Help with understanding download speeds using large and small files

[andysaykao]

Hi There,

I am doing some throughput testing between two VMs on the same subnet which happen to also be on the same ESXi Host and share the same Nexus 1000v. Both VMs are using the latest VMXNET3 drivers and the network card properties shows the NIC speed as being 10Gbps.

VM1 <--- N1KV --> VM2

VM1 = FTP Server

VM2 = FTP Client

Test #1:

When running a FTP test and pulling down a 1.5GB file, VM2's network graph is showing that it can download roughly about 250-300,000Kbytes/sec (or 1.5-2.0Gbits/sec). Not too bad !

Test #2:

I then created 1,500 x 1MB small files and ran the same FTP test again. This time to download all 1,500 x 1MB files, VM2's network graph is only peaking at about 30-40,000KBytes/sec (or 200-300Kbits/sec). Urghhh....

I'm just wondering why the large difference in throughput between FTP-ing a large 1.5GB file as opposed to FTP-ing 1,500x1MB files. Is this the expected behaviour? Should we be getting better throughtput with Test #2  - if so, what do we need to do to improve this?

Thanks.

Andy

[rais]

FTPing a large file will give enough time for the TCP window size to get utilized to its peak. 

Small 1 Meg files are too small for tcp to use its window size to a full potential. Moreover, FTP is not going to transfer all these files simultaneously. It has to do it one by one. 

IPERF can also be used to check the available bandwidth. 

HTH