Tuesday, February 20, 2018

Notes from FAST18

I attended the technical sessions of Usenix's File And Storage Technology conference this week. Below the fold, notes on the papers that caught my attention.

I'm a sucker for papers reporting failures in the field at scale such as Fail-Slow at Scale: Evidence of Hardware Performance Faults in Large Production Systems by Haryadi Gunawi and 20 other authors from 12 companies, national labs and University data centers:
Fail-slow hardware is an under-studied failure mode. We present a study of 101 reports of fail-slow hardware incidents, collected from large-scale cluster deployments in 12 institutions. We show that all hardware types such as disk, SSD, CPU, memory and network components can exhibit performance faults.
Unlike previous failure analysis papers at FAST, all this team had to go on was anecdotes from victims, but they managed to extract useful observations:
  • Varying root causes: Fail-slow hardware can be induced by internal causes such as firmware bugs or device errors/wear-outs as well as external factors such as configuration, environment, temperature, and power issues.
  • Faults convert from one form to another: Fail-stop, -partial, and -transient faults can convert to fail-slow faults (e.g., the overhead of frequent error masking of corrupt data can lead to performance degradation).
  • Varying symptoms: Fail-slow behavior can exhibit a permanent slowdown, transient slowdown (up-and-down performance), partial slowdown (degradation of sub-components), and transient stop (e.g., occasional reboots).
  • A long chain of root causes: Fail-slow hardware can be induced by a long chain of causes (e.g., a fan stopped working, making other fans run at maximal speeds, causing heavy vibration that degraded the disk performance).
  • Cascading impacts: A fail-slow hardware can collapse the entire cluster performance; for example, a degraded NIC made many jobs lock task slots/containers in healthy machines, hence new jobs cannot find enough free slots.
  • Rare but deadly (long time to detect): It can take hours to months to pinpoint and isolate a fail-slow hardware due to many reasons (e.g., no full-stack visibility, environment conditions, cascading root causes and impacts).
And make useful suggestions:
  • To vendors: When error masking becomes more frequent (e.g., due to increasing internal faults), more explicit signals should be thrown, rather than running with a high overhead. Device-level performance statistics should be collected and reported (e.g., via S.M.A.R.T) to facilitate further studies.
  • To operators: 39% root causes are external factors, thus troubleshooting fail-slow hardware must be done online. Due to the cascading root causes and impacts, full-stack monitoring is needed. Fail-slow root causes and impacts exhibit some correlation, thus statistical correlation techniques may be useful (with full-stack monitoring).
  • To systems designers: While software systems are effective in handling fail-stop (binary) model, more research is needed to tolerate fail-slow (non-binary) behavior. System architects, designers and developers can fault-inject their systems with all the root causes reported in this paper to evaluate the robustness of their systems.
Reading the paper's litany of horror stories induces a feeling of "there but for the grace of God go I".

Xiaosong Ma of the Qatar Computing Research Institute gave an exemplary presentation of RAID+: Deterministic and Balanced Data Distribution for Large Disk Enclosures by Guangyan Zhang, her, and 4 other authors from Tsinghua University. They addressed the problem of building RAID configurations in large disk enclosures in ways that spread the workload evenly across all the drives:
Unlike systems conducting randomized placement, RAID+ employs deterministic addressing enabled by the mathematical properties of mutually orthogonal Latin squares, based on which it constructs 3-D data templates mapping a logical data volume to uniformly distributed disk blocks across all disks. While the total read/write volume remains unchanged, with or without disk failures, many more disk drives participate in data service and disk reconstruction. Our evaluation with a 60-drive disk enclosure using both synthetic and real-world workloads shows that RAID+ significantly speeds up data recovery while delivering better normal I/O performance and higher multi-tenant system throughput.
Sudoku puzzles are examples of mutually orthogonal Latin squares.

Clay Codes: Moulding MDS Codes to Yield an MSR Code by Myna Vajha et al addressed the use of erasure codes at scale:
To ensure availability of data, failure-tolerance schemes such as Reed-Solomon (RS) or more generally, Maximum Distance Separable (MDS) erasure codes are used. However, while MDS codes offer minimum storage overhead for a given amount of failure tolerance, they do not meet other practical needs of today's data centers.
They built on earlier work by Min Ye and Alexander Barg, two of the other authors, who showed a theoretical construction of a Minimum Storage Regenerating (MSR) code that had the desirable properties of an MDS code. The paper described a practical implementation of this theory:
Clay (short for Coupled-Layer) codes are MSR codes that offer a simplified construction for decoding/repair by using pairwise coupling across multiple stacked layers of any single MDS code. ... Clay codes provide the first practical implementation of an MSR code that offers (a) low storage overhead, (b) simultaneous optimality in terms of three key parameters: repair bandwidth, sub-packetization level and disk I/O, (c) uniform repair performance of data and parity nodes and (d) support for both single and multiple-node repairs, while permitting faster and more efficient repair.
Their code has been implemented in Ceph:
A particular example code, with storage overhead 1.25x, is shown to reduce repair network traffic by a factor of 2.9 in comparison with RS codes and similar reductions are obtained for both repair time and disk read.
As often happens at FAST, The University of Wisconsin - Madison provided one of the two Best Papers, Protocol-Aware Recovery for Consensus-Based Storage by Ramnatthan Alagappan et al:. Their paper at the previous FAST used fault injection to:
characterize eight popular distributed storage systems and uncover numerous bugs related to file-system fault tolerance. We find that modern distributed systems do not consistently use redundancy to recover from file-system faults: a single file-system fault can cause catastrophic outcomes such as data loss, corruption, and unavailability.
This year's paper showed how to fix the problems they revealed last year:
We introduce protocol-aware recovery (PAR), a new approach that exploits protocol-specific knowledge to correctly recover from storage faults in distributed systems. We demonstrate the efficacy of PAR through the design and implementation of corruption-tolerant replication (CTRL), a PAR mechanism specific to replicated state machine (RSM) systems. We experimentally show that the CTRL versions of two systems, LogCabin and ZooKeeper, safely recover from storage faults and provide high availability, while the unmodified versions can lose data or become unavailable. We also show that the CTRL versions have little performance overhead.
The both the details of the failures, based on 6 sample scenarios, and the techniques for avoiding them are fascinating.

Another paper using fault injection was Towards Robust File System Checkers by Om Rameshwar Gatla et al. They write (my emphasis):
File systems may become corrupted for many reasons despite various protection techniques. Therefore, most file systems come with a checker to recover the file system to a consistent state. However, existing checkers are commonly assumed to be able to complete the repair without interruption, which may not be true in practice.
This is an understatement. A very common cause of file system corruption is power failure. The time with the highest probability of power failure is shortly after power is restored, when the checker will be running. The authors:
demonstrate via fault injection experiments that checkers of widely used file systems may leave the file system in an uncorrectable state if the repair procedure is interrupted unexpectedly.
They go on:
To address the problem, we first fix the ordering issue in the undo logging of e2fsck, and then build a general logging library (i.e., rfsck-lib) for strengthening checkers. To demonstrate the practicality, we integrate rfsck-lib with existing checkers and create two new checkers: (1) rfsck-ext, a robust checker for Ext-family file systems, and (2) rfsck-xfs, a robust checker for XFS file system, both of which require only tens of lines of modification to the original versions. Both rfsck-ext and rfsck-xfs are resilient to faults in our experiments. Also, both checkers incur reasonable performance overhead (i.e., up to 12%) comparing to the original unreliable versions. Moreover, rfsck-ext outperforms the patched e2fsck by up to nine times while achieving the same level of robustness.
One reason I like this paper is because I've had ext filesystems rendered unrecoverable by a rapid sequence of power failures!

One last paper I especially liked for its simple yet creative solution was Towards Web-based Delta Synchronization for Cloud Storage Services by He Xiao et al. rsync can vastly improve data transfer rates. They implemented WebRsync, using the rsync protocol in JavaScript so that browser clients for cloud storage services could use it, but:
Our measurements show that WebRsync severely suffers from the inefficiency of JavaScript execution inside web browsers, thus leading to frequent stagnation and even hanging.
The solution is obvious once they explain it:
Given that the computation burden on the web browser mainly stems from data chunk search and comparison, we reverse the traditional delta sync approach by lifting all chunk search and comparison operations from the client side to the server side. Inevitably, this brings considerable computation overhead to the servers. Hence, we further leverage locality-aware chunk matching and lightweight checksum algorithms to reduce the overhead. The resulting solution, WebR2sync+, outpaces WebRsync by an order of magnitude, and is able to simultaneously support 6800--8500 web clients' delta sync using a standard VM server instance based on a Dropbox-like system architecture.

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