On Sat, 2011-08-06 at 21:40 +0200, Eugen Leitl wrote:
> I've recently figured out how to make low-end hardware (e.g. HP N36L)
> work well as zfs hybrid pools. The system (Nexenta Core + napp-it)
> exports the zfs pools as CIFS, NFS or iSCSI (Comstar).
>
> 1) is this a good idea?
>
> 2) any of you are running vserver guests on iSCSI targets? Happy with it?
>
Yes, we have been using iSCSI to hold vserver guests for a couple of
years now and are generally unhappy with it. Besides our general
distress at Nexenta, there is the constraint of the Linux file system.
Someone please correct me if I'm wrong because this is a big problem for
us. As far as I know, Linux file system block size cannot exceed the
maximum memory page size and is limited to no more than 4KB. iSCSI
appears to acknowledge every individual block that is sent. That means
the most data one can stream without an ACK is 4KB. That means the
throughput is limited by the latency of the network rather than the
bandwidth.
Nexenta is built on OpenSolaris and has a significantly higher internal
network latency than Linux. It is not unusual for us to see round trip
times from host to Nexenta well upwards of 100us (micro-seconds). Let's
say it was even as good as 100us. One could send up to 10,000 packets
per second * 4KB = 40MBps maximum throughput for any one iSCSI
conversation. That's pretty lousy disk throughput.
Other than that, iSCSI is fabulous because it appears as a local block
device. We typically mount a large data volume into the VServer host
and the mount rbind it into the guest file systems. A magically well
working file server without a file server or the hassles of a network
file system. Our single complaint other than about Nexenta themselves
is the latency constrained throughput.
Any one have a way around that? Thanks - John
Received on Sat Aug 6 21:30:20 2011