> > 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.
Even if Linux filesystems generally stick to a block size of 4kB, that doesn't mean all transfers are maximum 4kB. If that would have been the case, Linux would be quite useless for a server. I/O operations are queued and if, for instance, a read() call requests 8MB, that's done in a single operation. > 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. That's why, back in 1992, the sliding window protocol was created (http://tools.ietf.org/html/rfc1323), so that a peer won't wait for a TCP ACK before resuming operation. Vennlige hilsener / Best regards roy -- Roy Sigurd Karlsbakk (+47) 97542685 r...@karlsbakk.net http://blogg.karlsbakk.net/ -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
