Ubuntu Disk performance bounces a lot, please help

2012-05-17  Source: Original Site  Category:Ubuntu  Views:0 

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I have a NFS server with a raid device for data. I recognize that the throughput differs a lot through time. On a GiEthernet it ranges from [1-90] MBps with an average ranging from [15-60] MBps.

I wanted to test the disk speed, as I think the NFS server is not loaded that much.

I found weird behavior, please see my tries below.

[[email protected] md0]# time dd if=/dev/zero of=test.tst bs=4k count=81920
81920+0 records in
81920+0 records out
335544320 bytes (336 MB) copied, 2.577 seconds, 130 MB/s

real 0m10.580s
user 0m0.010s
sys 0m0.641s

[[email protected] md0]# time dd if=/dev/zero of=test.tst bs=4k count=81920
81920+0 records in
81920+0 records out
335544320 bytes (336 MB) copied, 0.493557 seconds, 680 MB/s

real 0m10.874s
user 0m0.003s
sys 0m0.616s

I have 2 questions, I hope someone can clarify them:

Q1- What causes this slow/toggling performance between 130 and 680 MBps ? Can I configure bigger buffers for such an issue?

Q2- I see the real time far beyond that reported by 'dd', will applications (NFS server) see this real time and suffer much less performance?

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