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An Efficient Distributed File/Blob/Key-Value Store for Billions of Small Files

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Pomegranate File System Documentation

It is a distributed file system, but not only a file system!

Wiki Page

Introduction

Pomegranate File System (abbr. PFS) is originally proposed for large scale small file access. It contains many optimizations for small objects.

  • Automatic small file aggregation based on file system directory
  • Tabular directory model, support metadata deduplication
  • Automatic migrating file creations in a cluster
  • Metadata store and small file data store is designed for flash device
  • Support POSIX, REST interface
  • Has C/Python bindings

Architecture

To exploit fast storage devices to accelerate small file performace, e.g. SSD, PFS adopts a 3-tier storage architecture.

The first tier is memory caching layer, which is used for metadata caching to reduce metadata latency. Metadata latency has significant impacts on small file I/O latency. Decreasing metadata latency can efficient improve the small file performace.

The second tier is flash caching layer, which is used for durability of metadata and small data. Flash device has lower I/O latency. Thus, it is suitable for small data access.

The third tier is disk store layer, which is designed for longer durability of all data. It use data replication for data reliability and deduplication for efficient space consumption.

Tabular Directory Model

In many Web 2.0 applications, objects (e.g. photos, videos, docs, ...) are saved in several different forms. For example, in a photo gallery web site, photoes that updated by users are transformed to several resolutions. These different object forms that derived from the same (original) object contains almost the same metadata. Thus, if we save these different forms into different files, then we would have many metadata duplication in distributed file system. We define this issue as N-Form issue.

To overcome the above N-Forms issue, we propose to introduce powerful directory model to traditional file system. In PFS, we use tabular directory model to keep file system metadata. With one file name, users can save many different object forms in different columns' cells. File metadata is a special table column of the directory table.

By adopting tabular directory model, the metadata duplication of N-Form issue can be overcomed. Besides this benefit, the new directory model grouped the file data which has the same property or usage purpose in the same column. Thus, we can do more efficient file placements and aggregations.

File Aggregation

In Web 2.0 applications, objects are mainly in small size. For example, social network web pages contain many small sized photoes and short video segments. The typical size of these objects are less than 10MB. Many traditional distributed file systems are designed for HPC applications, which targets at large file I/O optimization. Thus, for small files, many of these I/O optimizations are not as efficient as that for large files.

To optimize small file I/O, we propose to do file aggregation based on tabular directory model. For files that in the same directory, we do file aggregations automatically. For each directory column, we generate an aggregated large file. File content is cached and then write sequentially to low level SSD. File aggregation can maximally utilize low level I/O bandwidth.

Extendible Metadata Service

There are so many objects to store in Web 2.0 applications. User generated objects, such as uploaded photoes, videos, documents, are tremendous. To manage these massive objects in a file system means that we need a expandable metadata service.

In PFS, we exploid the extendible hash technology to distribute file metadata across many cache servers. Metadata can migrate from one server to other server when there are too many cached file entries. The cache server can be add in or remove out at any time with little latency. File metadata is redistributed automatically on server changes.

Development Cycle

A new OBJECT STORE LAYER for large files is under developing.

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