I have two servers, with similar disks.

The first server has 4x300GB Seagate Cheetah 15K.5 model nº ST3300655SS 3.5'' RAID 10. The second 4x300GB Seagate Savvio 15K.3 model nº ST9300653SS 2.5'' RAID 10.

I'm running the following command to test the write speed:

dd if=/dev/zero of=/tmp/output bs=8k count=10k; rm -f /tmp/output

On the first server with 3.5 disks I get about 1,3 GB/s write speed, while on the other only 640MB/s

Both servers run Cent OS. The faster one, CentOS 5.7 and the slower Cent OS 6.4

What could be the reason for so big difference, if both disks are 15k rpm?

  • Just a guess - the second server is 3 Gb SAS. – Greg Askew Apr 11 '13 at 23:11
  • no. Both area 300gb – valter Apr 11 '13 at 23:18
  • I'm referring to the storage bus transfer rate - either 3 Gbps or 6 Gbps. Give that one server has 2x transfer rate, I would check on that. – Greg Askew Apr 11 '13 at 23:22
  • the RAID controllers used (and their config) can affect the resulting transfer speeds more than the disks. so you might want to provide that info – Alex P. Apr 11 '13 at 23:46

According to the data sheets the sustained data rate for the Cheetah is 73-125 MB/s and for the Savvio 151 to 202.

So while data rates of 640 MB/s for four Savvio disks seem to be reasonably within the specs, 1,3 GB/s for 4 Cheetahs obviously are not. You are likely facing a situation where caching effects (probably controller-side) are contorting the benchmarks.

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