EMC Centera的内容寻址技术
EMC公司Centera网络存储系统是世界上第一款针对固定内容的CAS网络存储解决方案。Centera的CentraStar软件操作环境采用了一种创造性的内容寻址系统来简化存储管理,确保存储内容的惟一性,提供了固定内容存储需求从TB级至PB级的可扩展性。Centera网络存储系统大大降低了管理整个存储系统的开销。
Centera网络存储系统实现了软硬件的完美结合,非常理想地解决了固定内容存储需求。对于网络存储客户而言,Centera系统的重要价值在于它的软件系统,通过丰富的API,用户可以非常容易地实现对整个网络存储系统的使用和管理。
当存储一个数据对象时,Centera首先根据所存储数据的二进制内容,按照特定算法计算出一个128比特的奇偶校验,接着,Centera把这一比特序列转换成一个独特的27个字符的标识符,叫做内容地址。这个内容地址源自所存储数据片断的内容本身,同样对于数据片断而言也是惟一的标志或称做数字标签、数字指纹,如下图所示。
Centera所实现的内容寻址技术可以有效地隔离对存储数据的非法访问。所存储数据的内容地址并不是对该数据的目录、文件名或数据类型的简单映像。客户端上的用户不具有Centera上的任何账号,更不可能对Centera进行浏览或管理。客户端用户存取Centera上的数据的惟一途径是通过存储系统的应用软件来进行——由该应用软件通过内容描述符文件(CDF)的27个字符内容地址来具体完成存取动作。
内容寻址(Content Addressing)是Centera区别于其他网络存储技术的关键所在,而其他网络存储技术如SAN、NAS等都是基于位置寻址(Location Addressing)的,基于内容寻址的网络存储技术降低了整个存储系统理解、管理、操纵存储介质上的信息的物理或逻辑位置的难度。
内容寻址技术的优点主要包括以下几点:
● 确保内容的可靠性
一个内容对象有且仅有一个内容地址,对所存储内容的任何修改都会被系统检测到,因为这个修改会产生一个不同的内容地址。
● 提供了全局惟一、位置独立的标识符
通过内容地址对所存储的内容进行寻址就导致了一个与存储内容相独立的内容索引,并且这个内容地址也与操作系统、文件系统和应用软件相独立。
● 单一实例存储
Centera只维护所存储内容的一个副本和一个映像,假设一个存取操作试图为30个不同的客户端用户存储同样内容的数据,那么对于所有这30个客户端用户而言,每一个用户的内容描述符文件CDF中的元数据是不同的,但数据对象本身只在存储系统上保存一份。由于采用特定的算法,每一个所存储内容的片断只有惟一一个内容地址与之对应,如下图所示。这种情形就为整个网络存储系统带来了前所未有的容量节省和简单管理,这也就是Centera之所以如此备受业界关注的重要原因之一。
正如SAN完全为优化块数据、而NAS完全为优化文件数据而设计一样,CAS完全为固定内容存储而设计。
内容寻址存储CAS提供了一种全新的高效的信息管理手段,完全适合于固定内容存储需求。内容寻址技术大大降低了应用软件操纵和管理存储介质上所存储信息的物理地址的难度。内容寻址存储的信息单元地址的计算是这样的:首先对所存储内容片断执行一个特定算法,从而得到一个数据单元的惟一ID(也称数字签名);由这一ID与存储信息的元数据(metadata)一起构成所访问数据的实际有效地址。应用程序或用户只有通过数据单元的ID才能存取固定内容数据。通过对所存储数据片断的内容执行特定算法而得到数据单元惟一ID的另外一个特别之处在于,当有相同内容数据要存储时,系统会对所存储内容的本身按照特定算法进行解析,进而得到同样数据的同样标识符,这就避免了同样数据的重复存储。内容寻址存储CAS的这一特别之处对于固定内容而言其重要性不言而喻。
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