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From James Cheng <wushuja...@gmail.com>
Subject Re: [DISCUSS] KIP-346 - Limit blast radius of log compaction failure
Date Wed, 01 Aug 2018 18:35:47 GMT
I’m a little confused about something. Is this KIP focused on log cleaner exceptions in general,
or focused on log cleaner exceptions due to disk failures?

Will max.uncleanable.partitions apply to all exceptions (including log cleaner logic errors)
or will it apply to only disk I/o exceptions?

I can understand taking the disk offline if there have been “N” I/O exceptions. Disk errors
are user fixable (by replacing the affected disk). It turns an invisible (soft?) failure into
a visible hard failure. And the I/O exceptions are possibly already causing problems, so it
makes sense to limit their impact.

But I’m not sure if it makes sense to take a disk offline after”N” logic errors in the
log cleaner. If a log cleaner logic error happens, it’s rarely user fixable. And it will
likely several partitions at once, so you’re likely to bump up against the max.uncleanable.partitions
limit more quickly. If a disk was taken due to logic errors, I’m not sure what the user
would do.

-James

Sent from my iPhone

> On Aug 1, 2018, at 9:11 AM, Stanislav Kozlovski <stanislav@confluent.io> wrote:
> 
> Yes, good catch. Thank you, James!
> 
> Best,
> Stanislav
> 
>> On Wed, Aug 1, 2018 at 5:05 PM James Cheng <wushujames@gmail.com> wrote:
>> 
>> Can you update the KIP to say what the default is for
>> max.uncleanable.partitions?
>> 
>> -James
>> 
>> Sent from my iPhone
>> 
>>> On Jul 31, 2018, at 9:56 AM, Stanislav Kozlovski <stanislav@confluent.io>
>> wrote:
>>> 
>>> Hey group,
>>> 
>>> I am planning on starting a voting thread tomorrow. Please do reply if
>> you
>>> feel there is anything left to discuss.
>>> 
>>> Best,
>>> Stanislav
>>> 
>>> On Fri, Jul 27, 2018 at 11:05 PM Stanislav Kozlovski <
>> stanislav@confluent.io>
>>> wrote:
>>> 
>>>> Hey, Ray
>>>> 
>>>> Thanks for pointing that out, it's fixed now
>>>> 
>>>> Best,
>>>> Stanislav
>>>> 
>>>>> On Fri, Jul 27, 2018 at 9:43 PM Ray Chiang <rchiang@apache.org>
wrote:
>>>>> 
>>>>> Thanks.  Can you fix the link in the "KIPs under discussion" table on
>>>>> the main KIP landing page
>>>>> <
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals#
>>> ?
>>>>> 
>>>>> I tried, but the Wiki won't let me.
>>>>> 
>>>>> -Ray
>>>>> 
>>>>>> On 7/26/18 2:01 PM, Stanislav Kozlovski wrote:
>>>>>> Hey guys,
>>>>>> 
>>>>>> @Colin - good point. I added some sentences mentioning recent
>>>>> improvements
>>>>>> in the introductory section.
>>>>>> 
>>>>>> *Disk Failure* - I tend to agree with what Colin said - once a disk
>>>>> fails,
>>>>>> you don't want to work with it again. As such, I've changed my mind
>> and
>>>>>> believe that we should mark the LogDir (assume its a disk) as offline
>> on
>>>>>> the first `IOException` encountered. This is the LogCleaner's current
>>>>>> behavior. We shouldn't change that.
>>>>>> 
>>>>>> *Respawning Threads* - I believe we should never re-spawn a thread.
>> The
>>>>>> correct approach in my mind is to either have it stay dead or never
>> let
>>>>> it
>>>>>> die in the first place.
>>>>>> 
>>>>>> *Uncleanable-partition-names metric* - Colin is right, this metric
is
>>>>>> unneeded. Users can monitor the `uncleanable-partitions-count` metric
>>>>> and
>>>>>> inspect logs.
>>>>>> 
>>>>>> 
>>>>>> Hey Ray,
>>>>>> 
>>>>>>> 2) I'm 100% with James in agreement with setting up the LogCleaner
to
>>>>>>> skip over problematic partitions instead of dying.
>>>>>> I think we can do this for every exception that isn't `IOException`.
>>>>> This
>>>>>> will future-proof us against bugs in the system and potential other
>>>>> errors.
>>>>>> Protecting yourself against unexpected failures is always a good
thing
>>>>> in
>>>>>> my mind, but I also think that protecting yourself against bugs in
the
>>>>>> software is sort of clunky. What does everybody think about this?
>>>>>> 
>>>>>>> 4) The only improvement I can think of is that if such an
>>>>>>> error occurs, then have the option (configuration setting?) to
>> create a
>>>>>>> <log_segment>.skip file (or something similar).
>>>>>> This is a good suggestion. Have others also seen corruption be
>> generally
>>>>>> tied to the same segment?
>>>>>> 
>>>>>> On Wed, Jul 25, 2018 at 11:55 AM Dhruvil Shah <dhruvil@confluent.io>
>>>>> wrote:
>>>>>> 
>>>>>>> For the cleaner thread specifically, I do not think respawning
will
>>>>> help at
>>>>>>> all because we are more than likely to run into the same issue
again
>>>>> which
>>>>>>> would end up crashing the cleaner. Retrying makes sense for transient
>>>>>>> errors or when you believe some part of the system could have
healed
>>>>>>> itself, both of which I think are not true for the log cleaner.
>>>>>>> 
>>>>>>> On Wed, Jul 25, 2018 at 11:08 AM Ron Dagostino <rndgstn@gmail.com>
>>>>> wrote:
>>>>>>> 
>>>>>>>> <<<respawning threads is likely to make things worse,
by putting you
>>>>> in
>>>>>>> an
>>>>>>>> infinite loop which consumes resources and fires off continuous
log
>>>>>>>> messages.
>>>>>>>> Hi Colin.  In case it could be relevant, one way to mitigate
this
>>>>> effect
>>>>>>> is
>>>>>>>> to implement a backoff mechanism (if a second respawn is
to occur
>> then
>>>>>>> wait
>>>>>>>> for 1 minute before doing it; then if a third respawn is
to occur
>> wait
>>>>>>> for
>>>>>>>> 2 minutes before doing it; then 4 minutes, 8 minutes, etc.
up to
>> some
>>>>> max
>>>>>>>> wait time).
>>>>>>>> 
>>>>>>>> I have no opinion on whether respawn is appropriate or not
in this
>>>>>>> context,
>>>>>>>> but a mitigation like the increasing backoff described above
may be
>>>>>>>> relevant in weighing the pros and cons.
>>>>>>>> 
>>>>>>>> Ron
>>>>>>>> 
>>>>>>>> On Wed, Jul 25, 2018 at 1:26 PM Colin McCabe <cmccabe@apache.org>
>>>>> wrote:
>>>>>>>> 
>>>>>>>>>> On Mon, Jul 23, 2018, at 23:20, James Cheng wrote:
>>>>>>>>>> Hi Stanislav! Thanks for this KIP!
>>>>>>>>>> 
>>>>>>>>>> I agree that it would be good if the LogCleaner were
more tolerant
>>>>> of
>>>>>>>>>> errors. Currently, as you said, once it dies, it
stays dead.
>>>>>>>>>> 
>>>>>>>>>> Things are better now than they used to be. We have
the metric
>>>>>>>>>>      kafka.log:type=LogCleanerManager,name=time-since-last-run-ms
>>>>>>>>>> which we can use to tell us if the threads are dead.
And as of
>>>>> 1.1.0,
>>>>>>>> we
>>>>>>>>>> have KIP-226, which allows you to restart the log
cleaner thread,
>>>>>>>>>> without requiring a broker restart.
>>>>>>>>>> 
>>>>>>> 
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-226+-+Dynamic+Broker+Configuration
>>>>>>>>>> <
>>>>>>> 
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-226+-+Dynamic+Broker+Configuration
>>>>>>>>> 
>>>>>>>>>> I've only read about this, I haven't personally tried
it.
>>>>>>>>> Thanks for pointing this out, James!  Stanislav, we should
probably
>>>>>>> add a
>>>>>>>>> sentence or two mentioning the KIP-226 changes somewhere
in the
>> KIP.
>>>>>>>> Maybe
>>>>>>>>> in the intro section?
>>>>>>>>> 
>>>>>>>>> I think it's clear that requiring the users to manually
restart the
>>>>> log
>>>>>>>>> cleaner is not a very good solution.  But it's good to
know that
>>>>> it's a
>>>>>>>>> possibility on some older releases.
>>>>>>>>> 
>>>>>>>>>> Some comments:
>>>>>>>>>> * I like the idea of having the log cleaner continue
to clean as
>>>>> many
>>>>>>>>>> partitions as it can, skipping over the problematic
ones if
>>>>> possible.
>>>>>>>>>> 
>>>>>>>>>> * If the log cleaner thread dies, I think it should
automatically
>> be
>>>>>>>>>> revived. Your KIP attempts to do that by catching
exceptions
>> during
>>>>>>>>>> execution, but I think we should go all the way and
make sure
>> that a
>>>>>>>> new
>>>>>>>>>> one gets created, if the thread ever dies.
>>>>>>>>> This is inconsistent with the way the rest of Kafka works.
 We
>> don't
>>>>>>>>> automatically re-create other threads in the broker if
they
>>>>> terminate.
>>>>>>>> In
>>>>>>>>> general, if there is a serious bug in the code, respawning
threads
>> is
>>>>>>>>> likely to make things worse, by putting you in an infinite
loop
>> which
>>>>>>>>> consumes resources and fires off continuous log messages.
>>>>>>>>> 
>>>>>>>>>> * It might be worth trying to re-clean the uncleanable
partitions.
>>>>>>> I've
>>>>>>>>>> seen cases where an uncleanable partition later became
cleanable.
>> I
>>>>>>>>>> unfortunately don't remember how that happened, but
I remember
>> being
>>>>>>>>>> surprised when I discovered it. It might have been
something like
>> a
>>>>>>>>>> follower was uncleanable but after a leader election
happened, the
>>>>>>> log
>>>>>>>>>> truncated and it was then cleanable again. I'm not
sure.
>>>>>>>>> James, I disagree.  We had this behavior in the Hadoop
Distributed
>>>>> File
>>>>>>>>> System (HDFS) and it was a constant source of user problems.
>>>>>>>>> 
>>>>>>>>> What would happen is disks would just go bad over time.
 The
>> DataNode
>>>>>>>>> would notice this and take them offline.  But then, due
to some
>>>>>>>>> "optimistic" code, the DataNode would periodically try
to re-add
>> them
>>>>>>> to
>>>>>>>>> the system.  Then one of two things would happen: the
disk would
>> just
>>>>>>>> fail
>>>>>>>>> immediately again, or it would appear to work and then
fail after a
>>>>>>> short
>>>>>>>>> amount of time.
>>>>>>>>> 
>>>>>>>>> The way the disk failed was normally having an I/O request
take a
>>>>>>> really
>>>>>>>>> long time and time out.  So a bunch of request handler
threads
>> would
>>>>>>>>> basically slam into a brick wall when they tried to access
the bad
>>>>>>> disk,
>>>>>>>>> slowing the DataNode to a crawl.  It was even worse in
the second
>>>>>>>> scenario,
>>>>>>>>> if the disk appeared to work for a while, but then failed.
 Any
>> data
>>>>>>> that
>>>>>>>>> had been written on that DataNode to that disk would
be lost, and
>> we
>>>>>>>> would
>>>>>>>>> need to re-replicate it.
>>>>>>>>> 
>>>>>>>>> Disks aren't biological systems-- they don't heal over
time.  Once
>>>>>>>> they're
>>>>>>>>> bad, they stay bad.  The log cleaner needs to be robust
against
>> cases
>>>>>>>> where
>>>>>>>>> the disk really is failing, and really is returning bad
data or
>>>>> timing
>>>>>>>> out.
>>>>>>>>>> * For your metrics, can you spell out the full metric
in JMX-style
>>>>>>>>>> format, such as:
>>>>>>>>>> 
>>>>>>>> kafka.log:type=LogCleanerManager,name=uncleanable-partitions-count
>>>>>>>>>>              value=4
>>>>>>>>>> 
>>>>>>>>>> * For "uncleanable-partitions": topic-partition names
can be very
>>>>>>> long.
>>>>>>>>>> I think the current max size is 210 characters (or
maybe
>> 240-ish?).
>>>>>>>>>> Having the "uncleanable-partitions" being a list
could be very
>> large
>>>>>>>>>> metric. Also, having the metric come out as a csv
might be
>> difficult
>>>>>>> to
>>>>>>>>>> work with for monitoring systems. If we *did* want
the topic names
>>>>> to
>>>>>>>> be
>>>>>>>>>> accessible, what do you think of having the
>>>>>>>>>>      kafka.log:type=LogCleanerManager,topic=topic1,partition=2
>>>>>>>>>> I'm not sure if LogCleanerManager is the right type,
but my
>> example
>>>>>>> was
>>>>>>>>>> that the topic and partition can be tags in the metric.
That will
>>>>>>> allow
>>>>>>>>>> monitoring systems to more easily slice and dice
the metric. I'm
>> not
>>>>>>>>>> sure what the attribute for that metric would be.
Maybe something
>>>>>>> like
>>>>>>>>>> "uncleaned bytes" for that topic-partition? Or
>>>>> time-since-last-clean?
>>>>>>>> Or
>>>>>>>>>> maybe even just "Value=1".
>>>>>>>>> I haven't though about this that hard, but do we really
need the
>>>>>>>>> uncleanable topic names to be accessible through a metric?
 It
>> seems
>>>>>>> like
>>>>>>>>> the admin should notice that uncleanable partitions are
present,
>> and
>>>>>>> then
>>>>>>>>> check the logs?
>>>>>>>>> 
>>>>>>>>>> * About `max.uncleanable.partitions`, you said that
this likely
>>>>>>>>>> indicates that the disk is having problems. I'm not
sure that is
>> the
>>>>>>>>>> case. For the 4 JIRAs that you mentioned about log
cleaner
>> problems,
>>>>>>>> all
>>>>>>>>>> of them are partition-level scenarios that happened
during normal
>>>>>>>>>> operation. None of them were indicative of disk problems.
>>>>>>>>> I don't think this is a meaningful comparison.  In general,
we
>> don't
>>>>>>>>> accept JIRAs for hard disk problems that happen on a
particular
>>>>>>> cluster.
>>>>>>>>> If someone opened a JIRA that said "my hard disk is having
>> problems"
>>>>> we
>>>>>>>>> could close that as "not a Kafka bug."  This doesn't
prove that
>> disk
>>>>>>>>> problems don't happen, but  just that JIRA isn't the
right place
>> for
>>>>>>>> them.
>>>>>>>>> I do agree that the log cleaner has had a significant
number of
>> logic
>>>>>>>>> bugs, and that we need to be careful to limit their impact.
 That's
>>>>> one
>>>>>>>>> reason why I think that a threshold of "number of uncleanable
logs"
>>>>> is
>>>>>>> a
>>>>>>>>> good idea, rather than just failing after one IOException.
 In all
>>>>> the
>>>>>>>>> cases I've seen where a user hit a logic bug in the log
cleaner, it
>>>>> was
>>>>>>>>> just one partition that had the issue.  We also should
increase
>> test
>>>>>>>>> coverage for the log cleaner.
>>>>>>>>> 
>>>>>>>>>> * About marking disks as offline when exceeding a
certain
>> threshold,
>>>>>>>>>> that actually increases the blast radius of log compaction
>> failures.
>>>>>>>>>> Currently, the uncleaned partitions are still readable
and
>> writable.
>>>>>>>>>> Taking the disks offline would impact availability
of the
>>>>> uncleanable
>>>>>>>>>> partitions, as well as impact all other partitions
that are on the
>>>>>>>> disk.
>>>>>>>>> In general, when we encounter I/O errors, we take the
disk
>> partition
>>>>>>>>> offline.  This is spelled out in KIP-112 (
>>>>>>>>> 
>>>>>>> 
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD
>>>>>>>>> ) :
>>>>>>>>> 
>>>>>>>>>> - Broker assumes a log directory to be good after
it starts, and
>>>>> mark
>>>>>>>>> log directory as
>>>>>>>>>> bad once there is IOException when broker attempts
to access (i.e.
>>>>>>> read
>>>>>>>>> or write) the log directory.
>>>>>>>>>> - Broker will be offline if all log directories are
bad.
>>>>>>>>>> - Broker will stop serving replicas in any bad log
directory. New
>>>>>>>>> replicas will only be created
>>>>>>>>>> on good log directory.
>>>>>>>>> The behavior Stanislav is proposing for the log cleaner
is actually
>>>>>>> more
>>>>>>>>> optimistic than what we do for regular broker I/O, since
we will
>>>>>>> tolerate
>>>>>>>>> multiple IOExceptions, not just one.  But it's generally
>> consistent.
>>>>>>>>> Ignoring errors is not.  In any case, if you want to
tolerate an
>>>>>>>> unlimited
>>>>>>>>> number of I/O errors, you can just set the threshold
to an infinite
>>>>>>> value
>>>>>>>>> (although I think that would be a bad idea).
>>>>>>>>> 
>>>>>>>>> best,
>>>>>>>>> Colin
>>>>>>>>> 
>>>>>>>>>> -James
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>>> On Jul 23, 2018, at 5:46 PM, Stanislav Kozlovski
<
>>>>>>>>> stanislav@confluent.io> wrote:
>>>>>>>>>>> I renamed the KIP and that changed the link.
Sorry about that.
>> Here
>>>>>>>> is
>>>>>>>>> the
>>>>>>>>>>> new link:
>>>>>>>>>>> 
>>>>>>> 
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-346+-+Improve+LogCleaner+behavior+on+error
>>>>>>>>>>> On Mon, Jul 23, 2018 at 5:11 PM Stanislav Kozlovski
<
>>>>>>>>> stanislav@confluent.io>
>>>>>>>>>>> wrote:
>>>>>>>>>>> 
>>>>>>>>>>>> Hey group,
>>>>>>>>>>>> 
>>>>>>>>>>>> I created a new KIP about making log compaction
more
>>>>>>> fault-tolerant.
>>>>>>>>>>>> Please give it a look here and please share
what you think,
>>>>>>>>> especially in
>>>>>>>>>>>> regards to the points in the "Needs Discussion"
paragraph.
>>>>>>>>>>>> 
>>>>>>>>>>>> KIP: KIP-346
>>>>>>>>>>>> <
>>>>>>> 
>>>>> 
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-346+-+Limit+blast+radius+of+log+compaction+failure
>>>>>>>>>>>> --
>>>>>>>>>>>> Best,
>>>>>>>>>>>> Stanislav
>>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> --
>>>>>>>>>>> Best,
>>>>>>>>>>> Stanislav
>>>>>> 
>>>>> 
>>>>> 
>>>> 
>>>> --
>>>> Best,
>>>> Stanislav
>>>> 
>>> 
>>> 
>>> --
>>> Best,
>>> Stanislav
>> 
> 
> 
> -- 
> Best,
> Stanislav

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