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Related work

Related work falls into two categories: first, analyses of traditional UFS-based systems and ways to beat their performance limitations, and second, analyses of the behavior of web proxies and how they can better use the underlying I/O and file systems.

The first set of research extends back to the original FFS work of McKusick et al. [13] which addressed the limitations of the System V file system by introducing larger block sizes, fragments, and cylinder groups. With increasing memory and buffer cache sizes, UNIX file systems were able to satisfy more reads out of memory. The FFS clustering work of McVoy and Kleiman [14] sought to improve write times by lazily writing the data to disk in contiguous extents called clusters. LFS [20] sought to improve write times by packing dirty file blocks together and writing to an on-disk log in large extents called segments. The LFS approach necessitates a cleaner daemon to coalesce live data and free on-disk segments. As well, new on-disk structures are required. Work in soft updates [6] and journalling [7,4] has sought to alleviate the performance limitations due to synchronous meta-data operations, such as file create or delete, which must modify file system structures in a specified order. Soft updates maintains dependency information in kernel memory to order disk updates. Journalling systems write meta-data updates to an auxiliary log using the write-ahead logging protocol. This differs from LFS, in which the log contains all data, including meta-data. LFS also addresses the meta-data update problem by ordering updates within segments.

The Bullet server [26,24] is the file system for Amoeba, a distributed operating system. The Bullet service supports entire file operations to read, create, and delete files. All files are immutable. Each file is stored contiguously, both on disk and in memory. Even though the file system API is similar to Hummingbird, the Bullet service does not perform clustering of files together, so it would not have the same type of performance improvement that Hummingbird has for a caching web proxy workload.

Kaashoek et al. [9] approaches high performance through developing server operating systems, where a server operating system is a set of abstractions and runtime support for specialized, high performance server applications. Their implementation of the Cheetah web server is similar to Hummingbird in one way: collocating an HTML page and its images on disk and reading them from disk as a unit. Web servers' data storage is represented naturally by the UFS file hierarchy. This is not true for caching web proxies as discussed in Section 2.2.

CacheFlow [2] builds a cache operating system called CacheOS with an optimized object storage system which minimizes the number of disk seek and I/O operations per object. Unfortunately, details of the object storage are not public. The Network Appliance filer [8] is a prime example of combination of an operating system and a specialized file system (WAFL) inside a storage appliance. Novell [15] has developed the Cache Object Store (COS) which they state is 10 times more efficient than typical file systems; few details on the design are available. The COS prefetches the components for a page when the page is requested, leading us to believe that the components are not stored contiguously as they are in Hummingbird.

Rousskov and Soloviev [21] studied the performance of Squid and its use of the file system. Markatos et al. [12] presents methods for web proxies to work around costly file system file opens, closes, and deletes. One of their methods, LAZY-READS, gathers read requests $n$-at-a-time, and issues them all at the same time to the disk; results are presented when $n$ is 10. This is similar to our clustering of locality sets, since a read for a cluster will, on average, access 8 files. We feel that Hummingbird in a more general solution to the decreasing the effect of costly file system operations on a web proxy.

Maltzahn et al. [10] compared the disk I/O of Apache and Squid and concluded that they were remarkably similar. In a later paper [11], they simulated the operation of several techniques for modifying Squid, one of which was to use a memory-mapped interface to access small files. Other techniques improved the locality of related files based on domain names. This paper reported a reduction of up to 70% in the number of disk operations relative to unmodified Squid. An inherent problem using one memory-mapped file to access all small objects is that it cannot scale to handle a very large number of objects. Like Hummingbird, using memory-mapped files requires modification to the proxy code.

Pai et al. [17] developed a kernel I/O system called IO-lite to permit sharing of ``buffer aggregates'' between multiple applications and kernel subsystems. This system solves the multiple buffering problem, but, like Hummingbird, applications must use a different interface that supersedes the traditional UNIX read and writes.

next up previous
Next: Conclusions Up: Storage management for web Previous: Polygraph results
Liddy Shriver 2001-05-01