I have done something similar in a home-rolled SAN, and am fairly pleased with the results. I wasn't sure from your post if you were looking to build the in-band virtualization, or if you you had a switch that was capable of doing that. I could not afford a specialized iscsi-switch, so I used stock ethernet switches and built my own in-band virtualization system. Hopefully my experience can be of some use to you.
I use IET (iSCSI Enterprise Target) on boxes with internal disk arrays to make 4 iscsi targets. Then I connect this across bonded interfaces and twp IP fabrics to an in-band virtualizer (two, actually, running in fail-over mode). The job of this virtualizer is similar to what you are looking for, I think. Our virtualizer acts as an iscsi initiator to the 4 iscsi target devices, and joins them into one large raid5 device. I then use LVM to carve out the "virtual" disks, and present them as iscsi targets to other systems. If you did the same thing with raid1 (mirrored) targets, then an iscsi request would read in a round-robin fashion from the two in-sync targets at your remote locations. The problem that I ran into was in keeping performance high. You end up passing the data through a lot of layers (hardware->iscsi->multipath->software raid->lvm->iscsi->multipath->OS). If you want to ENSURE that your data lands safely on the disk, then you have to turn off the software caching along the way, and this really hurt my performance. I now run with caching enabled in the virtualizer iscsi target, and performance is much better. But if you do that you need to make sure that your virtualizer does not crash, or you will loose the dirty blocks in the cache. I was looking for a generalized solution to hook to many different servers. If you are only dealing with one application then you should be able to get pretty good performance by careful tuning. The good side of going in-band is that it is generally easier and less invasive, since there are no agents to install in every OS to manage the out-of-band controls. But the downside is that all your data must flow through the in-band system and that can slow things down. Now, if you are looking at doing this inside of an iscsi-capable switch, then the same issues apply. An iscsi-capable switch is like my setup, but they have wrapped my virtualizer and my stock ethernet switches into one device (which is far more specialized than my generic devices). You still should look at how they handle caching in the switch, and how well you can tune and protect that. One other thought -- is this necessary? If you are just looking to read off of two iscsi targets that have the same data, then it might be simpler just to connect your server to the two iscsi targets in the SAN and use raid/multipath. That should take the OS system call to read the data, and split that into iscsi commands that pull from each of the targets. This takes some setup on the server, but it would be much simpler! -Ty! varun wrote: > I am thinking of a design to improve performance of SAN. To do this i > am thinking of an in band switch based virtualization which recieves > an ISCSI request and splits the request into two and simultaneously > reads data from two insync disk at remote locations. tell me is this > solution feasible !! and i want software based solution with minimum > hardware involvement ! tell me how out of band storage virtualization > can help me in this ?? > > > -- -===========================- Ty! Boyack NREL Unix Network Manager [EMAIL PROTECTED] (970) 491-1186 -===========================- --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to firstname.lastname@example.org To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/open-iscsi -~----------~----~----~----~------~----~------~--~---