the read speed should be 250Mb/s (31.25MB/s)
First off, it is very unlikely you will achieve this performance level with 4 7200 rpm HDDs with a random read access pattern. Even if your disks will be able to read larger block sizes ~ 16-64 KB, the maximum for I/O operations per second for a 7.2k disk is ~100 for non-sequential access. In my experience, you might be looking at ~10-20 MB/s in the end, if you've done everything right.
Should we combine the HDD's to one big directory?
You clearly will benefit from striping as offered by
dmraid or hardware RAID controllers for RAID0. Note that in this operation mode you will lose all data once even a single disk fails.
If you have read and write requests in parallel (even if the percentage of writes is rather low compared to reads), you will benefit from a RAID controller's write-back functionality. Consider buying a controller with a BBU for better operational consistency (controllers without a BBU will lose the content of their cache in case of a power failure and might tear your file system apart).
Should the files all be placed in the same directory? (we're talking about 25,000,000 files or so)
Certainly not. Many file systems perform poorly with a large number of files (>50,000) in a single directory. Avoid this condition for portability reasons. If you absolutely must, take a look at filesystems which are known to perform well under these conditions.
What Filesystem should we use?
It depends. Test your load with the modern bunch and see if you get at inefficiencies. You also likely find yourself looking for tuneables and tweeks to reduce the number of disk seeks per file access for each of them (like mounting with
In the end, you might want to double the number of disks to use in order to increase performance and introduce redundancy in a RAID10 setup.