虽然VTL结合磁带有相辅相成的效果，但在实际应用中，却面临性能、可用容量限制与节能等四大问题。
VTL的性能限制
单论顺序访问速度，其实当前的高速磁带机传输速率并不逊于磁盘阵列，而大型的磁带柜还能通过多个读写头执行多数据流并行备份（Multiple concurrent backup sessions），成倍增加了写入速度。但限制就在于磁带只能顺序读取，如果要还原特定的档案而不是要还原整卷磁带，就必须从头倒带卷到特定位置，非常耗费时间。
相比之下，VTL实际上就是个磁盘阵列，由于磁盘属于可随机读写的媒体，可随时读取磁盘区中的任意位置，还原特定数据的速度远高于磁带。更大的优势在于VTL可模拟任意数量的读写头，因此执行多数据流并行备份时，不会受到读写头数量的限制。一套VTL可同时连接多部备份服务器，同时执行多组备份程序大幅缩短备份时间。
但在底层，VTL是通过软件将磁盘阵列挂载到前端服务器上的LUN仿真成磁带格式，在多个备份数据流同时工作的情况下，很可能会发生多个数据流同时写入同一个控制器或同一个LUN，导致磁盘阵列负担不平衡而造成性能瓶颈。
另外，由于VTL是完全仿真磁带，而内建硬件压缩已经是磁带机的标准功能，因此VTL仿真出来的虚拟磁带也必须具备对应的压缩功能。但问题在于VTL提供的压缩功能多半是利用软件方式完成，非常占用VTL处理器资源，以致严重影响VTL的备份效率。
磁带转存作业占用备份服务器资源
在VTL结合磁带的架构下，VTL是居于缓冲的角色，数据最终还是要转存到后端的磁带上。但关键在于要如何执行转存磁带的作业。
传统的作法都是由前端备份服务器的备份软件，来执行将数据从VTL转存到磁带的作业。具体方法有几种：一种是先把数据从VTL还原到备份服务器上，然后再转存到磁带设备上；另一种则是通过备份软件的磁带复制（clone）功能。在备份软件看来，VTL仿真出来的磁带和磁带没什么不同，因此可事先在VTL上设定与磁带相同规格的虚拟磁带，然后再以磁带复制功能，将VTL中的数据转到磁带上。
无论哪种方式，都需通过前端的备份服务器执行，因此会占用备份服务器的作业时间与处理资源，当备份服务器执行VTL转存磁带作业时，就没办法执行原来的正常备份工作。
但当前企业的数据量都很大，对备份服务器被占用均十分敏感，不见得能允许备份服务器花很多时间去执行转存磁带作业。
容量瓶颈
如前所述，由于VTL是作为磁带的缓冲，显而易见的，如果VTL的容量越大，则留存在VTL磁盘中的备份数据也越多，因此在需要还原数据时，能够降低从最后端的磁带中寻找数据的机率，从而大幅提高还原效率。
虽然多数VTL产品都预留有扩充容量的准备，但比起可通过购买新磁带无限扩充容量的磁带设备，VTL容量仍是相当有限，因此一般的作法是通过政策设定，只在VTL中保留1个月内的备份数据，超过时限的数据就转存{}到磁带，以把VTL的空间让给新的数据。
但对许多要求每天均执行全备份的用户来说，这样的标准仍不易达成，要让VTL保留最近1个月份的数据，将需要非常大的磁盘空间，因此用户不是被迫为VTL购买极大的容量，就是只能在VTL上保留更少天数的数据，以致必须更频繁的执行转存磁带作用。
VTL需持续运行，难以节电
VTL相较于磁带设备虽有性能与可靠性上的优势，但就当前越来越受重视的电力消耗方面来看，却是处于劣势。
磁带柜、自动上带机之类的磁带设备内部主要组件都是机械装置，机械手臂、磁带运输、磁带机磁头等部件，都只有在实际工作时才会被驱动，平时处在待命状态下消耗的功率极低；而且多数企业都只有在下班等离峰时间才会启动磁带设备执行备份，因此磁带设备一天中几乎只有8到10多个小时会全功率运转。
相比下VTL的磁盘阵列就必须维持不间断的运作，尤其像是控制器与风扇等组件的供电都是不能中断的，虽然也可以平时把VTL关机，待要备份／还原时再行开机，但一来重新启动需要时间，二来又会牵涉到前端备份服务器的设置；原先设置好的VTL在关机后就会从备份服务器的备份装置中脱机，VTL重新启动后，还须在备份服务器上重新设定，十分麻烦。
34 条回复
有几个不同的意见供参加：
1。现在很多厂家提供硬件压缩，4：1压缩可以提供压缩比，而且通过硬件，不再影响性能，因为硬件压缩可以达到1000GB/s
2.物理磁带库的归档也可以通过虚拟磁带库本身完成，这样不占用备份服务器的性能，而且速度会很快，并且实现全自动的物理归档功能。
3。节电模式：和存储的电源系统相关，有的电源系统可以做到虚拟磁带库没有使用时，只开一个电源，保持低供电，当有应用时，可以同时开多个电源
您觉得呢？
1000GB/s，也就是1TB/s，怎么可能会有这么高的处理能力，用什么芯片啊？
什么芯片我不懂，就是Hifn的压缩卡，他的高端压缩卡，可以达到这个速度。有些虚拟磁带库厂商就直接OEM他的卡片，在卡片上进行再开发。
不好意思，错了是1000MB/s, 1GB/s，不是1TB/s，没有那么快的速度，不好意思，写错了。。。。。。。。。。。。
Sorry
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