Comment 14 for bug 129488

I suggest to mark this as "fixed". I did a few simple tests with hdparm -t /dev/xxx and while I am not convinced that this is necessarily the best, let on alone the only relevant measure of performance, I found no limitation in Karmic on a current 250 GB 7200 rpm laptop hard drive with the default setting of 256 sectors (as Phillip Susi indicated, this is the default for all block devices, both LVM and non-LVM). This agrees with Jack Wasey's observations. I get ~90+ MB/s for readaheads from 64 to 8192 sectors, and I have no reason to believe that there are is a setting within that range that would be significantly different from the others (on my system, with the hdparm test; there is some variability in repeated measurements even with the same settings). At 32 sectors there is a slight drop to ~85 MB/s, and settings of 2 and 8 sectors give ~27 and ~40 MB/s, respectively, which shows that the readahead settings do have an effect. To set the rumor to rest that the lvchange command has no effect (which was true in 2004 and perhaps later) - I tried with both lvchange -r and blockdev --setra, and they have the same effect - i.e. basically no change in performance between 64 and 8192 (highest I have tried), and a drop below 32. (Using blockdev --setra on the physical device a logical volume resides on, e.g. /dev/sdxy, rather than of the logical volume itself has no effect, by the way.)
Long story short, in my experience the default settings are fine and the lvchange command works as expected if you feel the need to change them. With SSD's this may have even less impact.