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I'm benchmarking disk write performance using this command:

dd if=/dev/zero of=/mnt/test.dmp bs=1M count=256 conv=fdatasync

After a random time and random number of attempts, the file system performance drops to 1/4 of the initial value (unmout, reformat file system and mount to retry).

I'm using Linux guest Ubuntu 14.04 (kernel 3.13.0-65-generic x86_64 and ). I used several different file systems (ext4, ext2, vfat)

It's not clear for me what is happening. I initially thought that it was due to some file system cache getting filled up, but I can't find a clear pattern.

Update 1:

Hardware: Dell PowerEdge R710, 64GB RAM, PERC H200, 2 SATA disks

Update 2:

dd in VMWare ESXi (via SSH) gives a constant 20 MBytes/s write rate on our storage. Guest is Linux 3.13.0-65-generic x86_64.

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    In order for us to help you on this question, you should add some pertinent detail like the hardware involved, the type of underlying storage in your VMware install and anything else in the environment that could impact I/O. – ewwhite Mar 21 '16 at 17:15
  • you could try to monitor the writeback buffer watch "grep -i writeback /proc/meminfo" to see if it's OS cache. It could also be the disk controller cache. Also use much greater file sizes and write directly on the block device. There is no point in formating to write zeros. – Bogdan Mar 23 '16 at 15:15

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