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PostPosted: Mon Dec 06, 2010 2:54 pm 
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If you're downloading from an external site and you see a very low download speed, such as 40 KB/sec, you may wonder if there is a problem with your VDS.

The answer is that there likely is not!

We do not throttle our servers, and all our VDSes sit on full gigE connections that are only lightly utilized. We have extensively tested our connection and VDSes to make sure that they can use the full gigE; on a daily basis, we see customers that briefly use very large chunks of the bandwidth available, testing it further.

One thing that we have noticed in our testing is that many popular download sites are very slow at times, or have very slow mirrors (that are slow all the time). For instance, some Ubuntu mirrors are slow and can only achieve 40 KB/sec.

To see if it is the download site you are using, we recommend testing several others. For instance, try downloading these large files from mirrors that are typically fast (often >10 MB/sec when tested from a VDS in the same state).

http://mirrors.kernel.org/gentoo/releas ... b-10.1.iso (California)
http://gentoo.cites.uiuc.edu/pub/gentoo ... b-10.1.iso (Illinois)
http://mirrors.rit.edu/gentoo/releases/ ... b-10.1.iso (New York)
http://gentoo-distfiles.mirrors.tds.net ... b-10.1.iso (New York)
http://mirror.de.leaseweb.net/centos/6. ... n-DVD1.iso (Frankfurt)

Another thing that many people don't realize is the role that latency plays in transfer speed. Because of the way that TCP flow control works, if the latency between two endpoints is very large, TCP slow start means that transfers take a little while to "spin up" to full speed, and larger receive and send windows are needed to handle all of the extra in-flight data. If you find that your downloads are only at a few hundred KB/sec when you expect them to be in the multiple MB/sec range, finding a closer mirror might help.

If you are still concerned that your VDS may not be operating properly, please contact us, and we can help with additional troubleshooting. We have seen a couple of cases where a Xen setting needed to be tweaked, or a MAC needed to be adjusted.


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PostPosted: Thu Apr 14, 2011 9:48 pm 
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On certain versions of Linux, this is the tweak that you may need to apply to speed up transfers. It won't hurt to do it on any version of Linux, so it's always worth a try:

Code:
ethtool -K eth0 tso off

This is needed sometimes because of a bug in older versions of the Xen PVHVM client-side drivers, as well as because of a bug on the Xen hypervisor side for fully emulated (non-PVHVM) operation.

More specifically, with TSO (TCP Segmentation Offload) enabled, the network adapter is supposed to handle splitting up large blocks of data into smaller MTU-sized packets, but Xen doesn't do it correctly in some cases, leading to the large packets going directly onto the wire. Some of the networking equipment on the internet supports an MTU of only 1500, and none of it allows very large (>10K) packets, which means that many of those very large packets end up being dropped. Dropped packets need to be retransmitted, killing transfer speeds.


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