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May 22, 2009

最近NetApp与Data Domain(NASDAQ:DDUP)达成收购协议,DDUP两天上涨了42.87%,在消息传出的第一天,收购交易值就从15亿美元跃升至17.5亿美元。这里聊聊DDUP的技术。

DDUP是数据备份领域的公司。早期的数据备份通常是备份公司内部的数据,备份介质是磁带,备份周期长,数据量相对小。万一需要恢复系统数据,对实效的要求也没那么紧。所以用磁带这种速度慢,每GB成本低的介质,很合适。

在互联网时代,备份的数据量大,数据服务的人数剧增,备份的周期缩短,需要恢复数据的时候,对恢复的速度要求很高,因为数据通常是服务全球的。同时,由于硬盘技术的飞跃性发展,每GB的成本迅速降低。硬盘的存取速度远远超过磁带,这就为使用硬盘替代磁带作为备份介质,提供了可能性。

硬盘的每GB成本虽然迅速降低,但是依然比磁带要高很多,另外海量数据在备份和恢复的时候,硬盘本身的机械结构,也是瓶颈。所以要针对备份数据的特征,将数据里面的重复部分(duplication)去掉,也就是DeDUPlication,这也就是DDUP的股票代号的来源。有人戏称因为DDUP的技术来自Princeton的Kai Li教授,这个DDUP是“Day Day UP”,天天向上的意思,也未可知,不过作为一支股票,Day Day Up的确是好事情。

备份数据里面存在大量的重复部分,很多人把数据比作游泳池加水:每天加的水并不多,但是整个的游泳池很大。如果每天备份的话,每次都备份一个游泳池就太不合算了,应该是备份每天加的那些水。具体到实际操作,因为数据并非简单的添加,而是对很多原有的数据进行修改,所以也并不是备份增量那么简单。

另外一个难点,就是对磁盘上面的海量数据进行操作的时候,如何才能高效的寻找到数据,并对数据进行有效操作。在计算机体系结构里面,存储层次从CPU的cache到内存,再到硬盘,每个层次的存取速度,都是上面一个层次的千分之一,如何利用有限的内存来处理海量的低速硬盘,里面有很多的技巧。这点在我和彼岸以前合作翻译的Google BigTable的论文里面,也有提及。这里面的技术,很多都是目前文件系统和数据库使用的技巧,但是技巧如何合理组合,就是一个技术公司成功的秘密之一了。

在DDUP所在的数据备份领域,还有其他几家公司,DDUP的一个显著不同,是他们使用的硬件,就是普通的PC。我曾经问过DDUP的工程师,他们为何不用特制的硬件。这个问题也是很显然的:因为从DDUP的应用来说,他们要处理的数据单位很大,行话叫数据通路很宽,通用CPU的数据通路都很窄。对方的回应是:由于特制硬件,特别是特制的芯片,如基于FPGA的可编程芯片,虽然可以提供足够的数据通路宽度,但是由于摩尔定律对硬件速度的提升,以及英特尔对CPU质量的保证,使得使用通用CPU比用特制硬件的开发周期大大减少,只要专注开发自己的软件,18个月以后,就可以得到免费的性能提升,几年之内就可以超越使用特制芯片的对手。

这里列出的几个DDUP所在行业的门槛和竞争技巧,其实也都是计算机行业里面众所周知的东西,如何巧妙组合,形成一个公司的竞争力,才是我关心的问题。

3 Comments »

  1. 我想到的另一点:

    如果更大程度的利用,由摩尔定律所揭示的、以18个月为周期的、免费性能提升,在一定程度上也能提升公司的竞争力。

    貌似不少coding团队都忽略了这一点。

    Comment by Microwish — June 9, 2009 @ 11:38 pm

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