We got new RHEL 6 and I got some wierd performance when using dd. What I see is that dd with odirect is faster than when not using it. It's 5 times faster which dosn't make sense.

So I looked at /proc/meminfo and see these new parameters. Can't seem to find any doc. Can someone please help me understand?


DirectMap4k: 5568 kB
DirectMap2M: 2082816 kB
DirectMap1G: 48234496 kB

Not sure if this has any impact. This is a baremetal machine with 24 CPU and 48G RAM.

results from dd:

[root@ppa301 .mohit]# dd if=/dev/zero of=/data/testfile bs=128k count=80k oflag=direct
81920+0 records in
81920+0 records out
10737418240 bytes (11 GB) copied, 16.2958 s, 659 MB/s

[root@ppa301 .mohit]# dd if=/dev/zero of=/data/testfile bs=128k count=80k
81920+0 records in
81920+0 records out
10737418240 bytes (11 GB) copied, 52.2661 s, 205 MB/s