MongoDB Readahead warning

Question

I'm receiving this warning when starting my MongoDB shell:

Wed Mar 20 22:40:49.850 [initandlisten]

Wed Mar 20 22:40:49.850 [initandlisten] ** WARNING: Readahead for /data is set to 2048KB

Wed Mar 20 22:40:49.850 [initandlisten] ** We suggest setting it to 256KB (512 sectors) or less

Wed Mar 20 22:40:49.850 [initandlisten] ** http://dochub.mongodb.org/core/readahead

I've used:

df /data

To find where the drive is that my data is mounted and subsequently used:

sudo blockdev --setra 256 /dev/mapper/vg0-data

to set the readahead to the 256 that was specified. I've confirmed this has worked with:

sudo blockdev --getra /dev/mapper/vg0-data

I'm still however getting the same error when starting my shell?

UPDATE

I've restarted the mongodb server instance and am still receiving this error.

UPDATE 2

My linux server is hosted on AWS and therefore uses virtual volumes. I have set the readahead value for all those volumes to 256 and still receive this error.

UPDATE 3

This is a blockdev --report of the Primary instance of MongoDB

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0      8589934592   /dev/xvda1
rw   256   512  4096          0     10737418240   /dev/xvdh8
rw   256   512  4096          0     10737418240   /dev/xvdh7
rw   256   512  4096          0     10737418240   /dev/xvdh6
rw   256   512  4096          0     10737418240   /dev/xvdh5
rw   256   512  4096          0     10737418240   /dev/xvdh4
rw   256   512  4096          0     10737418240   /dev/xvdh3
rw   256   512  4096          0     10737418240   /dev/xvdh2
rw   256   512  4096          0     10737418240   /dev/xvdh1
rw  4096   512  4096          0     42944430080   /dev/md127
rw  4096   512  4096          0     38646317056   /dev/dm-0
rw  4096   512  4096          0      2143289344   /dev/dm-1
rw  4096   512  4096          0      2143289344   /dev/dm-2

This is a blockdev --report of the Secondary instance of MongoDB

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0      8589934592   /dev/xvda1
rw   256   512  4096          0     10737418240   /dev/xvdh8
rw   256   512  4096          0     10737418240   /dev/xvdh7
rw   256   512  4096          0     10737418240   /dev/xvdh6
rw   256   512  4096          0     10737418240   /dev/xvdh5
rw   256   512  4096          0     10737418240   /dev/xvdh4
rw   256   512  4096          0     10737418240   /dev/xvdh3
rw   256   512  4096          0     10737418240   /dev/xvdh2
rw   256   512  4096          0     10737418240   /dev/xvdh1
rw  4096   512  4096          0     42944430080   /dev/md127
rw  4096   512  4096          0     38646317056   /dev/dm-0
rw  4096   512  4096          0      2143289344   /dev/dm-1
rw  4096   512  4096          0      2143289344   /dev/dm-2

This is a blockdev --report of the Arbiter instance of MongoDB

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0      8589934592   /dev/xvda1

Answer 1

The answer was to run blockdev --setra in a startup script. Every time the system reboots the read-ahead values revert back to default.

In my case I just figured out what my logical drives were with blockdev --report, I then run blockdev --setra on every drive in the "start" section of the MongoDB startup script in init.d.

Hope this helps anybody else having similar issues.