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磁盘负责三方面，一个将数据写入到日志文件ldf中，然后在check point后写入到mdf, 接着就是大量的读操作。
先写入ldf,是因为sql server的预写日志机制引起的，此机制保证了数据在写入到mdf中前，一定会在ldf中保留一份，这样以后恢复等操作都可进行，故可以说，影响写性能的主要是写入到ldf中的操作。磁盘一边负责随机取数据，一边负责写入数据，磁头转来变去，性能低下，而将ldf文件放在不同的磁盘中，就独立了写操作。
注意：磁盘应是物理上的。

Other logical drive will not benefit because ultimately, it’s the same drive controller and the same physical disc. Hence, if the heads are busy doing random reads for a logical drive D, they won’t be able to perform serial writes on logical drive E on the same physical drive.

Hence, the log should be given the priviledge of it’s own separate physical drive.
引自：
Performance Best practice – Transaction log must on a different drive. But WHY?
Write-Ahead Transaction Log