Monday, July 13, 2009

Content Addressable Storage -- The Next Step

Recently, I was writing an slightly modified version of implementation of Content Addressable Storage (CAS) for our on-line image servers. I found it a perfect choice to apply CAS to image servers, where images are uploaded once and never changed, and where disk access throughput is a main bottleneck to many large-scale on-line image service providers.

As explained on wikipedia, CAS is a mechanism for storing information that can be retrieved based on its content, not its storage location. It is typically used for high-speed storage and retrieval of fixed content.

CAS Characteristics:
1. Storage is identified by its content
2. Designed to make the searching for a given document content very quick
3. Works most efficiently on data that does not change often

Currently, most servers store images as common files and rely on filesystem's mechanism to optimize reading speed. However, filesystems are limited by POSIX standards and are optimized only for everyday usage. For large-scale image servers (or any application storing large amount of images), CAS will be a better choice (for the following reasons).

1. deduplication: both block-level CAS and file-level CAS achieve efficient storage utilization by storing same data only once.
2. Aggregation: log-structured extent-based on-disk object-stores, storing many images continuously in a single large file, resulting only one disk seek if index files are saved/managed in-memory (verses 2~3 disk seeks per read for common files).

My CAS implementation is written by the fact that most images are small files (~10MB) and there are huge amount of them on image servers. The implementation should speed up accessing of these image files. I am planing some benchmarks this week and hope to post them here soon.

After that, I will be writing a paper illustrating the opportunistic of applying CAS system to large-scale image servers. Then, I also want to explore the possibility of changing the interface (to, maybe dbus?) and make it runnable on common desktop, because current gnome/kde also read a lot of thumbnails. Maybe I can make it quicker somehow. Who knows :)

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