Thanks Weston for posting here! I appreciate this a lot, as it gives us the opportunity to discuss modern formats in depth with the authors themselves, who probably know the design trade-offs they took best and thus can give us a deeper understanding what certain features would mean for Parquet.
I read both your linked posts. I read them with the mindset as if they were the documentation for a file format that I myself would need to add to our engine, so I always double checked whether I would agree with your reasoning and where I would see problems in the implementation. I ended up with some points where I cannot follow your reasoning, yet, or where I feel clarification would be good. It would be nice if you could go a bit into detail here: Regarding your "parallelism without row groups" post [2]: 1. Do I understand correctly that you basically replace row groups with files. Thus, the task for reading row groups in parallel boils down to reading files in parallel. Your post does *not* claim that the new format would be able to parallelize *inside* a row group/file, correct? 2. I do not fully understand what the proposed parallelism has to do with the file format. As you mention yourself, files and row groups are basically the same thing. As such, couldn't you do the same "Decode Based Parallelism" also with Parquet as it is today? E.g., the file reader in our engine looks basically exactly like what you propose, employing what you call Mini Batches and not reading a whole row group as a whole (which could lead to out of memory in case a row group contains an insane amount of rows, so it is a big no no anyway for us). It seems that the shortcomings of the code listed in "Our First Parallel File Reader" is solely a shortcoming of that code, not of the underlying format. Regarding [1]: 3. This one is mostly about understanding your rationales: As one main argument for abolishing row groups, you mention that sizing them well is hard (I fully agree!). But since you replace row groups with files, don't you have the same problem for the file again? Small row groups/files are bad due to small I/O requests and metadata explosion, agree! So let's use bigger ones. Here you argue that Parquet readers will load the whole row group into memory and therefore suffer memory issues. This is a strawman IMHO, as this is just a shortcoming of the reader, not of the format. Nothing in the Parquet spec forces a reader to read a row group at once (and in fact, our implementation doesn't do this for exactly the reasons you mentioned). Just like in LanceV2, Parquet readers can opt to read only a few pages ahead of the decoding. On the writing side, I see your point that a Lance V2 writer never has to buffer more than a page and this is great! However, this seems to be just a result of allowing pages to not be contiguous, not of the fact that row groups were abolished. You could still support multiple row groups with non-contiguous pages and reap all the benefits you mention. Your post intermingles the two design choices "contiguous pages yes/no" and "row groups as horizontal partitions within a file yes/no". I would argue that the two features are basically fully orthogonal. You can have one without the other and vice versa. So all in all, do I see correctly that your main argument here basically is "don't force pages to be contiguous!". Doing away with row groups is just added bonus for easier maintenance, as you can just use files instead of row groups. 4. Considering contiguous pages and I/O granularity: The format basically proposes to have pages as the only granularity below a file (+ metadata & footer), while Parquet has two granularities: Row group, or rather Column Chunk, and Page. You argue that a page in Lance V2 should basically be as big as is necessary for good I/O performance (say, 8 MiB for Amazon S3). Thus, the Parquet counterpart of a Lance v2 page would actually be - at least in terms of I/O efficiency - a Parquet Column Chunk. A Parquet page can instead be quite small, as it does not need to be the grain of the I/O but just the grain of the encoding. The fact that Parquet has these two grains has advantages when considering a scan vs. a point look-up. When doing a scan, we can load whole column chunks at once, having large I/O requests to not overwhelm the I/O with too many requests. When doing a point access, we can use the page & offset index to find and load only the one page (per column) in which the row we are looking for is located. As such, it seems that the two grains that Parquet has benefit us, as they give us flexibility of both being able to scan with large requests and doing point accesses without too much read amplification by using small single-page requests. With Lance V2, either I make large pages to make scans take fewer I/O requests (e.g., 8 MiB), but then I will have large read amplification for point accesses, or I make my pages quite small to benefit point accesses, but then scans will need to emit tons of I/O operations, which is what you are trying to avoid. How does Lance V2 solve this challenge? Or did I understand the format wrong here? Cheers, Jan Am Di., 21. Mai 2024 um 18:07 Uhr schrieb Weston Pace <weston.p...@gmail.com >: > As the author of one of these new formats I'll chime in. The main issues I > have with parquet are: > > A. Pages in a column chunk must be contiguous (this is Lance's biggest > issue with parquet) > B. Encodings should be extensible > C. Flexibility in what is considered data / metadata > > I outline my reasoning for these in [1] and so I'll avoid repeating that > here. I think B has been discussed pretty thoroughly in this thread. > > As for C, a format should be flexible, and then it is pretty > straightforward. If a file is likely to be used in "search" (very > selective filters, ability to cache, etc.) then lots of data should be put > in the column metadata. If the file is mostly for cold full scans then > almost nothing should go in column metadata (either don't write the > metadata at all or, I guess, you can put it in the data pages). The format > shouldn't force a choice. > > Personally, I am more excited about A than I am about B & C (though I do > think both B & C should be addressed if going through the trouble of a new > format). Addressing A lets us get rid of row groups, allows for APIs such > as "array-at-a-time writing", lets us make large data pages, and generally > leads to more foolproof files. > > I agree with Andrew that any discussion of B & C is usually based on > assumptions rather than concrete measurements of reader performance. In > the scattered profiling I've done of parquet-cpp and parquet-rs I've found > that poor parquet reader performance typically has very little to do with B > & C. Actually, I would guess that the most widespread (though not > necessarily most important) obstacle to parquet has been user knowledge. > To get the best performance from a reader users need to be familiar not > just with the format but also with the features available in a particular > reader. I think simplifying the user experience should be a secondary goal > for any new changes. > > At the risk of arrogant self-promotion I would recommend people read [1] > for inspiration if nothing else. I'm also hoping to detail design > decisions and tradeoffs that we come across (starting in [2] and continuing > throughout the summer). > > [1] https://blog.lancedb.com/lance-v2/ > [2] > > https://blog.lancedb.com/file-readers-in-depth-parallelism-without-row-groups/ > > On Mon, May 20, 2024 at 11:06 AM Parth Chandra <par...@apache.org> wrote: > > > Hi Parquet team, > > > > It is very exciting to see this effort. Thanks Micah for starting this. > > > > For most use case that our team sees the broad areas for improvement > > appear to be - > > 1) Optimizing for cloud storage (latency is high, seeks are expensive) > > 2) Optimized metadata reading - we've seen 30% (sometimes more) of > > Spark's scan operator time spent in reading footers. > > 3) Anything that improves support for data lakes. > > > > Also I'll be happy to help wherever I can. > > > > Parth > > > > On Sun, May 19, 2024 at 10:59 AM Xinli shang <sha...@uber.com.invalid> > > wrote: > > > > > Sorry I am late to the party! It's great to see this discussion! Thank > > you > > > everyone for the many good points and thank you, Micah, for starting > the > > > discussion and putting it together into a document, which is very > > helpful! > > > I agree with most of the points we discussed above, and we need to > > improve > > > Parquet and sometimes even speed up to catch up with industry changes. > > > > > > With that said, we need people to work on it, as Julien mentioned. The > > > document > > > < > > > > > > https://docs.google.com/document/d/19hQLYcU5_r5nJB7GtnjfODLlSDiNS24GXAtKg9b0_ls/edit > > > > > > > that Micah created covers pretty much everything we discussed here. I > > > encourage all of us to contribute by raising questions, providing > > > suggestions, adding missing functionality, etc. Once we reach a > consensus > > > on each topic, we can create different tracks and working streams to > kick > > > off the implementations. > > > > > > I believe continuously improving Parquet would benefit the industry > more > > > than creating a new format, which could add friction. These improvement > > > ideas are exciting opportunities. If you, your team members, or friends > > > have time and interest, please encourage them to contribute. > > > > > > Our Parquet community meeting is next week, on May 28, 2024. We can > have > > > discussions there if you can join. Currently, it is scheduled for 7:00 > am > > > PDT, but I can change it according to the majority's availability. > > > > > > On Fri, May 17, 2024 at 3:58 PM Rok Mihevc <rok.mih...@gmail.com> > wrote: > > > > > > > Hi all, > > > > > > > > I've discussed with my colleagues and we would dedicate two engineers > > for > > > > 4-6 months on tasks related to implementing the format changes. We're > > > > already active in design discussions and can help with C++, Rust and > C# > > > > implementations. I thought it'd be good to state this explicitly > FWIW. > > > > > > > > Our main areas of interest are efficient reads for tables with wide > > > schemas > > > > and faster random rowgroup access [1]. > > > > > > > > To workaround the wide schemas issue we actually implemented an > > internal > > > > tool [3] for storing index information into a separate file which > > allows > > > > for reading only the necessary subset of metadata. We would offer > this > > > > approach for consideration as a possible approach to solve the wide > > > schema > > > > problem. > > > > > > > > [1] https://github.com/apache/arrow/issues/39676 > > > > [2] https://github.com/G-Research/PalletJack > > > > > > > > Rok > > > > > > > > On Sun, May 12, 2024 at 12:59 AM Micah Kornfield < > > emkornfi...@gmail.com> > > > > wrote: > > > > > > > > > Hi Parquet Dev, > > > > > I wanted to start a conversation within the community about working > > on > > > a > > > > > new revision of Parquet. For context there have been a bunch of > new > > > > > formats [1][2][3] that show there is decent room for improvement > > across > > > > > data encodings and how metadata is organized. > > > > > > > > > > Specifically, in a new format revision I think we should be > thinking > > > > about > > > > > the following areas for improvements: > > > > > 1. More efficient encodings that allow for data skipping and SIMD > > > > > optimizations. > > > > > 2. More efficient metadata handling for deserialization and > > projection > > > > to > > > > > address areas when metadata deserialization time is not trivial > [4]. > > > > > 3. Possibly thinking about different encodings instead of > > > > > repetition/definition for repeated and nested field > > > > > 4. Support for optimizing semi-structured data (e.g. JSON or > Variant > > > > type) > > > > > that can shred elements into individual columns (a recent thread in > > > > Iceberg > > > > > mentions doing this at the metadata level [5]) > > > > > > > > > > I think the goals of V3 would be to provide existing API > > compatibility > > > as > > > > > broadly as possible (possibly with some performance loss) and > expose > > > new > > > > > API surface areas where appropriate to make use of new elements. > New > > > > > encodings could be backported so they can be made use of without > > > metadata > > > > > changes. I think unfortunately that for points 2 and 3 we would > want > > > to > > > > > break file level compatibility. More thought would be needed to > > > consider > > > > > whether 4 could be backported effectively. > > > > > > > > > > This is a non-trivial amount of work to get good coverage across > > > > > implementations, so before putting together more formal proposal it > > > would > > > > > be nice to know if: > > > > > > > > > > 1. If there is an appetite in the general community to consider > > these > > > > > changes > > > > > 2. If anybody from the community is interested in collaborating on > > > > > proposals/implementation in this area. > > > > > > > > > > Thanks, > > > > > Micah > > > > > > > > > > [1] https://github.com/maxi-k/btrblocks > > > > > [2] https://github.com/facebookincubator/nimble > > > > > [3] https://blog.lancedb.com/lance-v2/ > > > > > [4] https://github.com/apache/arrow/issues/39676 > > > > > [5] > https://lists.apache.org/thread/xnyo1k66dxh0ffpg7j9f04xgos0kwc34 > > > > > > > > > > > > > > > > > > -- > > > Xinli Shang > > > > > >
