Keyun Cheng

Kurma: Secure Geo-Distributed Multi-Cloud Storage Gateways

Download

SYSTOR, 2019

Summary

This paper introduces Kruma, a cloud storage gateway system which allows applications requiring NAS as the storage interface. Kruma replicates metadata across geo-distributed gateways and maintains a unified file-system namespace. Kruma stores encrypted data blocks in clouds and can provide data integrity checking.

Details

Previous works

Selected

• SCFS provides freshness without using Merkle trees, but requires a trusted and centralized metadata service running on a cloud
• Hybris targets for KV store and with only one gateway

Novelty of the paper

System Architecture

• Designed for enterprise uses

• Geo-distributed storage gateways, each for one office/department

Features

• Support replication, EC, secret sharing

• Geo-distributed storage gateways, each for one office/department
  • Metadata are stored in trusted gateways
  • Each Kurma gateway maintains a copy of the whole filesystem metadata
  • Metadata changes made by a Kurma gateway are asynchronously replicated to all other gateways using Hedwig
  • NFS close-to-open consistency
  • For clients across geo-distributed gateways, Kurma provides FIFO consistency
  • Metadata are stored in ZooKeeper (in-memory store), each gateway maintains a replica of the entire FS metadata in a ZooKeeper instance
• Caching
  • Each gateway has local storage (persistent write-back cache) for cache
  • The cache stores plaintext
  • The cache also maintains additional metadata in stable storage so that dirty data can be recovered after crashes; the metadata includes a list of dirty files and the dirty extents of each file
  • For consistency between local cache and remote data, it compares locally saved remote-time and the latest remote-time from cloud
  • Use Write-back-wait time (WBWT) to sync and async write to cloud
• Security
  • to check data freshness, Kruma broadcasts block version numbers among all gateways.
• File Sharing
  • Metadata changes are broadcasts using Hedwig in a “all-to-all broadcast” manner. Hedwig is a pub-sub system optimized for communication across data-centers, and it protects its communication using SSL.
  • Across gateways, Kurma provides FIFO consistency. E.g. directy operation should be executed before file creation in that directory
  • Kruma add ObjectID to detect conflicts in async operations
• Cloud ranking by latency

Strength

• Designed and targeted for enterprise use

Weakness

• Extra cost for real deployment (multiple gateways)

• For the cache design

  • Write-back-write-time falls back to sync mode when operating over large files