Read an interesting article (Ex-Apple engineers raise … data storage startup) and research paper (Git is for data) about a of group of ML engineers from Apple forming a new “data storage” startup targeted at MLOps teams just like Apple. It turns out that MLops has some very unique data requirements that go way beyond just data storage.
The paper discusses some of the unusual data requirements for MLOps such as:
- Infrequent updates – yes there are some MLOps datasets where updates are streamed in but the vast majority of MLOps datasets are updated on a slower cadence. The authors think monthly works for most MLOps teams
- Small changes/lots of copies – The changes to MLOps data are relatively small compared to the overall dataset size and usually consist of data additions, record deletions, label updates, etc. But uncommon to most data, MLOps data are often subsetted or extracted into smaller datasets used for testing, experimentation and other “off-label” activities.
- Variety of file types – depending on the application domain, MLOps file types range all over the place. But there’s often a lot of CSV files in combination with text, images, audio, and semi-structured data (DICOM, FASTQ, sensor streams, etc.). However within a single domain, MLOps file types are pretty much all the same.
- Variety of file directory trees – this is very MLOps team and model dependent. Usually there are train/validate/test splits to every MLOps dataset but what’s underneath each of these can vary a lot and needs to be user customizable.
- Data often requires pre-processing to be cleansed and made into something appropriate and more useable by ML models
- Code and data must co-evolve together, over time – as data changes, the code that uses them change. Adding more data may not cause changes to code but models are constantly under scrutiny to improve performance, accuracy or remove biases. Bias elimination often requires data changes but code changes may also be needed.
It’s that last requirement, MLOps data and code must co-evolve and thus, need to be versioned together that’s most unusual. Data-code co-evolution is needed for reproducibility, rollback and QA but also for many other reasons as well.
In the paper they show a typical MLOps data pipeline.
Versioning can also provide data (and code) provenance, identifying the origin of data (and code). MLOps teams undergoing continuous integration need to know where data and code came from and who changed them. And as most MLOps teams collaborate in the development, they also need a way to identify data and code conflicts when multiple changes occur to the same artifact.
Source version control
Code has had this versioning problem forever and the solution became revision control systems (RCS) or source version control (SVC) systems. The most popular solutions for code RCS are Git (software) and GitHub (SaaS). Both provide repositories and source code version control (clone, checkout, diff, add/merge, commit, etc.) as well as a number of other features that enable teams of developers to collaborate on code development.
The only thing holding Git/GitHub back from being the answer to MLOps data and code version control is that they don’t handle large (>1MB) files very well.
The solution seems to be adding better data handling capabilities to Git or GitHub. And that’s what XetHub has created for Git.
XetHub’s “Git is Data” paper (see link above) explains what they do in much detail, as to how they provide a better data layer to Git, but it boils down to using Git for code versioning and as a metadata database for their deduplicating data store. They are using a Merkle trees to track the chunks of data in a deduped dataset.
How XetHub works
XetHub support (dedupe) variable chunking capabilities for their data store. This allows them to use relatively small files checked into Git to provide the metadata to point to the current (and all) previous versions of data files checked into the system.
Their mean chunk size is ~4KB. Data chunks are stored in their data store. But the manifest for dataset versions is effectively stored in the Git repository.
The paper shows how using a deduplicated data store can support data versioning.
XetHub uses a content addressable store (CAS) to store the file data chunk(s) as objects or BLOBs. The key to getting good IO performance out of such a system is to have small chunks but large objects.
They map data chunks to files using a CDMT (content defined merkle tree[s]). Each chunk of data resides in at least two different CDMTs, one associated with the file version and the other associated with the data storage elements.
XetHub’s variable chunking approach is done using a statistical approach and multiple checksums but they also offer one specialized file type chunking for CSV files. As it is, even with their general purpose variable chunking method, they can offer ~9X dedupe ratio for text data (embeddings).
They end up using Git commands for code and data but provide hooks (Git filters) to support data cloning, add/checkin, commits, etc.). So they can take advantage of all the capabilities of Git that have grown up over the years to support code collaborative development but use these for data as well as code.
In addition to normal Git services for code and data, XetHub also offers a read-only, NFSv3 file system interface to XetHub datases. Doing this eliminates having to reconstitute and copy TB of data from their code-data repo to user workstations. With NFSv3 front end access to XetHub data, users can easily incorporate data access for experimentation, testing and other uses.
Results from using XetHub
XetHub showed some benchmarks comparing their solution to GIT LFS, another Git based large data storage solution. For their benchmark, they used the CORD-19 (and ArXiv paper, and Kaggle CORD-I9 dataset) which is a corpus of all COVID-19 papers since COVID started. The corpus is updated daily, released periodically and they used the last 50 versions (up to June 2022) of the research corpus for their benchmark.
Each version of the CORD-19 corpus consists of JSON files (research reports, up to 700K each) and 2 large CSV files one with paper information and the other paper (word?) embeddings (a more useable version of the paper text/tables used for ML modeling).
For CORD-19, XetHub are able to store all the 2.45TB of research reports and CSV files in only 287GB of Git (metadata) and datastore data, or with a dedupe factor 8.7X. With XetHub’s specialized CSV chunking (Xet w/ CSV chunking above), the CORD-19 50 versions can be stored in 87GB or with a 28.8X dedupe ratio. And of that 87GB, only 82GB is data and the rest ~5GB is metadata (of which 1.7GB is the merle tree).
In the paper, they also showed the cost of branching this data by extracting and adding one version which consisted of a 75-25% (random) split of a version. This split was accomplished by changing only the two (paper metadata and paper word embeddings) CSV files. Adding this single split version to their code-data repository/datastore only took an additional 11GB of space An aligned split (only partitioning on a CSV record boundary, unclear but presumably with CSV chunking), only added 185KB.
XETHUB Potential Enhancements
XetHub envisions many enhancements to their solution, including adding other specific file type chunking strategies, adding a “time series” view to their NFS frontend to view code/data versions over time, finer granularity data provenance (at the record level rather than at the change level), and RW NFS access to data. Further, XetHub’s dedupe metadata (on the Git repo) only grows over time, supporting updates and deletes to dedupe metadata would help reduce data requirements.
Read the paper to find out more.
- From Wikipedia Version Control article, By Revision_controlled_project_visualization.svg: *Subversion_project_visualization.svg: Traced by User:Stannered, original by en:User:Sami Keroladerivative work: Moxfyre (talk)derivative work: Echion2 (talk) – Revision_controlled_project_visualization.svg, CC BY-SA 3.0
- From XetHub’s Git is Data paper.
- From XetHub’s Git is Data paper Figure 1.
- From XetHub’s Git is Data paper Figure 3.
- From XetHub’s Git is Data paper Figure 4.