CPU usage 100% and Transmission Web GUI becomes unresponsive/inaccessible when trying to download large single file (24GB)

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Load a torrent into Transmission with a large single file. In my case it was 
a 24GB file.
2. Start the torrent

What is the expected output? What do you see instead?
I expect torrent to download normally. Instead, this torrent caused 
Transmission to lock up and the Web GUI is almost inaccessible.

System Info shows 100% CPU usage, access to the Router Config page appears to 
be unaffected. Tried to stop and start Tranmission service in USB Applications 
page but whenever Transmission is restarted, CPU usage immediately jumps to 
100% and the Transmission GUI remains unresponsive.

I needed multiple retries before the Transmission GUI will appear and when it 
does, it needs about 2-3 minutes to fully load. Once I stop the offending 
torrent job, everything returns to normal.

What version of the product are you using? On what operating system?
RT-N56U 3.x.3.9-095, downloading onto a 120GB HDD

Original issue reported on code.google.com by tze...@gmail.com on 1 Jul 2015 at 4:10

[GoogleCodeExporter]

Please use Ext4 filesystem + preallocation=2. 

Do not use NTFS or exFAT.

Original comment by andy.pad...@gmail.com on 1 Jul 2015 at 8:10

[GoogleCodeExporter]

I'm using ext4, though preallocation=1. Will test again with 2.

Original comment by tze...@gmail.com on 2 Jul 2015 at 1:15

[GoogleCodeExporter]

Tried with preallocation=2. No difference. I let it run for about 12 minutes 
before I decided to stop it. CPU usage maxed out with 0% progress on torrent, 
preallocation files not appearing.

Tried with torrent with relatively smaller sizes, no more than 5GB per file. 
Works just fine, preallocation files appear within a minute or two. CPU usages 
ranges from 30%-50%.

Original comment by tze...@gmail.com on 2 Jul 2015 at 1:50