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Nvme-Based Caching in Video-Delivery Cdns NVMe-Based Caching In Video-Delivery CDNs Thomas Colonna 1901942 Double Degree INSA Rennes Supervisor: Sébastien Lafond Faculty of Science and Engineering Åbo Akademi University 2020 In HTTP-based video-delivery CDNs (content delivery networks), a critical compo- nent is caching servers that serve clients with content obtained from an origin server. These caches store the content they obtain in RAM or onto disks for serving additional clients without fetching them from the origin. For most use cases, access to the disk remains the limiting factor, thus requiring a significant amount of RAM to avoid these accesses and achieve good performance, but increasing the cost. In this master’s thesis, we benchmark various approaches to provide storage such as regular disks and NVMe-based SSDs. Based on these insights, we design a caching mod- ule for a web server relying on kernel-bypass, implemented using the reference framework SPDK. The outcome of the master’s thesis is a caching module leveraging specific proper- ties of NVMe disks, and benchmark results for the various types of disks with the two approaches to caching (i.e., regular filesystem based or NVMe-specific). Contents 1 Introduction 1 2 Background 3 2.1 Caching in the context of CDNs . .3 2.2 Performances of the different disk models . .4 2.2.1 Hard-Disk Drive . .4 2.2.2 Random-Access Memory . .5 2.2.3 Solid-State Drive . .6 2.2.4 Non-Volatile Main Memory . .6 2.2.5 Performance comparison of 2019-2020 storage devices . .6 2.3 Analysing Nginx . .7 2.3.1 Event processing . .7 2.3.2 Caching with Nginx . .7 2.3.3 Overhead . .8 2.3.4 Kernel bypasses . .8 3 Related Work 11 3.1 Kernel bypass for network . 11 3.2 Kernel bypass for storage . 11 3.3 Optimizing the existing access to the storage . 11 3.4 Optimizing the application . 12 3.5 Optimizing the CDN . 12 4 Technical Choices 13 4.1 First measurements . 13 4.2 Data visualization . 13 4.3 Disks performances comparison . 14 5 NVMe-specific caching module 21 5.1 Architecture of the module . 21 5.1.1 Asynchronous programming . 21 5.2 Integration with Nginx . 23 5.3 Experimental protocol . 24 5.4 Results . 26 6 Conclusion 27 6.1 Future work . 27 1. Introduction Broadpeak is a company created in 2010, designing and providing components for con- tent delivery networks. The company focuses on content delivery networks for video content, like IPTV, Cable and on demand video. Broadpeak provides it services to con- tent providers and internet services providers like Orange. The classical ways of consuming videos and films, broadcasts and DVDs have be- come less popular since the emergence of services providing on-demand video via the internet. To provide this kind of service, some technologies have been created, such as the HLS [1] and DASH [2] protocols. These protocols were developed using norms and protocols already existing for the web, allowing video streaming to use a large part of the infrastructure created for the web. HLS and DASH contributed to popularizing the on- demand video streaming by allowing a large number of devices (smartphones, laptops, television...) to access the system and by creating a standard ensuring the interoperability between all the devices and servers in the network. Now, the focus is on creating systems able to manage the increasing number of users switching from traditional television to OTT (Over The Top) streaming, because of the advantages of this technology: choice, replay, time-shifting... The example of the con- tainment because of the COVID-19 shows the importance of having optimized and scal- able systems to be able to manage the huge demand for content. In the context of the video distribution, one key element is the Content Delivery Network (CDN). The role of the CDN is to deliver the content once the user is connected to it. Because video con- tent is huge (e.g., 1.8 GB for an hour of video delivered at 4 Mbps), the pressure on the CDN throughput is higher for video than for any other content. Note that this trend is reinforcing as shifting to large screens and Ultra HD means that video bitrate is doubled. In addition, video CDNs have stringent performance requirements. Indeed, a CDN must be able to store this huge amount of data and at the same time answer the requests of the users at a constant rate. The constant rate is important, because even the slightest fluctu- ation can lead to a pause in the reading for one user or more, which has a huge negative impact on the user experience. Video Streaming relies heavily on I/O operations. I/O operations are the bottleneck of 1 a Content Delivery Network (CDN). To increase the performance of the system, we can increase the number of servers in the CDN and, thus, its cost and energy consumption. Improving the I/O performances allows reducing the number of servers without losing performances, leading to less cost, and less energy consumption. Concerning the I/O, with the evolution of storage device technology, the software overhead, negligible until now, is becoming huge compared to the time spent in the disk, especially the overhead related to the isolation between the kernel and user-space. This isolation requires that every time a storage system is accessed, a context switch is done [3]. The POSIX API used to access files is also an element that slows down the process. In this master’s thesis, we analyze the performance of a streaming server using stan- dard modules of Nginx. We show the time distribution between the kernel and the disk, and compare it to a configuration where an NVMe disk is used to illustrate the impact of the overhead (Chapter 4). We present a new Nginx caching module using SPDK [4], aiming to eliminate this overhead when using NVMe disks (Chapter 5). The design of our module is similar to EvFS [5], but we specialized it for caching purposes, and we do not expose a POSIX API. We moved the API to interact with the storage device outside the kernel, reducing the overhead due to the file system API. We also designed it knowing the specific workload of the caching server to maximize the lifetime of the flash memory of the disks. There are several projects aiming to achieve a similar objective: Bluestore [6] and NVMeDirect [7] are two examples. 2 2. Background 2.1 Caching in the context of CDNs A Content Delivery Network (CDN) is a network composed of servers located in various geographical places [8]. Its purpose is to quickly deliver content such as web pages, files, videos... The infrastructure of a CDN is conceived to reduce the network load and the server load, leading to an improvement of the performances of the applications using it [9]. The infrastructure of a CDN heavily relies on the principle of caching. Web caching is a technology developed to allow websites and applications to face the increasing number of users of the internet [11, 10, 12, 13]. Web caching can reduce the bandwidth consump- tion, improve the load-balancing, reduce network latency and provide higher availability. To achieve caching we need at least two servers, the first one, the origin and the second one the caching server. The principle is that when a user requests some content to the server, the request first goes through the caching server to check if the content demanded is already in the cache. If it is, the caching server provides the content to the user. If not, the caching server requests the content to the origin server that stores the data, provides it to the user and keeps a copy of the content for the next time someone requests this content, as illustrated in figure 2.1. Using the same principle, a CDN is composed of one or several origin servers which store the content to be delivered, and multiples caching all around the geographical area it delivers, as shown in figure 2.2. When a user requests a content to the CDN the request goes through the closest caching server. Thus, the latency is greatly reduced, because the data does not need to travel all around the world, and the workload is spread between all the servers. This infrastructure is largely used by the industry because of its efficiency, its relia- bility and its performances. However, in some cases, this is not enough, like in a video CDN. In the context of a video CDN, objects are large, so the request payload is larger and the number of requests is reduced. Since there are fewer requests to serve, the load of 3 Figure 2.1: caching diagram. the CPU is reduced. Because the content to deliver is larger, a video CDN requires more storage capacity. To improve the performances of these systems we can leverage on the technology used to store the data. 2.2 Performances of the different disk models 2.2.1 Hard-Disk Drive The Hard-Disk Drive (HDD) is a permanent storage technology made of rotating magnetic disks that store the information. To read or write data with this kind of storage, the disk needs to place the reading head on the correct location and start to read the data. The main advantage of this kind of storage is that it is a well-known technology and we can produce devices that can store a huge amount of the information. HDDs have the best capacity/price ratio making it appealing for use in caching servers.
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