The Curious Case of the Slow File Transfer

Bart Jacobs — Mon, 11 Feb 2013 06:01:42 +0000

Let’s get started with a bit of storytelling…

It all started back in at the end of Q3 2011, I was setting up a new Citrix XenApp 6.5 farm for a migration project at the company I was outsourced at. Back then XenApp 6.5 had been released just a few months earlier.

Everything was looking fine and the migration was going smoothly. A few weeks after the last user was migrated we began seeing some strange behavior. Most of that was solved by implementing new Citrix Access Gateway (CAG) VPX’s, but one issue remained: accessing client drive mappings was extremely slow. To make matters worse, the application being used needed to transfer PDF’s (not even large ones, mostly around 200KB) from client drives to the server. Although the copy eventually would always complete, performance was far less than what is considered acceptable. At the beginning of the weirdness, we did see some rare picadm.sys messages in the event log, but they went away later on.

So after some initial troubleshooting and searching numerous Citrix forums, we came up with the only solution left at that stage: open a support call at Citrix.

Despite the tests we already performed we were initially met with some skepticism. It took quite some convincing that our problem was very real.

Enough for storytelling, let’s get to the facts. These are the results of our first round of testing:

Protocol	10mb c->s	300mb c->s	10mb s->c	300mb s->c
ICA	1.39 s	45.47 s	2.86 s	92.73 s
RDP	0.64 s	15.31 s	1.82 s	43.19s

C->S stands for Client to Server transfer

S->C stands for Server to Client transfer

Tests were performed on a LAN-connection, with a firewall between Client and Server.

Just looking at the numbers, you’ll probably get the level of impact of this bug.

However, our luck was turning. For the first time, we did have evidence of the problem in our hands.

After looking at various parameters and tuning, and many months later(!), the problem was identified as being driver-related, and the picadm.sys driver in particular. Yes, that’s right, everything started out with sporadic picadm.sys warnings didn’t it? Citrix Development came up with a fix, actually a double fix: client and server side. They send me private binaries to test with, and the results were amazing.

First round of tests, done with a 12.1 ICA client:

Server078 – not patched server

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slow file transfer – Carl Webster

The Curious Case of the Slow File Transfer