Disk I/O on Xen VPSes
I just got my Linode 360 (only two extras are two extra IPs) and set up a LAMP stack, and Transmission for BitTorrent with the Clutch WebUI.
Torrenting, as I've read about on these forums, is a murder on the disk I/O. I've been getting warnings, and my torrents have slowed down to unreasonable speeds (downloading at 583 bytes/second with 20 seeds and no leeches?
I moved the torrents to an ext2 partition to avoid journaling, as somebody suggested. I hope this'll help. But in the meantime, this 8GB move has killed my I/O even more.
So pretty much the I/O system seems to be killing me, and I have some questions about it:
1. How exactly does the Xen I/O control system work?
2. Somebody suggested moving the alert threshold up for Xen. What is a reasonable disk I/O alert threshold for a Xen Linode 360?
3. Assuming Xen throttles me, how long would it last for before I would get back to normal?
4. Could it really be throttling me down to 4 kB/s disk I/O or even less?
Anybody who can please help answer these. Thanks.
3 Replies
@Falselife:
I assumed the slowness was because of disk I/O being throttled.
You might try the following to see if your IO is really being limited (you might adjust the count down if you are really as slow as you think)````
$ dd if=/dev/urandom of=testfile bs=1024 count=1024
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB) copied, 0.164511 s, 6.4 MB/s
````
> 2. Somebody suggested moving the alert threshold up for Xen. What is a reasonable disk I/O alert threshold for a Xen Linode 360?
Adjusting the alert threshold will not change the performance, it is just a warning for you to know what is happening on your linode.
@Stever:
You might try the following to see if your IO is really being limited (you might adjust the count down if you are really as slow as you think)````
$ dd if=/dev/urandom of=testfile bs=1024 count=1024
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB) copied, 0.164511 s, 6.4 MB/s
</quote> Hm, I did this, and got:
dd if=/dev/urandom of=testfile bs=1024 count=1024 1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB) copied, 0.184902 s, 5.7 MB/s
That doesn't seem that throttled from yours. I also tried a wget from my local computer for a 32MB file on the server (on the torrent disk image, actually) and got:
Length: 33,981,351 (32M) [application/download]
100%[=======================================================================>] 33,981,351 477.28K/s ETA 00:00
11:57:31 (511.62 KB/s) - `test.mov' saved [33981351/33981351]
````So now I'm just confused. Does Xen only throttle the process that hogged the I/O (in this case Transmission)? Or maybe does the Dallas datacenter throttle BitTorrent? It worked fine for the first day or so, which is making me suspicious. I can get fine speeds for those same torrents from my home computer running Transmission, so it isn't a problem with the torrents or with Transmission.
@Stever:
Adjusting the alert threshold will not change the performance, it is just a warning for you to know what is happening on your linode. Oh, I know, I just meant I'm not sure when I should be getting worried.
@Falselife:
Torrenting, as I've read about on these forums, is a murder on the disk I/O.
…
So pretty much the I/O system seems to be killing me, and I have some questions about it:
1. How exactly does the Xen I/O control system work?
…
Anybody who can please help answer these. Thanks.
I've used rtorrent without any problems on a Linode360. Does it get bad throughput with that bittorrent app aswell?