Linux – Read Ahead value automatically getting updated at boot

block-devicebootlinuxrc.localstartup

OS version: CentOS Linux release 7.2.1511 (Core)

I have the following commands in my rc.local file:

/usr/sbin/blockdev --report > /root/test_boot
/usr/sbin/blockdev --setra 256 /dev/vdb
/usr/sbin/blockdev --report >> /root/test_boot

After booting the /root/test_boot file has following content:

cat test_boot

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0     10737418240   /dev/vda
rw   256   512  4096       2048      8588886016   /dev/vda1
rw   256   512  4096          0     53687091200   /dev/vdb
RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0     10737418240   /dev/vda
rw   256   512  4096       2048      8588886016   /dev/vda1
rw   256   512  4096          0     53687091200   /dev/vdb

whereas the command

blockdev –report

gives a different value:

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw  8192   512  4096          0     10737418240   /dev/vda
rw  8192   512  4096       2048      8588886016   /dev/vda1
rw  8192   512  4096          0     53687091200   /dev/vdb

So it seems that some other kernel program is overwriting the value at boot. What could be the program? Is there some way that I can execute the blockdev –setra command at the very end of the boot process?

Note: I've also tried putting the commands in init.d, systemd and crontab. Same thing happens all the times.

Best Answer

Disable readahead value in tuned.conf.

Run these commands to reload the tuned.conf file:

# tuned-adm profile
# tuned-adm active virtual-guest

After this the values will load from /etc/rc.d/rc.local file.

Related Solutions

Linux – Mount RootFS error: VFS: Cannot open root device “mtdblock4” or unknown-block(0,0)

Do you have the correct module built inside the kernel for that device type and the file system on it?

You could try using an initramfs and mount that device from there.

Linux Readahead Settings – LVM, Device-Mapper, Software Raid, and Block Devices

How does the RA setting get passed down the virtual block device chain?

It depends. Let's assume you are inside Xen domU and have RA=256. Your /dev/xvda1 is actual LV on the dom0 visible under /dev/dm1. So you have RA(domU(/dev/xvda1)) = 256 and RA(dom0(/dev/dm1)) = 512 . It will have such effect that dom0 kernel will access /dev/dm1 with another RA than domU's kernel. Simple as that.

Another sittutation will occur if we assume /dev/md0(/dev/sda1,/dev/sda2) sittuation.

blockdev --report | grep sda
rw   **512**   512  4096          0   1500301910016   /dev/sda
rw   **512**   512  4096       2048      1072693248   /dev/sda1
rw   **512**   512  4096    2097152   1499227750400   /dev/sda2
blockdev --setra 256 /dev/sda1
blockdev --report | grep sda
rw   **256**   512  4096          0   1500301910016   /dev/sda
rw   **256**   512  4096       2048      1072693248   /dev/sda1
rw   **256**   512  4096    2097152   1499227750400   /dev/sda2

Setting /dev/md0 RA won't affect /dev/sdX blockdevices.

rw   **256**   512  4096       2048      1072693248   /dev/sda1
rw   **256**   512  4096    2097152   1499227750400   /dev/sda2
rw   **512**   512  4096          0      1072627712   /dev/md0

So generally in my opinion kernel accesses blockdevice in the manner that is actually set. One logical volume can be accessed via RAID (that it's part of) or devicemapper device and each with another RA that will be respected.

So the answer is - the RA setting is IMHO not passed down the blockdevice chain, but whatever the top level device RA setting is, will be used to access the constituent devices

Does dm-0 trump all because that is the top level block device you are actually accessing?

If you mean deep propagation by "trump all" - as per my previous comment I think that you may have different RA's for different devices in the system.

Would lvchange -r have an impact on the dm-0 device and not show up here?

Yes but this is a particular case. Let's assume that we have /dev/dm0 which is LVM's /dev/vg0/blockdevice. If you do:

lvchange -r 512 /dev/vg0/blockdevice

the /dev/dm0 will also change because /dev/dm0 and /dev/vg0/blockdevice is exactly the same block device when it comes to kernel access.

But let's assume that /dev/vg0/blockdevice is the same as /dev/dm0 and /dev/xvda1 in Xen domU that is utilizing it. Setting the RA of /dev/xvda1 will take effect but dom0 will see still have it's own RA.

What do you use, equivalent to the sector size above to determine the actual readahead value for a virtual device:

I typically discover RA by experimenting with different values and testing it with hdparm .

The stripe size of the RAID (for md0)?

Same as above.

Does the FS play a part (I am primarily interested in ext4 and XFS)?

Sure - this is a very big topic. I recommend You start here http://archives.postgresql.org/pgsql-performance/2008-09/msg00141.php

Best Answer

Related Solutions

Linux – Mount RootFS error: VFS: Cannot open root device “mtdblock4” or unknown-block(0,0)

Linux Readahead Settings – LVM, Device-Mapper, Software Raid, and Block Devices

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