If you're already enjoying the increased read speeds on your HTC EVO's microSD card after changing the read-ahead value to 128 (tutorial here), then you'll be ecstatic to hear that increased values up to 1024 have been confirmed to boost speeds even more.
G&E reader Neil H did "a bunch of writing, testing, and posting larger caches" after the initial 128 improvements were reported and discovered that while the 8GB card bundled with the EVO can only benefit from the 128 or 256 values, 16GB/32GB Class 4 and higher cards can use 512 and 1024.
He even created flashable .zips for us so the higher values changes will survive a reboot. Just download the one you want and flash it on your rooted EVO (tutorial here; don't forget to make sure that you have a recent Nandroid backup just in case): 256 value .zip | 512 value .zip | 1024 value .zip
Neil H says the .zips will work on all ROMs (Sense or AOSP) as long as the kernel you're using doesn't already have the modified value built into it. He also notes that "a lot of the speed increases depend on the ratio of used-to-empty space on the card," which means that a card with less free space will be a bit slower than the same card with more free space.
He and his friends tested the new values on Class 2, 4, and 10 cards. Their results are below.
16GB Class 2 card
- Stock value: 3 MB/s read
- 128 value: 5 MB/s read
- 256 value: 10 MB/s read
- 512 value: 10MB/s read
- 1024 value: 8 MB/s read (reduced speed because "the cache was so large compared to what [he] was loading into it that it took away some speed" while the cache was "filling")
16GB Class 4 card
- 512 value: 12 MB/s read
32GB Class 10 card (17GB filled)
- 256 value: 20 MB/s read
- 512 value: 24 MB/s read
- 1024 value: 28 MB/s read
The higher values actually made no difference on my Class 6 card (11 MB/s read), but maybe you'll have better luck. And since it's not in the Market anymore, here's the free version of SD Card Speed Test.
Thanks, Neil H!