c# More File Reads Vs. More File Open/Close speed

I have an application which accesses a flat file filesystem whose structure I have to decide. My filesystem stores data for users for each day of the year for every year and each day's data has information of the form HH:mm, value. My data access is usually limited to about 1 month at a time. My directory structure options are,

1. File开发者_JAVA技巧System > UserA > 2010_01_01, 2010_01_02, i.e., for each user, store each day's data as a file. and,

2. FileSystem > UserA > 2010_01, 2010_02, i.e., for each user, store a months data as a file.

using 1 would mean when I fetching user data I have more frequent file open/close and fewer file reads(to get to the line I need) while using 2 implies less frequent file open/close and more file reads(to get to the line I need). What would I be better off using?

Note: I realize with 1 that it is possible for me to access different files in different threads and probably speeding things up.

Consider that each file open is actually several reads, and that large reads are not much more expensive than small reads. Also, hard drives are physically limited to accessing a single part of the disk at a time, so multiple threads will likely not help you unless each thread reads from a different disk.