documentation of /sys/vm/max-readahead

[Morten Sylvest Olsen <mso@medical-insight.com>]

Hi list.

Am I the first to battle performance related issues from the setting of
/proc/sys/vm/max-readahead? It seems there is quite a few threads floating
around the net related to performance of the md driver with external disks
connected over FC.

But I was pretty confused because /proc/mdstat reports the bogus old
device read_ahead value, and the md driver silently (well, not really)
uses /sys/vm/max-readahead to calculate a window size which makes quite a
difference in my setup where I have some scsi disks on a relatively
high-latency SAN. Possibly this issue only shows itself when running
RAID-1 where the chunksize is not used (there seem to be a bug in Jacob's
howto in section 5.10 on that point)

Maybe the issue only arises on RedHat because they for some reason chose
to make the 31 default value of /proc/sys/vm/max-readahead in their EL3
release, where the older default of 127 is more resonable?

I don't mind writing the blurb myself, if I'm not totally off course in my
error diagnostics.

-- Morten
--
Morten Sylvest Olsen, System Developer
Medical Insight A/S, Hovedgaden 451 C,2640 Hedehusene, Denmark
Phone:+4546550444, Mobile:+4551573092,Mail: mso@medical-insight.com