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I have a question about how data moves through a SAN.

For example: Within an iSCSI SAN if Server A is mounted with a SAN disk and wants to copy data to Server B which is also mounted with a SAN disk, does the data transfer happen through the SAN raid/disks or the data moving through network server a --> server b?

In file level storage when using let say's file server in windows environment as long as I understand if we move data from client a to client b using file sharing from file server the data moved using network client a --> client b hence if in client a network slower than client b network the copy process would be as slow as client a network correct?

Sorry for my bad English.

Joseph Kern
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dchochan
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2 Answers2

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Yes, you are right. Data flow path:

  • hostA asks SAN for data
  • hostA sends the data (over smb?) to hostB
  • hostB tels SAN to write down the data

SAN doesn't understand file systems, network protocols between hostA/hostB. So the slowest link will limit the transfer.

As a solution to this (and many others) problem, there are network filesystems like Ceph.

neutrinus
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  • thank you very much for your answer sir neutrinus, I plan to implement SAN to multimedia lab so each client will be assign disk from isolated iscsi san network. If it happened over san when transfer data from each host would be blazing fast. To bad. – dchochan Sep 10 '15 at 10:31
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All data transfer between the Server and the SAN happen over the network.

All data transfers between two Servers happen over the network.

If two servers mount the same disk, the iSCSI communication still happens over the network, but you may reduce the overhead of the server to server communications. But now you might have locking issues on the shared filesystem.

Network transfer in any case (client to client, server to client) is always dominated by the slowest network link speed.

Joseph Kern
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  • Wow thank you very much for your explanation sir Joseph, I though it will be write directly to raid disk. – dchochan Sep 10 '15 at 10:28
  • Nope. Only the operating system knows where the data goes. The SAN is just there to accept iSCSI information from the server. – Joseph Kern Sep 10 '15 at 10:31
  • sir joseph, a little question. would it be make sense to apply SAN network over multimedia lab with each client had only one nick? – dchochan Sep 10 '15 at 10:33
  • It just depends on NIC speed, switch back plane throughput, how many disks you have, what your workload looks like, etc. Are you talking about running iSCSI traffic directly to the clients? This is an interesting proposition, but I don't think you'll be saving too much bandwidth by doing directly attached iSCSI storage to the clients. Are you having issues with latency? – Joseph Kern Sep 10 '15 at 10:36
  • Also check out this question; http://serverfault.com/questions/81723/what-is-the-difference-between-san-nas-and-das – Joseph Kern Sep 10 '15 at 10:37
  • actually currently still do not have issue with latency but I want to analysis it for my essay so far I know it would improve server infrastructure but do not know if some one had implement it for client. – dchochan Sep 10 '15 at 10:56
  • planning using at least 4 sata 1 TB, workload around 25 pc then network using 1 gig switch and san server using iscsi with 2 gig or 4 gi(still planning). I though it can improve network speed especially when open large multimedia file by program like 3d max, maya or unity because currently the multimedia lab using file server to transport large multimedia file e.g animation, game file etc and using das for local storage. – dchochan Sep 10 '15 at 10:56
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/28982/discussion-between-dchochan-and-joseph-kern). – dchochan Sep 10 '15 at 10:56
  • Again you'll have to deal with file locking (especially in the case of things like 3d animation files). So iSCSI would probably not be appropriate here. Better to ensure that you have a redundant file server and fast LAN speeds to the desktop. You can also shovel off the rendering to a server farm. Seen this in production before. – Joseph Kern Sep 10 '15 at 18:14
  • Wow you had been seen these scheme in production before :). Look like I still had some confusion regarding SAN technology, my plan is to assign each client for testing 8 client SAN disk in hope it can improve network throughput because student often submitted the task using file sharing to lecturer and if many people submitted file at same time network some time become crowded therefore if SAN transfer file using raid directly I though it can reduce the traffic, to bad it can not. Anyway thank's for your explanation sir @Joseph Kern – dchochan Sep 11 '15 at 01:28