Hi @Piotr and @Jingsong Li I have read access to the document, but I'm not sure whether the owner of this document wants to make it public. Actually, the doc is for FLINK-23342 and there is a candidate design very similar to this FLIP, but only for the shared state. Like Yun said, the previous design is not taken because of the code complexity, however I think it is acceptable after implementing the POC[1]. I think we could focus on the current plan, WDTY?
[1] POC of this FLIP: https://github.com/Zakelly/flink/commit/98538185182e33739828ee36ab96dcf2aebad80c On Fri, Apr 7, 2023 at 8:13 PM Zakelly Lan <zakelly....@gmail.com> wrote: > > Hi Piotr, > > Thanks for your comments! > > (1) Sorry for the misleading, let me make it more clear. It is a > concurrent checkpoint senario. Yes, the assumption you said needs to > be followed, but the state handles here refer to the original SST > files, not the underlying file. In this FLIP when checkpoint N and N+1 > are running concurrently, they reuse files from checkpoint N-1, and > some of the files may be deleted when checkpoint N completes while > checkpoint N+1 is still writing on it. There is no such problem for > original shared states without file merging because when a state > handle (or sst file here) from checkpoint N-1 is not referenced by > checkpoint N, it will not be referenced by checkpoint N+1. So the > subsumption of sst files from checkpoint N-1 are safe. > For above example, when reaching step "d.", File 1 reached the size > threshold and will not be used. The Chk-2 and Chk-3 are running > concurrently, and the File 3 is being written by Chk-2, so it can not > be used by Chk-3 (As described in section 4.6). Here comes the > problem. > > (2) Please correct me if I'm wrong. The purpose of the > `RecoverableWriter` is to provide a reliable file writer even > tolerable with job failure and recovery. The implementation varies > among the file systems, some of which involves writing into temporary > files (such as HDFS). As a result, it may produce more RPC requests to > the DFS. > The goal of this FLIP is to reduce the pressure on DFS, especially the > number of files and RPC requests. Currently the TMs are NOT using the > RecoverableWriter to persist/upload the state files, and a file > closing is enough. The section 4.1.1 is trying to omit this file > closing but ensure file visibility in some DFS, thus reducing pressure > on DFS. That's why I said the problems they want to solve are > different. I'm not sure if I made myself clear. > > > Thanks! > > Best regards, > Zakelly > > On Fri, Apr 7, 2023 at 8:08 PM Zakelly Lan <zakelly....@gmail.com> wrote: > > > > Hi Yun, > > > > Thanks for your suggestions! > > > > I have read the FLINK-23342 and its design doc as you provided. First > > of all the goal of this FLIP and the doc are similar, and the design > > of this FLIP is pretty much like option 3. The main difference is that > > we imply the concept of 'epoch' in the folder path for each > > granularity. For shared state, the folder for each subtask is like > > "${checkpointBaseDir}/shared/subtask-{index}-{parallelism}", so if the > > ${checkpointBaseDir} changes (when user restart a job manually) or the > > ${parallelism} changes (when rescaling), there will be a re-uploading, > > and the JM takes care of the old artifacts. The folder path for > > private state is in the form of > > "${checkpointBaseDir}/tm-owned/${tmResourceId}" and the division of > > responsibilities between JM and TM is similar. The design of this FLIP > > inherits all the advantages of the design of option 3 in that doc, and > > also avoids extra communication for epoch maintenance. As for the code > > complexity, you may check the POC commit[1] and find that the > > implementation is pretty clean and is a totally new code path making > > nearly no influence on the old one. Comparing the number of lines of > > code change with what's currently done for merging channel state[2] > > (5200 vs. 2500 additions), I think it is acceptable considering we are > > providing a unified file merging framework, which would save a lot of > > effort in future. WDYT? > > > > Thanks. > > > > Best regards, > > Zakelly > > > > [1] POC of this FLIP: > > https://github.com/Zakelly/flink/commit/98538185182e33739828ee36ab96dcf2aebad80c > > [2] Commit for FLINK-26803 (Merge the channel state files) : > > https://github.com/apache/flink/commit/8be94e6663d8ac6e3d74bf4cd5f540cc96c8289e > > > > > > On Fri, Apr 7, 2023 at 7:22 PM Yanfei Lei <fredia...@gmail.com> wrote: > > > > > > Thanks for your explanation Zakelly. > > > (1) Keeping these merging granularities for different types of files > > > as presets that are not configurable is a good idea to prevent > > > performance degradation. > > > > > > (2) > > > > For the third option, 64MB is an acceptable target size. The RocksDB > > > > state backend in Flink also chooses 64MB as the default target file > > > > size. > > > > > > Does this result in a larger space amplification? Maybe a more > > > suitable value can be determined through some experimental statistics > > > after we implement this feature. > > > > > > Best, > > > Yanfei > > > > > > Jingsong Li <jingsongl...@gmail.com> 于2023年4月7日周五 17:09写道： > > > > > > > > Hi Yun, > > > > > > > > It looks like this doc needs permission to read? [1] > > > > > > > > [1] > > > > https://docs.google.com/document/d/1NJJQ30P27BmUvD7oa4FChvkYxMEgjRPTVdO1dHLl_9I/edit# > > > > > > > > Best, > > > > Jingsong > > > > > > > > On Fri, Apr 7, 2023 at 4:34 PM Piotr Nowojski <pnowoj...@apache.org> > > > > wrote: > > > > > > > > > > Hi, > > > > > > > > > > +1 To what Yun Tang wrote. We don't seem to have access to the design > > > > > doc. > > > > > Could you make it publicly visible or copy out its content to another > > > > > document? > > > > > > > > > > Thanks for your answers Zakelly. > > > > > > > > > > (1) > > > > > Yes, the current mechanism introduced in FLINK-24611 allows for > > > > > checkpoint > > > > > N, to only re-use shared state handles that have been already > > > > > referenced by > > > > > checkpoint N-1. But why do we need to break this assumption? In your > > > > > step, > > > > > "d.", TM could adhere to that assumption, and instead of reusing > > > > > File-2, it > > > > > could either re-use File-1, File-3 or create a new file. > > > > > > > > > > (2) > > > > > Can you elaborate a bit more on this? As far as I recall, the purpose > > > > > of > > > > > the `RecoverableWriter` is to support exactly the things described in > > > > > this > > > > > FLIP, so what's the difference? If you are saying that for this FLIP > > > > > you > > > > > can implement something more efficiently for a given FileSystem, then > > > > > why > > > > > can it not be done the same way for the `RecoverableWriter`? > > > > > > > > > > Best, > > > > > Piotrek > > > > > > > > > > czw., 6 kwi 2023 o 17:24 Yun Tang <myas...@live.com> napisał(a): > > > > > > > > > > > Hi Zakelly, > > > > > > > > > > > > Thanks for driving this work! > > > > > > > > > > > > I'm not sure did you ever read the discussion between Stephan, > > > > > > Roman, > > > > > > Piotr, Yuan and I in the design doc [1] in nearly two years ago. > > > > > > > > > > > > From my understanding, your proposal is also a mixed state > > > > > > ownership: some > > > > > > states are owned by the TM while some are owned by the JM. If my > > > > > > memory is > > > > > > correct, we did not take the option-3 or option-5 in the design doc > > > > > > [1] for > > > > > > the code complexity when implements the 1st version of changelog > > > > > > state-backend. > > > > > > > > > > > > Could you also compare the current FLIP with the proposals in the > > > > > > design > > > > > > doc[1]? From my understanding, we should at least consider to > > > > > > comapre with > > > > > > option-3 and option-5 as they are all mixed solutions. > > > > > > > > > > > > > > > > > > [1] > > > > > > https://docs.google.com/document/d/1NJJQ30P27BmUvD7oa4FChvkYxMEgjRPTVdO1dHLl_9I/edit# > > > > > > > > > > > > Best > > > > > > Yun Tang > > > > > > > > > > > > ------------------------------ > > > > > > *From:* Zakelly Lan <zakelly....@gmail.com> > > > > > > *Sent:* Thursday, April 6, 2023 16:38 > > > > > > *To:* dev@flink.apache.org <dev@flink.apache.org> > > > > > > *Subject:* Re: [DISCUSS] FLIP-306: Unified File Merging Mechanism > > > > > > for > > > > > > Checkpoints > > > > > > > > > > > > Hi Piotr, > > > > > > > > > > > > Thanks for all the feedback. > > > > > > > > > > > > (1) Thanks for the reminder. I have just seen the FLINK-24611, the > > > > > > delayed > > > > > > deletion by JM resolves some sync problems between JM and TM, but > > > > > > I'm > > > > > > afraid it is still not feasible for the file sharing in this FLIP. > > > > > > Considering a concurrent checkpoint scenario as follows: > > > > > > a. Checkpoint 1 finishes. 1.sst, 2.sst and 3.sst are written in > > > > > > file 1, > > > > > > and 4.sst is written in file 2. > > > > > > b. Checkpoint 2 starts based on checkpoint 1, including 1.sst, > > > > > > 2.sst > > > > > > and 5.sst. > > > > > > c. Checkpoint 3 starts based on checkpoint 1, including 1.sst, > > > > > > 2.sst > > > > > > and 5.sst as well. > > > > > > d. Checkpoint 3 reuses the file 2, TM writes 5.sst on it. > > > > > > e. Checkpoint 2 creates a new file 3, TM writes 5.sst on it. > > > > > > f. Checkpoint 2 finishes, checkpoint 1 is subsumed and the file > > > > > > 2 is > > > > > > deleted, while checkpoint 3 still needs file 2. > > > > > > > > > > > > I attached a diagram to describe the scenario. > > > > > > [image: concurrent cp.jpg] > > > > > > The core issue is that this FLIP introduces a mechanism that allows > > > > > > physical files to be potentially used by the next several > > > > > > checkpoints. JM > > > > > > is uncertain whether there will be a TM continuing to write to a > > > > > > specific > > > > > > file. So in this FLIP, TMs take the responsibility to delete the > > > > > > physical > > > > > > files. > > > > > > > > > > > > (2) IIUC, the RecoverableWriter is introduced to persist data in > > > > > > the "in > > > > > > progress" files after each checkpoint, and the implementation may > > > > > > be based > > > > > > on the file sync in some file systems. However, since the sync is a > > > > > > heavy > > > > > > operation for DFS, this FLIP wants to use flush instead of the sync > > > > > > with > > > > > > the best effort. This only fits the case that the DFS is considered > > > > > > reliable. The problems they want to solve are different. > > > > > > > > > > > > (3) Yes, if files are managed by JM via the shared registry, this > > > > > > problem > > > > > > is solved. And as I mentioned in (1), there are some other corner > > > > > > cases > > > > > > hard to resolve via the shared registry. > > > > > > > > > > > > The goal of this FLIP is to have a common way of merging files in > > > > > > all use > > > > > > cases. For shared state it merges at subtask level, while for > > > > > > private state > > > > > > (and changelog files, as I replied to Yanfei), files are merged at > > > > > > TM > > > > > > level. So it is not contrary to the current plan for the unaligned > > > > > > checkpoint state (FLINK-26803). You are right that the unaligned > > > > > > checkpoint > > > > > > state would be merged with the operator's state file, so overall, > > > > > > it is > > > > > > slightly better than what's currently done. > > > > > > > > > > > > > > > > > > Thanks again for the valuable comments! > > > > > > > > > > > > Best regards, > > > > > > Zakelly > > > > > > > > > > > > > > > > > > > > > > > > On Wed, Apr 5, 2023 at 8:43 PM Piotr Nowojski <pnowoj...@apache.org> > > > > > > wrote: > > > > > > > > > > > > Hi, > > > > > > > > > > > > Thanks for coming up with the proposal, it's definitely valuable. > > > > > > I'm still > > > > > > reading and trying to understand the proposal, but a couple of > > > > > > comments > > > > > > from my side. > > > > > > > > > > > > (1) > > > > > > > Ownership of a single checkpoint file is transferred to TM, while > > > > > > > JM > > > > > > manages the parent directory of these files. > > > > > > > > > > > > Have you seen https://issues.apache.org/jira/browse/FLINK-24611 > > > > > > before? I > > > > > > don't fully remember why, but we have rejected the idea of moving > > > > > > the file > > > > > > ownership to TM and instead reworked the shared file registry in a > > > > > > way that > > > > > > I think should be sufficient for file sharing. Could you elaborate > > > > > > why we > > > > > > need to move the file ownership to TM, and why is the current > > > > > > mechanism not > > > > > > sufficient? > > > > > > > > > > > > (2) > > > > > > > File visibility is needed when a Flink job recovers after a > > > > > > > checkpoint is > > > > > > materialized. In some DFS, such as most object storages, a file is > > > > > > only > > > > > > visible after it is closed > > > > > > > > > > > > Is that really the case? > > > > > > `org.apache.flink.core.fs.FileSystem#createRecoverableWriter` seems > > > > > > to be > > > > > > addressing exactly this issue, and the most frequently used > > > > > > FileSystem (S3) > > > > > > AFAIK supports it with no problems? > > > > > > > > > > > > (3) > > > > > > > 4.1.2 Merge files within a subtask or a TM > > > > > > > Given that TMs are reassigned after restoration, it is difficult > > > > > > > to > > > > > > manage physical files that contain data from multiple subtasks > > > > > > scattered > > > > > > across different TMs (as depicted in Fig.3). There is no > > > > > > synchronization > > > > > > mechanism between TMs, making file management in this scenario > > > > > > challenging. > > > > > > > > > > > > I think this is solved in many places already via the shared state > > > > > > managed > > > > > > by the JM, as I mentioned in (1). > > > > > > > > > > > > > > > > > > If I understand it correctly you are proposing to have a common > > > > > > interface/way of merging small files, in all use cases, that would > > > > > > work > > > > > > only across a single subtask? That's contrary to what's currently > > > > > > done for > > > > > > unaligned checkpoints, right? But if this generic mechanism was to > > > > > > be used > > > > > > for unaligned checkpoints, unaligned checkpoint state would have > > > > > > been > > > > > > merged with the operators state file, so all in all there would be > > > > > > no > > > > > > regression visible to a user? The limit is that we always have at > > > > > > least a > > > > > > single file per subtask, but in exchange we are getting a simpler > > > > > > threading > > > > > > model? > > > > > > > > > > > > Bets, > > > > > > Piotrek > > > > > > > > > > > > wt., 4 kwi 2023 o 08:51 Zakelly Lan <zakelly....@gmail.com> > > > > > > napisał(a): > > > > > > > > > > > > > Hi Yanfei, > > > > > > > > > > > > > > Thank you for your prompt response. > > > > > > > > > > > > > > I agree that managing (deleting) only some folders with JM can > > > > > > > greatly > > > > > > > relieve JM's burden. Thanks for pointing this out. > > > > > > > > > > > > > > In general, merging at the TM level is more effective since there > > > > > > > are > > > > > > > usually more files to merge. Therefore, I believe it is better to > > > > > > > merge files per TM as much as possible. However, for shared > > > > > > > state, > > > > > > > merging at the subtask level is the best choice to prevent > > > > > > > significant > > > > > > > data transfer over the network after restoring. I think it is > > > > > > > better > > > > > > > to keep these merging granularities for different types of files > > > > > > > as > > > > > > > presets that are not configurable. WDYT? > > > > > > > > > > > > > > As for the DSTL files, they are merged per TM and placed in the > > > > > > > task-owned folder. These files can be classified as shared state > > > > > > > since > > > > > > > they are shared across checkpoints. However, the DSTL file is a > > > > > > > special case that will be subsumed by the first checkpoint of the > > > > > > > newly restored job. Therefore, there is no need for new TMs to > > > > > > > keep > > > > > > > these files after the old checkpoint is subsumed, just like the > > > > > > > private state files. Thus, it is feasible to merge DSTL files per > > > > > > > TM > > > > > > > without introducing complex file management across job attempts. > > > > > > > So > > > > > > > the possible performance degradation is avoided. > > > > > > > > > > > > > > The three newly introduced options have recommended defaults. For > > > > > > > upcoming versions, this feature is turned off by default. For the > > > > > > > second option, SEGMENTED_ACROSS_CP_BOUNDARY is the recommended > > > > > > > default > > > > > > > as it is more effective. Of course, if encountering some DFS that > > > > > > > does > > > > > > > not support file visibility until the file is closed, it is > > > > > > > possible > > > > > > > to fall back to another option automatically. For the third > > > > > > > option, > > > > > > > 64MB is an acceptable target size. The RocksDB state backend in > > > > > > > Flink > > > > > > > also chooses 64MB as the default target file size. > > > > > > > > > > > > > > > > > > > > > Thank you again for your quick response. > > > > > > > > > > > > > > > > > > > > > Best regards, > > > > > > > Zakelly > > > > > > > > > > > > > > > > > > > > > On Mon, Apr 3, 2023 at 11:27 PM Yanfei Lei <fredia...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > > Hi Zakelly, > > > > > > > > > > > > > > > > Thanks for driving this, this proposal enables the files > > > > > > > > merging of > > > > > > > > different types of states to be grouped under a unified > > > > > > > > framework. I > > > > > > > > think it has the added benefit of lightening the load on JM. As > > > > > > > > FLINK-26590[1] described, triggered checkpoints can be delayed > > > > > > > > by > > > > > > > > discarding shared state when JM manages a large number of > > > > > > > > files. After > > > > > > > > this FLIP, JM only needs to manage some folders, which greatly > > > > > > > > reduces > > > > > > > > the burden on JM. > > > > > > > > > > > > > > > > In Section 4.1, two types of merging granularities(per subtask > > > > > > > > and per > > > > > > > > task manager) are proposed, the shared state is managed by per > > > > > > > > subtask > > > > > > > > granularity, but for the changelog state backend, its DSTL > > > > > > > > files are > > > > > > > > shared between checkpoints, and are currently merged in batches > > > > > > > > at the > > > > > > > > task manager level. When merging with the > > > > > > > > SEGMENTED_WITHIN_CP_BOUNDARY > > > > > > > > mode, I'm concerned about the performance degradation of its > > > > > > > > merging, > > > > > > > > hence I wonder if the merge granularities are configurable? > > > > > > > > Further, > > > > > > > > from a user perspective, three new options are introduced in > > > > > > > > this > > > > > > > > FLIP, do they have recommended defaults? > > > > > > > > > > > > > > > > > > > > > > > > [1] https://issues.apache.org/jira/browse/FLINK-26590 > > > > > > > > > > > > > > > > Best, > > > > > > > > Yanfei > > > > > > > > > > > > > > > > Zakelly Lan <zakelly....@gmail.com> 于2023年4月3日周一 18:36写道： > > > > > > > > > > > > > > > > > > > > > > > > > > Hi everyone, > > > > > > > > > > > > > > > > > > I would like to open a discussion on providing a unified file > > > > > > > > > merging > > > > > > > > > mechanism for checkpoints[1]. > > > > > > > > > > > > > > > > > > Currently, many files are uploaded to the DFS during > > > > > > > > > checkpoints, > > > > > > > > > leading to the 'file flood' problem when running > > > > > > > > > intensive workloads in a cluster. To tackle this problem, > > > > > > > > > various > > > > > > > > > solutions have been proposed for different types > > > > > > > > > of state files. Although these methods are similar, they lack > > > > > > > > > a > > > > > > > > > systematic view and approach. We believe that it is > > > > > > > > > better to consider this problem as a whole and introduce a > > > > > > > > > unified > > > > > > > > > framework to address the file flood problem for > > > > > > > > > all types of state files. A POC has been implemented based on > > > > > > > > > current > > > > > > > > > FLIP design, and the test results are promising. > > > > > > > > > > > > > > > > > > > > > > > > > > > Looking forward to your comments or feedback. > > > > > > > > > > > > > > > > > > Best regards, > > > > > > > > > Zakelly > > > > > > > > > > > > > > > > > > [1] > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/FLINK/FLIP-306%3A+Unified+File+Merging+Mechanism+for+Checkpoints > > > > > > > > > > > > > > > > > > >
