LSMs do not guarantee that sequential key/value pairs are stored in the same SSTable so reading "n" k/v pairs, in the worst case, could require seeking through "n" different SSTables. There are datastores that use LSMs such as HBase that provide guarantees on top of LSMs to facilitate efficient range reads and, of course, many LSM compaction algorithms improve range reads, but the basic LSM is optimized only for single k/v reads. That's the reason, for example, why Cassandra doesn't even offer range reads such as "SELECT * FROM mytable WHERE KEY > x AND key < (x + k)".