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Storage Area Networks

Storage Area Networks. The Basics. Storage Area Networks. SANS are designed to give you: • More disk space • Multiple server access to a single disk pool • Better performance • Option of disk distributed across multiple locations. Direct Attached Storage.

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Storage Area Networks

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  1. Storage Area Networks The Basics

  2. Storage Area Networks • SANS are designed to give you: • • More disk space • • Multiple server access to a single disk pool • • Better performance • • Option of disk distributed across multiple locations

  3. Direct Attached Storage • Classically, for storage we had a single box with a bunch of disks attached: Public Network LUN1 LUN2 LUN0 Server SCSI Bus

  4. Attached Storage • The server speaks to the SCSI disks using a command language: • • Read from LUN0, Block 123 • • Write to LUN1, Block 456 • All this goes over the SCSI bus, which is directly attached to the server; only that server has access to the bus • The server would create a filesystem on the disk(s) and could then make the disk available to other computers via NFS, Samba, etc.

  5. Network Attached Storage • This idea is easily extended to an appliance approach. Configure a utility box with some disk that does only NFS or Samba/SMB, place on network Public Network NAS Server NFS Client NFS Server SCSI Bus

  6. NAS and Servers • Redundant web servers share the same data--but they both talk to the same NFS server Public Network NAS Server Web server, data NFS mounted NFS Server SCSI Bus Web server, data NFS mounted

  7. Attached Storage • We can also do things like place a RAID array on the NAS server. • This works, but it has some limitations: • • If the server goes down, there is no access to the disk • • File sharing goes through the network storage server and across the network, which can be slow • • Limitations on location of disks--must be near server, within range of the disk bus • • Adding or subtracting disk space can be difficult • What we want is a shared disk pool that all servers can access

  8. Storage Area Network • What we want is something that looks like this: NFS Client Public Ethernet Net SAN Participants Disk Pool

  9. Storage Area Network • Notice: • • You can take down a server and still maintain access to the disk pool via the other SAN participants • • Disk added to the pool is available to all servers, not just one • • Shared, high speed access to the disk pool; can run clustered copies of SQL database or web server if the SQL databases or web servers are also SAN participants • • Can still serve up the disk pool via an NFS or SMB server on a SAN-connected box • • “serverless backups”--just send command to copy blocks from disk A to disk B. Snapshots easier, shortened backup windows--you can have a SAN particpant handle moving a volume to tape

  10. Storage Area Network • So how does this work? It’s a scaled up version of the old system. • • The commands being sent are the same disk standard commands: either SCSI or ATA disk bus commands, READ, WRITE, etc. • • The network connecting the SAN servers to the disk is typically (but not always) higher speed, eg FibreChannel • • Some extra glue to allow for concurrent access by more than one server--need a shared filesystem • • Special filesystems to allow for concurrent access

  11. Storage Area Network • A popular choice: • • SCSI for the bus commands (commands sent over the wire) • • Fiber Channel for the SAN network • • EMC or similar for the glue volume software • Fiber Channel is 2+ Gbit/sec, and can be deployed across up to a 500m distance (sometimes) and up to 70 KM with special equipment

  12. Storage Area Network • Another option is to use gigabit ethernet for the SAN networking. • • Cheap! Commodity equipment, don’t need to learn new Fiber Channel stuff, reuse existing gear • • But also lower performance--fibre channel has higher BW, and can use more of it.

  13. ATA Over Ethernet • AoE uses ethernet plus ATA bus commands rather than SCSI. Low cost; but since ethernet frames are not routable all devices must be on the same network

  14. iSCSI • iSCSI uses SCSI bus commands over ethernet, encapsulated inside of TCP/IP • • Cheap hardware! • • Well supported in Linux, Solaris, and Windows world • • Because the SCSI is inside of TCP/IP, it is routable--which means you can do a SAN across wide area networks (with lower performance due to latency) and do things like mirror for disaster backup, or across campus on high performance networks • • Processing TCP/IP takes some overhead; some use TCP offload chips

  15. iSCSI • Each “disk”/LUN is a RAID array that understands iSCSI. NFS Client Public Ethernet Net

  16. iSCSI • The green network is a dedicated (usually) gigabit ethernet network that carries the SCSI commands encapsulated inside TCP/IP. The red network connects the SAN participants to other clients not on the SAN • Important point: TCP/IP is routable. That means that (modulo latency) the devices can be located anywhere. We could have a iSCSI SAN participant in Root Hall, and one in Spanagel. The Root iSCSI server can access the disk pool in Spanagel • We could also have a volume located at Fleet Numeric in the same SAN • The price we pay for this is having to process the TCP/IP overhead as iSCSI commands go up the network protocol stack. This can be alleviatedin part by TCP offload chips

  17. Volume Software • Remember, the iSCSI targets are just block devices. iSCSI says nothing about concurrent access or multiple hosts accessing the same devices • For that we need a SAN Filesystem. This deconflicts concurrent access by hosts to the block devices

  18. Volume Software NFS Client Public Ethernet Net Vol2 Vol1

  19. SAN Software • The “volume software” allows you to build a concurrent access filesystem out of one or more LUNs

  20. iSCSI • Example: Five compute servers need read access to one weather data set. If the servers are all on the SAN, they can directly access the data • Example: backup. Copy disk blocks directly, then have a tape drive SAN participant copy to tape • Example: storage expansion. Just add more disk, and it is available to all SAN participants

  21. Competitors • iSCSI’s competitor is for the most part fibre channel. The concept of fiber channel is almost identical, but the SCSI commands are simply encapsulated in a fibre channel frame • Fibre channel is typically higher performance--more data can be pushed across FC, and there is much less overhead processing FC frames • BUT it is higher cost • ATA Over Ethernet is very similar to FC in concept--directly inserting the ATA commands in ethernet frames. But it seems to have less market penetration

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