> > > >> wrote:
>> > > >> > >
>> > > >> > > > Hey Jay,
>> > > >> > > >
>> > > >> > > > Thanks for your comments!
>> > > >> > > >
>> > > >> > > > Yeah recovery is different from the sanity check.
They are
>> > > >> correlated
>> > > >> > in
>> > > >> > > > the sense that there may still be corrupted index
files even
>> > after
>> > > >> > clean
>> > > >> > > > broker shutdown. And in that case if we delay the
sanity
>> check
>> > > then
>> > > >> we
>> > > >> > > may
>> > > >> > > > delay the log recovery. The main goal of this KIP
is to
>> optimize
>> > > the
>> > > >> > > sanity
>> > > >> > > > check related work so that it does not delay the
broker
>> startup
>> > > >> much.
>> > > >> > > >
>> > > >> > > > The KIP mentioned that the sanity check is done
using log
>> > recovery
>> > > >> > > > background thread. The name "recovery" is mentioned
mainly
>> > because
>> > > >> the
>> > > >> > > > background thread number is determined using the
existing
>> > > >> > > > config num.recovery.threads.per.data.dir. I have
updated
>> the KIP
>> > > to
>> > > >> > make
>> > > >> > > > this less confusing.
>> > > >> > > >
>> > > >> > > > It makes a ton of sense to optimize the broker
startup time
>> in a
>> > > >> data
>> > > >> > > > driven fashion. The currently optimize is done
kind of in
>> this
>> > > >> fashion.
>> > > >> > > The
>> > > >> > > > broker log shows that LogManager.loadLogs() takes
a long
>> time in
>> > > >> large
>> > > >> > > > clusters. Then I started broker with cold cache
and
>> repeatedly
>> > get
>> > > >> > thread
>> > > >> > > > dump to see what are broker threads are doing during
>> > > >> > > LogManager.loadLogs().
>> > > >> > > > Most of the threads are working on sanityCheck()
and this
>> > > motivates
>> > > >> the
>> > > >> > > > change in this KIP. Previously broker shutdown
time was
>> > > investigated
>> > > >> > in a
>> > > >> > > > similar data driven fashion and optimized with
KAFKA-6172 and
>> > > >> > KAFKA-6175.
>> > > >> > > > It seems that the current KIP can reduces the rolling
bounce
>> > time
>> > > >> of a
>> > > >> > > > large cluster by 50% -- there may be room for further
>> > improvement
>> > > >> but
>> > > >> > > maybe
>> > > >> > > > those do not require as big a change (with the
caveat
>> described
>> > in
>> > > >> the
>> > > >> > > KIP)
>> > > >> > > > as suggested in this KIP.
>> > > >> > > >
>> > > >> > > > It is not clear whether it is safe to just read
the latest
>> > segment
>> > > >> > > without
>> > > >> > > > sanity checking all previous inactive segment of
a given
>> > partition
>> > > >> if
>> > > >> > > > transaction is used. Otherwise we probably want
to always
>> skip
>> > the
>> > > >> > sanity
>> > > >> > > > check of inactive segments without introducing
a new config.
>> > Maybe
>> > > >> the
>> > > >> > > > developers familiar with the transaction can comment
on that?
>> > > >> > > >
>> > > >> > > > Thanks,
>> > > >> > > > Dong
>> > > >> > > >
>> > > >> > > >
>> > > >> > > > On Mon, Mar 19, 2018 at 7:21 PM, Jay Kreps <jay@confluent.io
>> >
>> > > >> wrote:
>> > > >> > > >
>> > > >> > > > > Optimizing startup seems really valuable but
I'm a little
>> > > >> confused by
>> > > >> > > > this.
>> > > >> > > > >
>> > > >> > > > > There are two different things:
>> > > >> > > > > 1. Recovery
>> > > >> > > > > 2. Sanity check
>> > > >> > > > >
>> > > >> > > > > The terminology we're using is a bit mixed
here.
>> > > >> > > > >
>> > > >> > > > > Recovery means checksumming the log segments
and rebuilding
>> > the
>> > > >> index
>> > > >> > > on
>> > > >> > > > a
>> > > >> > > > > hard crash. This only happens on unflushed
segments, which
>> is
>> > > >> > generally
>> > > >> > > > > just the last segment. Recovery is essential
for the
>> > correctness
>> > > >> > > > guarantees
>> > > >> > > > > of the log and you shouldn't disable it. It
only happens on
>> > hard
>> > > >> > crash
>> > > >> > > > and
>> > > >> > > > > is not a factor in graceful restart. We can
likely
>> optimize it
>> > > but
>> > > >> > that
>> > > >> > > > > would make most sense to do in a data driven
fashion off
>> some
>> > > >> > > profiling.
>> > > >> > > > >
>> > > >> > > > > However there is also a ton of disk activity
that happens
>> > during
>> > > >> > > > > initialization (lots of checks on the file
size, absolute
>> > path,
>> > > >> > etc). I
>> > > >> > > > > think these have crept in over time with people
not really
>> > > >> realizing
>> > > >> > > this
>> > > >> > > > > code is perf sensitive and java hiding a lot
of what is and
>> > > isn't
>> > > >> a
>> > > >> > > file
>> > > >> > > > > operation. One part of this is the sanityCheck()
call for
>> the
>> > > two
>> > > >> > > > indexes.
>> > > >> > > > > I don't think this call reads the full index,
just the last
>> > > entry
>> > > >> in
>> > > >> > > the
>> > > >> > > > > index, right?. There should be no need to
read the full
>> index
>> > > >> except
>> > > >> > > > during
>> > > >> > > > > recovery (and then only for the segments being
recovered).
>> I
>> > > >> think it
>> > > >> > > > would
>> > > >> > > > > make a ton of sense to optimize this but I
don't think that
>> > > >> > > optimization
>> > > >> > > > > needs to be configurable as this is just a
helpful sanity
>> > check
>> > > to
>> > > >> > > detect
>> > > >> > > > > common non-sensical things in the index files,
but it isn't
>> > part
>> > > >> of
>> > > >> > the
>> > > >> > > > > core guarantees, in general you aren't supposed
to lose
>> > > committed
>> > > >> > data
>> > > >> > > > from
>> > > >> > > > > disk, and if you do we may be able to fail
faster but we
>> > > >> > fundamentally
>> > > >> > > > > can't really help you. Again I think this
would make the
>> most
>> > > >> sense
>> > > >> > to
>> > > >> > > do
>> > > >> > > > > in a data driven way, if you look at that
code I think it
>> is
>> > > doing
>> > > >> > > crazy
>> > > >> > > > > amounts of file operations (e.g. getAbsolutePath,
file
>> sizes,
>> > > >> etc). I
>> > > >> > > > think
>> > > >> > > > > it'd make most sense to profile startup with
a cold cash
>> on a
>> > > >> large
>> > > >> > log
>> > > >> > > > > directory and do the same with an strace to
see how many
>> > > redundant
>> > > >> > > system
>> > > >> > > > > calls we do per segment and what is costing
us and then cut
>> > some
>> > > >> of
>> > > >> > > this
>> > > >> > > > > out. I suspect we could speed up our startup
time quite a
>> lot
>> > if
>> > > >> we
>> > > >> > did
>> > > >> > > > > that.
>> > > >> > > > >
>> > > >> > > > > For example we have a bunch of calls like
this:
>> > > >> > > > >
>> > > >> > > > >     require(len % entrySize == 0,
>> > > >> > > > >
>> > > >> > > > >             "Index file " + file.getAbsolutePath
+ " is
>> > corrupt,
>> > > >> > found
>> > > >> > > "
>> > > >> > > > +
>> > > >> > > > > len +
>> > > >> > > > >
>> > > >> > > > >             " bytes which is not positive
or not a
>> multiple of
>> > > >> 8.")
>> > > >> > > > > I'm pretty such file.getAbsolutePath is a
system call and I
>> > > assume
>> > > >> > that
>> > > >> > > > > happens whether or not you fail the in-memory
check?
>> > > >> > > > >
>> > > >> > > > > -Jay
>> > > >> > > > >
>> > > >> > > > >
>> > > >> > > > > On Sun, Feb 25, 2018 at 10:27 PM, Dong Lin
<
>> > lindong28@gmail.com
>> > > >
>> > > >> > > wrote:
>> > > >> > > > >
>> > > >> > > > > > Hi all,
>> > > >> > > > > >
>> > > >> > > > > > I have created KIP-263: Allow broker
to skip sanity
>> check of
>> > > >> > inactive
>> > > >> > > > > > segments on broker startup. See
>> > > >> > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-
>> > > >> > > > > > 263%3A+Allow+broker+to+skip+sanity+check+of+inactive+
>> > > >> > > > > > segments+on+broker+startup
>> > > >> > > > > > .
>> > > >> > > > > >
>> > > >> > > > > > This KIP provides a way to significantly
reduce time to
>> > > rolling
>> > > >> > > bounce
>> > > >> > > > a
>> > > >> > > > > > Kafka cluster.
>> > > >> > > > > >
>> > > >> > > > > > Comments are welcome!
>> > > >> > > > > >
>> > > >> > > > > > Thanks,
>> > > >> > > > > > Dong
>> > > >> > > > > >
>> > > >> > > > >
>> > > >> > > >
>> > > >> > >
>> > > >> >
>> > > >>
>> > > >
>> > > >
>> > >
>> >
>>
>
>

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