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From Jun Rao <...@confluent.io>
Subject Re: [DISCUSS] KIP-113: Support replicas movement between log directories
Date Wed, 08 Mar 2017 21:24:50 GMT
Hi, Dong,

Thanks for the updated KIP. A few more comments below.

1.1 and 1.2: I am still not sure there is enough benefit of reusing
ReplicaFetchThread
to move data across disks.
(a) A big part of ReplicaFetchThread is to deal with issuing and tracking
fetch requests. So, it doesn't feel that we get much from reusing
ReplicaFetchThread
only to disable the fetching part.
(b) The leader replica has no ReplicaFetchThread to start with. It feels
weird to start one just for intra broker data movement.
(c) The ReplicaFetchThread is per broker. Intuitively, the number of
threads doing intra broker data movement should be related to the number of
disks in the broker, not the number of brokers in the cluster.
(d) If the destination disk fails, we want to stop the intra broker data
movement, but want to continue inter broker replication. So, logically, it
seems it's better to separate out the two.
(e) I am also not sure if we should reuse the existing throttling for
replication. It's designed to handle traffic across brokers and the
delaying is done in the fetch request. So, if we are not doing
fetching in ReplicaFetchThread,
I am not sure the existing throttling is effective. Also, when specifying
the throttling of moving data across disks, it seems the user shouldn't
care about whether a replica is a leader or a follower. Reusing the
existing throttling config name will be awkward in this regard.
(f) It seems it's simpler and more consistent to use a separate thread pool
for local data movement (for both leader and follower replicas). This
process can then be configured (e.g. number of threads, etc) and throttled
independently.

1.3 Yes, we will need some synchronization there. So, if the movement
thread catches up, gets the lock to do the swap, but realizes that new data
is added, it has to continue catching up while holding the lock?

2.3 The benefit of including the desired log directory in LeaderAndIsrRequest
during partition reassignment is that the controller doesn't need to track
the progress for disk movement. So, you don't need the additional
BrokerDirStateUpdateRequest. Then the controller never needs to issue
ChangeReplicaDirRequest.
Only the admin tool will issue ChangeReplicaDirRequest to move data within
a broker. I agree that this makes LeaderAndIsrRequest more complicated, but
that seems simpler than changing the controller to track additional states
during partition reassignment.

4. We want to make a decision on how to expose the stats. So far, we are
exposing stats like the individual log size as JMX. So, one way is to just
add new jmx to expose the log directory of individual replicas.

Thanks,

Jun


On Thu, Mar 2, 2017 at 11:18 PM, Dong Lin <lindong28@gmail.com> wrote:

> Hey Jun,
>
> Thanks for all the comments! Please see my answer below. I have updated the
> KIP to address most of the questions and make the KIP easier to understand.
>
> Thanks,
> Dong
>
> On Thu, Mar 2, 2017 at 9:35 AM, Jun Rao <jun@confluent.io> wrote:
>
> > Hi, Dong,
> >
> > Thanks for the KIP. A few comments below.
> >
> > 1. For moving data across directories
> > 1.1 I am not sure why we want to use ReplicaFetcherThread to move data
> > around in the leader. ReplicaFetchThread fetches data from socket. For
> > moving data locally, it seems that we want to avoid the socket overhead.
> >
>
> The purpose of using ReplicaFetchThread is to re-use existing thread
> instead of creating more threads and make our thread model more complex. It
> seems like a nature choice for copying data between disks since it is
> similar to copying data between brokers. Another reason is that if the
> replica to be moved is a follower, we don't need lock to swap replicas when
> destination replica has caught up, since the same thread which is fetching
> data from leader will swap the replica.
>
> The ReplicaFetchThread will not incur socket overhead while copying data
> between disks. It will read directly from source disk (as we do when
> processing FetchRequest) and write to destination disk (as we do when
> processing ProduceRequest).
>
>
> > 1.2 I am also not sure about moving data in the ReplicaFetcherThread in
> the
> > follower. For example, I am not sure setting replica.fetch.max.wait to 0
> >  is ideal. It may not always be effective since a fetch request in the
> > ReplicaFetcherThread could be arbitrarily delayed due to replication
> > throttling on the leader. In general, the data movement logic across
> disks
> > seems different from that in ReplicaFetcherThread. So, I am not sure why
> > they need to be coupled.
> >
>
> While it may not be the most efficient way to copy data between local
> disks, it will be at least as efficient as copying data from leader to the
> destination disk. The expected goal of KIP-113 is to enable data movement
> between disks with no less efficiency than what we do now when moving data
> between brokers. I think we can optimize its performance using separate
> thread if the performance is not good enough.
>
>
> > 1.3 Could you add a bit more details on how we swap the replicas when the
> > new ones are fully caught up? For example, what happens when the new
> > replica in the new log directory is caught up, but when we want to do the
> > swap, some new data has arrived?
> >
>
> If the replica is a leader, then ReplicaFetcherThread will perform the
> replacement. Proper lock is needed to prevent KafkaRequestHandler from
> appending data to the topicPartition.log on the source disks before this
> replacement is completed by ReplicaFetcherThread.
>
> If the replica is a follower, because the same ReplicaFetchThread which
> fetches data from leader will also swap the replica , no lock is needed.
>
> I have updated the KIP to specify both more explicitly.
>
>
>
> > 1.4 Do we need to do the .move at the log segment level or could we just
> do
> > that at the replica directory level? Renaming just a directory is much
> > faster than renaming the log segments.
> >
>
> Great point. I have updated the KIP to rename the log directory instead.
>
>
> > 1.5 Could you also describe a bit what happens when either the source or
> > the target log directory fails while the data moving is in progress?
> >
>
> If source log directory fails, then the replica movement will stop and the
> source replica is marked offline. If destination log directory fails, then
> the replica movement will stop. I have updated the KIP to clarify this.
>
>
> >
> > 2. For partition reassignment.
> > 2.1 I am not sure if the controller can block on ChangeReplicaDirRequest.
> > Data movement may take a long time to complete. If there is an
> outstanding
> > request from the controller to a broker, that broker won't be able to
> > process any new request from the controller. So if another event (e.g.
> > broker failure) happens when the data movement is in progress, subsequent
> > LeaderAnIsrRequest will be delayed.
> >
>
> Yeah good point. I missed the fact that there is be only one inflight
> request from controller to broker.
>
> How about I add a request, e.g. BrokerDirStateUpdateRequest, which maps
> topicPartition to log directory and can be sent from broker to controller
> to indicate completion?
>
>
>
> > 2.2 in the KIP, the partition reassignment tool is also used for cases
> > where an admin just wants to balance the existing data across log
> > directories in the broker. In this case, it seems that it's over killing
> to
> > have the process go through the controller. A simpler approach is to
> issue
> > an RPC request to the broker directly.
> >
>
> I agree we can optimize this case. It is just that we have to add new logic
> or code path to handle a scenario that is already covered by the more
> complicated scenario. I will add it to the KIP.
>
>
> > 2.3 When using the partition reassignment tool to move replicas across
> > brokers, it make sense to be able to specify the log directory of the
> newly
> > created replicas. The KIP does that in two separate requests
> > ChangeReplicaDirRequest and LeaderAndIsrRequest, and tracks the progress
> of
> > each independently. An alternative is to do that just in
> > LeaderAndIsrRequest.
> > That way, the new replicas will be created in the right log dir in the
> > first place and the controller just needs to track the progress of
> > partition reassignment in the current way.
> >
>
> I agree it is better to use one request instead of two to request replica
> movement between disks. But I think the performance advantage of doing so
> is negligible because we trigger replica assignment much less than all
> other kinds of events in the Kafka cluster. I am not sure that the benefit
> of doing this is worth the effort to add an optional string field in the
> LeaderAndIsrRequest. Also if we add this optional field in the
> LeaderAndIsrRequest, we probably want to remove ChangeReplicaDirRequest to
> avoid having two requests doing the same thing. But it means user script
> can not send request directly to the broker to trigger replica movement
> between log directories.
>
> I will do it if you are strong about this optimzation.
>
>
> >
> > 3. /admin/reassign_partitions: Including the log dir in every replica may
> > not be efficient. We could include a list of log directories and
> reference
> > the index of the log directory in each replica.
> >
>
> Good point. I have updated the KIP to use this solution.
>
>
> >
> > 4. DescribeDirsRequest: The stats in the request are already available
> from
> > JMX. Do we need the new request?
> >
>
> Does JMX also include the state (i.e. offline or online) of each log
> directory and the log directory of each replica? If not, then maybe we
> still need DescribeDirsRequest?
>
>
> >
> > 5. We want to be consistent on ChangeReplicaDirRequest vs
> > ChangeReplicaRequest.
> >
>
> I think ChangeReplicaRequest and ChangeReplicaResponse is my typo. Sorry,
> they are fixed now.
>
>
> >
> > Thanks,
> >
> > Jun
> >
> >
> > On Fri, Feb 3, 2017 at 6:19 PM, Dong Lin <lindong28@gmail.com> wrote:
> >
> > > Hey ALexey,
> > >
> > > Thanks for all the comments!
> > >
> > > I have updated the KIP to specify how we enforce quota. I also updated
> > the
> > > "The thread model and broker logic for moving replica data between log
> > > directories" to make it easier to read. You can find the exact change
> > here
> > > <https://cwiki.apache.org/confluence/pages/diffpagesbyversio
> > > n.action?pageId=67638408&selectedPageVersions=5&selectedPage
> Versions=6>.
> > > The idea is to use the same replication quota mechanism introduced in
> > > KIP-73.
> > >
> > > Thanks,
> > > Dong
> > >
> > >
> > >
> > > On Wed, Feb 1, 2017 at 2:16 AM, Alexey Ozeritsky <aozeritsky@yandex.ru
> >
> > > wrote:
> > >
> > > >
> > > >
> > > > 24.01.2017, 22:03, "Dong Lin" <lindong28@gmail.com>:
> > > > > Hey Alexey,
> > > > >
> > > > > Thanks. I think we agreed that the suggested solution doesn't work
> in
> > > > > general for kafka users. To answer your questions:
> > > > >
> > > > > 1. I agree we need quota to rate limit replica movement when a
> broker
> > > is
> > > > > moving a "leader" replica. I will come up with solution, probably
> > > re-use
> > > > > the config of replication quota introduced in KIP-73.
> > > > >
> > > > > 2. Good point. I agree that this is a problem in general. If is no
> > new
> > > > data
> > > > > on that broker, with current default value of
> > > replica.fetch.wait.max.ms
> > > > > and replica.fetch.max.bytes, the replica will be moved at only 2
> MBps
> > > > > throughput. I think the solution is for broker to set
> > > > > replica.fetch.wait.max.ms to 0 in its FetchRequest if the
> > > corresponding
> > > > > ReplicaFetcherThread needs to move some replica to another disk.
> > > > >
> > > > > 3. I have updated the KIP to mention that the read size of a given
> > > > > partition is configured using replica.fetch.max.bytes when we move
> > > > replicas
> > > > > between disks.
> > > > >
> > > > > Please see this
> > > > > <https://cwiki.apache.org/confluence/pages/diffpagesbyversio
> n.action
> > ?
> > > > pageId=67638408&selectedPageVersions=4&selectedPageVersions=5>
> > > > > for the change of the KIP. I will come up with a solution to
> throttle
> > > > > replica movement when a broker is moving a "leader" replica.
> > > >
> > > > Thanks. It looks great.
> > > >
> > > > >
> > > > > On Tue, Jan 24, 2017 at 3:30 AM, Alexey Ozeritsky <
> > > aozeritsky@yandex.ru>
> > > > > wrote:
> > > > >
> > > > >>  23.01.2017, 22:11, "Dong Lin" <lindong28@gmail.com>:
> > > > >>  > Thanks. Please see my comment inline.
> > > > >>  >
> > > > >>  > On Mon, Jan 23, 2017 at 6:45 AM, Alexey Ozeritsky <
> > > > aozeritsky@yandex.ru>
> > > > >>  > wrote:
> > > > >>  >
> > > > >>  >> 13.01.2017, 22:29, "Dong Lin" <lindong28@gmail.com>:
> > > > >>  >> > Hey Alexey,
> > > > >>  >> >
> > > > >>  >> > Thanks for your review and the alternative approach.
Here is
> > my
> > > > >>  >> > understanding of your patch. kafka's background
threads are
> > used
> > > > to
> > > > >>  move
> > > > >>  >> > data between replicas. When data movement is triggered,
the
> > log
> > > > will
> > > > >>  be
> > > > >>  >> > rolled and the new logs will be put in the new
directory,
> and
> > > > >>  background
> > > > >>  >> > threads will move segment from old directory to
new
> directory.
> > > > >>  >> >
> > > > >>  >> > It is important to note that KIP-112 is intended
to work
> with
> > > > >>  KIP-113 to
> > > > >>  >> > support JBOD. I think your solution is definitely
simpler
> and
> > > > better
> > > > >>  >> under
> > > > >>  >> > the current kafka implementation that a broker
will fail if
> > any
> > > > disk
> > > > >>  >> fails.
> > > > >>  >> > But I am not sure if we want to allow broker to
run with
> > partial
> > > > >>  disks
> > > > >>  >> > failure. Let's say the a replica is being moved
from
> > log_dir_old
> > > > to
> > > > >>  >> > log_dir_new and then log_dir_old stops working
due to disk
> > > > failure.
> > > > >>  How
> > > > >>  >> > would your existing patch handles it? To make
the scenario a
> > bit
> > > > more
> > > > >>  >>
> > > > >>  >> We will lose log_dir_old. After broker restart we can
read the
> > > data
> > > > >>  from
> > > > >>  >> log_dir_new.
> > > > >>  >
> > > > >>  > No, you probably can't. This is because the broker doesn't
have
> > > > *all* the
> > > > >>  > data for this partition. For example, say the broker has
> > > > >>  > partition_segement_1, partition_segment_50 and
> > > partition_segment_100
> > > > on
> > > > >>  the
> > > > >>  > log_dir_old. partition_segment_100, which has the latest
data,
> > has
> > > > been
> > > > >>  > moved to log_dir_new, and the log_dir_old fails before
> > > > >>  partition_segment_50
> > > > >>  > and partition_segment_1 is moved to log_dir_new. When broker
> > > > re-starts,
> > > > >>  it
> > > > >>  > won't have partition_segment_50. This causes problem if
broker
> is
> > > > elected
> > > > >>  > leader and consumer wants to consume data in the
> > > partition_segment_1.
> > > > >>
> > > > >>  Right.
> > > > >>
> > > > >>  >
> > > > >>  >> > complicated, let's say the broker is shtudown,
log_dir_old's
> > > disk
> > > > >>  fails,
> > > > >>  >> > and the broker starts. In this case broker doesn't
even know
> > if
> > > > >>  >> log_dir_new
> > > > >>  >> > has all the data needed for this replica. It becomes
a
> problem
> > > if
> > > > the
> > > > >>  >> > broker is elected leader of this partition in
this case.
> > > > >>  >>
> > > > >>  >> log_dir_new contains the most recent data so we will
lose the
> > tail
> > > > of
> > > > >>  >> partition.
> > > > >>  >> This is not a big problem for us because we already
delete
> tails
> > > by
> > > > >>  hand
> > > > >>  >> (see https://issues.apache.org/jira/browse/KAFKA-1712).
> > > > >>  >> Also we dont use authomatic leader balancing
> > > > >>  (auto.leader.rebalance.enable=false),
> > > > >>  >> so this partition becomes the leader with a low probability.
> > > > >>  >> I think my patch can be modified to prohibit the selection
of
> > the
> > > > >>  leader
> > > > >>  >> until the partition does not move completely.
> > > > >>  >
> > > > >>  > I guess you are saying that you have deleted the tails
by hand
> in
> > > > your
> > > > >>  own
> > > > >>  > kafka branch. But KAFKA-1712 is not accepted into Kafka
trunk
> > and I
> > > > am
> > > > >>  not
> > > > >>
> > > > >>  No. We just modify segments mtime by cron job. This works with
> > > vanilla
> > > > >>  kafka.
> > > > >>
> > > > >>  > sure if it is the right solution. How would this solution
> address
> > > the
> > > > >>  > problem mentioned above?
> > > > >>
> > > > >>  If you need only fresh data and if you remove old data by hands
> > this
> > > is
> > > > >>  not a problem. But in general case
> > > > >>  this is a problem of course.
> > > > >>
> > > > >>  >
> > > > >>  > BTW, I am not sure the solution mentioned in KAFKA-1712
is the
> > > right
> > > > way
> > > > >>  to
> > > > >>  > address its problem. Now that we have timestamp in the
message
> we
> > > > can use
> > > > >>  > that to delete old segement instead of relying on the log
> segment
> > > > mtime.
> > > > >>  > Just some idea and we don't have to discuss this problem
here.
> > > > >>  >
> > > > >>  >> >
> > > > >>  >> > The solution presented in the KIP attempts to
handle it by
> > > > replacing
> > > > >>  >> > replica in an atomic version fashion after the
log in the
> new
> > > dir
> > > > has
> > > > >>  >> fully
> > > > >>  >> > caught up with the log in the old dir. At at time
the log
> can
> > be
> > > > >>  >> considered
> > > > >>  >> > to exist on only one log directory.
> > > > >>  >>
> > > > >>  >> As I understand your solution does not cover quotas.
> > > > >>  >> What happens if someone starts to transfer 100 partitions
?
> > > > >>  >
> > > > >>  > Good point. Quota can be implemented in the future. It
is
> > currently
> > > > >>  > mentioned as as a potential future improvement in KIP-112
> > > > >>  > <https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3
> > > > >>  A+Handle+disk+failure+for+JBOD>.Thanks
> > > > >>  > for the reminder. I will move it to KIP-113.
> > > > >>  >
> > > > >>  >> > If yes, it will read a ByteBufferMessageSet from
> > > > topicPartition.log
> > > > >>  and
> > > > >>  >> append the message set to topicPartition.move
> > > > >>  >>
> > > > >>  >> i.e. processPartitionData will read data from the beginning
of
> > > > >>  >> topicPartition.log? What is the read size?
> > > > >>  >> A ReplicaFetchThread reads many partitions so if one
does some
> > > > >>  complicated
> > > > >>  >> work (= read a lot of data from disk) everything will
slow
> down.
> > > > >>  >> I think read size should not be very big.
> > > > >>  >>
> > > > >>  >> On the other hand at this point (processPartitionData)
one can
> > use
> > > > only
> > > > >>  >> the new data (ByteBufferMessageSet from parameters)
and wait
> > until
> > > > >>  >> (topicPartition.move.smallestOffset <=
> > > > topicPartition.log.smallestOff
> > > > >>  set
> > > > >>  >> && topicPartition.log.largestOffset ==
> > > > topicPartition.log.largestOffs
> > > > >>  et).
> > > > >>  >> In this case the write speed to topicPartition.move
and
> > > > >>  topicPartition.log
> > > > >>  >> will be the same so this will allow us to move many
partitions
> > to
> > > > one
> > > > >>  disk.
> > > > >>  >
> > > > >>  > The read size of a given partition is configured
> > > > >>  > using replica.fetch.max.bytes, which is the same size used
by
> > > > >>  FetchRequest
> > > > >>  > from follower to leader. If the broker is moving a replica
for
> > > which
> > > > it
> > > > >>
> > > > >>  OK. Could you mention it in KIP?
> > > > >>
> > > > >>  > acts as a follower, the disk write rate for moving this
replica
> > is
> > > at
> > > > >>  most
> > > > >>  > the rate it fetches from leader (assume it is catching
up and
> has
> > > > >>  > sufficient data to read from leader, which is subject to
> > > > round-trip-time
> > > > >>  > between itself and the leader. Thus this part if probably
fine
> > even
> > > > >>  without
> > > > >>  > quota.
> > > > >>
> > > > >>  I think there are 2 problems
> > > > >>  1. Without speed limiter this will not work good even for 1
> > > partition.
> > > > In
> > > > >>  our production we had a problem so we did the throuput limiter:
> > > > >>  https://github.com/resetius/kafka/commit/cda31dadb2f135743bf
> > > > >>  41083062927886c5ddce1#diff-ffa8861e850121997a534ebdde2929c6R713
> > > > >>
> > > > >>  2. I dont understand how it will work in case of big
> > > > >>  replica.fetch.wait.max.ms and partition with irregular flow.
> > > > >>  For example someone could have replica.fetch.wait.max.ms=10mi
> nutes
> > > and
> > > > >>  partition that has very high data flow from 12:00 to 13:00 and
> zero
> > > > flow
> > > > >>  otherwise.
> > > > >>  In this case processPartitionData could be called once per
> > 10minutes
> > > > so if
> > > > >>  we start data moving in 13:01 it will be finished next day.
> > > > >>
> > > > >>  >
> > > > >>  > But ff the broker is moving a replica for which it acts
as a
> > > leader,
> > > > as
> > > > >>  of
> > > > >>  > current KIP the broker will keep reading from log_dir_old
and
> > > append
> > > > to
> > > > >>  > log_dir_new without having to wait for round-trip-time.
We
> > probably
> > > > need
> > > > >>  > quota for this in the future.
> > > > >>  >
> > > > >>  >> >
> > > > >>  >> > And to answer your question, yes topicpartition.log
refers
> to
> > > > >>  >> > topic-paritition/segment.log.
> > > > >>  >> >
> > > > >>  >> > Thanks,
> > > > >>  >> > Dong
> > > > >>  >> >
> > > > >>  >> > On Fri, Jan 13, 2017 at 4:12 AM, Alexey Ozeritsky
<
> > > > >>  aozeritsky@yandex.ru>
> > > > >>  >> > wrote:
> > > > >>  >> >
> > > > >>  >> >> Hi,
> > > > >>  >> >>
> > > > >>  >> >> We have the similar solution that have been
working in
> > > production
> > > > >>  since
> > > > >>  >> >> 2014. You can see it here: https://github.com/resetius/ka
> > > > >>  >> >> fka/commit/20658593e246d2184906879defa2e763c4d413fb
> > > > >>  >> >> The idea is very simple
> > > > >>  >> >> 1. Disk balancer runs in a separate thread
inside scheduler
> > > pool.
> > > > >>  >> >> 2. It does not touch empty partitions
> > > > >>  >> >> 3. Before it moves a partition it forcibly
creates new
> > segment
> > > > on a
> > > > >>  >> >> destination disk
> > > > >>  >> >> 4. It moves segment by segment from new to
old.
> > > > >>  >> >> 5. Log class works with segments on both disks
> > > > >>  >> >>
> > > > >>  >> >> Your approach seems too complicated, moreover
it means that
> > you
> > > > >>  have to
> > > > >>  >> >> patch different components of the system
> > > > >>  >> >> Could you clarify what do you mean by topicPartition.log?
> Is
> > it
> > > > >>  >> >> topic-paritition/segment.log ?
> > > > >>  >> >>
> > > > >>  >> >> 12.01.2017, 21:47, "Dong Lin" <lindong28@gmail.com>:
> > > > >>  >> >> > Hi all,
> > > > >>  >> >> >
> > > > >>  >> >> > We created KIP-113: Support replicas
movement between log
> > > > >>  >> directories.
> > > > >>  >> >> > Please find the KIP wiki in the link
> > > > >>  >> >> > *https://cwiki.apache.org/conf
> > luence/display/KAFKA/KIP-113%
> > > > >>  >> >> 3A+Support+replicas+movement+between+log+directories
> > > > >>  >> >> > <https://cwiki.apache.org/conf
> > luence/display/KAFKA/KIP-113%
> > > > >>  >> >> 3A+Support+replicas+movement+between+log+directories>.*
> > > > >>  >> >> >
> > > > >>  >> >> > This KIP is related to KIP-112
> > > > >>  >> >> > <https://cwiki.apache.org/conf
> > luence/display/KAFKA/KIP-112%
> > > > >>  >> >> 3A+Handle+disk+failure+for+JBOD>:
> > > > >>  >> >> > Handle disk failure for JBOD. They are
needed in order to
> > > > support
> > > > >>  >> JBOD in
> > > > >>  >> >> > Kafka. Please help review the KIP. You
feedback is
> > > appreciated!
> > > > >>  >> >> >
> > > > >>  >> >> > Thanks,
> > > > >>  >> >> > Dong
> > > >
> > >
> >
>

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