The Xet team at Hugging Face is working on improving the efficiency of the Hub's storage architecture to make it easier and quicker for users to store and update data and models. As Hugging Face hosts nearly 11PB of datasets with Parquet files alone accounting for over 2.2PB of that storage, optimizing Parquet storage is of pretty high priority. 

Most Parquet files are bulk exports from various data analysis pipelines or databases, often appearing as full snapshots rather than incremental updates. Data deduplication becomes critical for efficiency when users want to update their datasets on a regular basis. Only by deduplicating can we store all versions as compactly as possible, without requiring everything to be uploaded again on every update. In an ideal case, we should be able to store every version of a growing dataset with only a little more space than the size of its largest version.

Our default storage algorithm uses byte-level Content-Defined Chunking (CDC), which generally dedupes well over insertions and deletions, but the Parquet layout brings some challenges. Here we run some experiments to see how some simple modifications behave on Parquet files, using a 2GB Parquet file with 1,092,000 rows from the FineWeb dataset and generating visualizations using our dedupe estimator.

Background

Parquet tables work by splitting the table into row groups, each with a fixed number of rows (for instance 1000 rows). Each column within the row group is then compressed and stored:

Intuitively, this means that operations which do not mess with the row grouping, like modifications or appends, should dedupe pretty well. So let's test this out!

Append

Here we append 10,000 new rows to the file and compare the results with the original version. Green represents all deduped blocks, red represents all new blocks, and shades in between show different levels of deduplication.

We can see that indeed we are able to dedupe nearly the entire file, but only with changes seen at the end of the file. The new file is 99.1% deduped, requiring only 20MB of additional storage. This matches our intuition pretty well.

Modification

Given the layout, we would expect that row modifications to be pretty isolated, but this is apparently not the case. Here we make a small modification to row 10000, and we see that while most of the file does dedupe, there are many small regularly spaced sections of new data!

A quick scan of the Parquet file format suggests that absolute file offsets are part of the Parquet column headers (see the structures ColumnChunk and ColumnMetaData)! This means that any modification is likely to rewrite all the Column headers. So while the data does dedupe well (it is mostly green), we get new bytes in every column header. 

In this case, the new file is only 89% deduped, requiring 230MB of additional storage.

Deletion

Here we delete a row from the middle of the file (note: insertion should have similar behavior). As this reorganizes the entire row group layout (each row group is 1000 rows), we see that while we dedupe the first half of the file, the remaining file has completely new blocks.

This is mostly because the Parquet format compresses each column aggressively. If we turn off compression we are able to dedupe more aggressively:

However the file sizes are nearly 2x larger if we store the data uncompressed. 

Is it possible to have the benefit of dedupe and compression at the same time?

Content-Defined Row Groups

One potential solution is to use not only byte-level CDC, but also apply it at the row level: we split row groups not based on absolute count (1000 rows), but on a hash of a provided “Key” column. In other words, I split off a row group whenever the hash of the key column % [target row count] = 0, with some allowances for a minimum and a maximum row group size. 

I hacked up a quick inefficient experimental demonstration here.

With this, we are able to efficiently dedupe across compressed Parquet files even as I delete a row. Here we clearly see a big red block representing the rewritten row group, followed by a small change for every column header.

Optimizing Parquet for Dedupe-ability

Based on these experiments, we could consider improving Parquet file dedupe-ability in a couple of ways:

1. Use relative offsets instead of absolute offsets for file structure data. This would make the Parquet structures position independent and easy to “memcpy” around, although it is an involving file format change that is probably difficult to do.

2. Support content defined chunking on row groups. The format actually supports this today as it does not require row groups to be uniformly sized, so this can be done with minimal blast radius. Only Parquet format writers would have to be updated.

While we will continue exploring ways to improve Parquet storage performance (Ex: perhaps we could optionally rewrite Parquet files before uploading? Strip absolute file offsets on upload and restore on download?), we would love to work with the Apache Arrow project to see if there is interest in implementing some of these ideas in the Parquet / Arrow code base.

In the meantime, we are also exploring the behavior of our data dedupe process on other common filetypes. Please do try our dedupe estimator and tell us about your findings!

This post was originally published on the Hugging Face blog. Follow our team on Hugging Face to keep up with all new updates!