不过，这种紧密的复制有很高的性能成本，并且难于实现，当群集接近某一规模时，还会有伸缩性的问题。
However, such tight replication has a considerable performance cost and implementation complexity, and may also have scalability problems as the cluster approaches a certain size.
他那神气是善良的，但又使人感到难于接近，他的目光从来不停留在别人的眼睛上。
He had a kindly but unapproachable air, and he never let his glance linger on the eyes of any one.
一些人估计通过电子方式产生的信用违约掉期接近万亿美元，而它们都没有正式文件证明而且难于理解。
By some estimates close to a trillion dollars in credit default swaps out there were created electronically and are now undocumented and little understood.
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