Re: [zfs-discuss] ZFS performance degradation when backups are running

[Jean Dion]

Normal iSCSI setup split network traffic at physical layer and not logical layer. That mean physical ports and often physical PCI bridge chip if you can.  That will be fine for small traffic but we are talking backup performance issues.  IP network and number of small files are very often the bottlenecks. If you want performance you do not put all your I/O across the same physical wire.  Once again you cannot go faster than the physical wire can support (CAT5E, CAT6, fibre).  No matter if it is layer 2 or not. Using VLAN on single port you "share" the bandwidth and not creating more Gbits speed with Layer 2. iSCSI best practice require separated physical network. Many books, white papers are written about this.  This is like any FC SAN implementation.   We always split the workload between disk and tape using more than one HBA.  Never forget , backup are intensive I/O and  will fill the entire I/O path. Jean gm_sjo wrote: 2008/9/30 Jean Dion <[EMAIL PROTECTED]>: Simple. You cannot go faster than the slowest link. That is indeed correct, but what is the slowest link when using a Layer 2 VLAN? You made a broad statement that iSCSI 'requires' a dedicated, standalone network. I do not believe this is the case. Any VLAN share the bandwidth workload and do not provide a dedicated bandwidth for each of them. That means if you have multiple VLAN coming out of the same wire of your server you do not have "n" time the bandwidth but only a fraction of it. Simple network maths. I can only assume that you are only referring to VLAN trunks, eg using a NIC on a server for both 'normal' traffic and having another virtual interface on it bound to a 'storage' VLAN. If this is the case then what you say is true, of course you are sharing the same physical link so ultimately that will be the limit. However, and this should be clarified before anyone gets the wrong idea, there is nothing wrong with segmenting a switch by using VLANs to have some ports for storage traffic and some ports for 'normal' traffic. You can have one/multiple NIC(s) for storage, and another/multiple NIC(s) for everything else (or however you please to use your interfaces!). These can be hooked up to switch ports that are on different physical VLANs with no performance degredation. It's best not to assume that every use of a VLAN is a trunk. Also iSCSI works better by using segregated IP network switches. Beware that some switches do not guaranty full 1Gbits speed on all ports when all active at the same time. Plan multiple uplinks if you have more than one switch. Once again you cannot go faster than the slowest link. I think it's fairly safe to assume that you're going to get per-port line-speed across anything other than the cheapest budget switches. Most SMB (and above) switches will be rated at say 48gbit/sec backplane on a 24 port item, for example. However, I am keen to see any benchmarks you may have that shows the performance difference between running a single switch with layer 2 vlans Vs. two seperate switches.

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