Need help understanding SANs

[mike2323]

Storage Area Networks are completely foreign to me. NAS and DAS's aren't. Can someone give me a breakdown of how a SAN works?

When a system is connected to the SAN's fiber switch, does the 'allocated' space act as a 'directly attached harddrive'? Are there typically multiple partitions with different format types (NTFS, ext3, etc etc)? I have absolutely no experience with them and would like to gain some understand of them.

 

[unhappy_mage]

Step one.

 

[mike2323]

It doesn't completely answer my question.

1) Does that mean the SAN is seen as just another hard drive to the system connected to the switch fabric?

2) Can multiple systems see the same drive? Or is the relationship 1-to-1?

 

[drizzt81]

from what I understand a SAN is a seperate network, it is attached to one (or more) servers that provide user access to the SAN. I would assume that SANs can be accessed by multiple servers (redundancy) and the number of drives that the user 'sees' is most likely configuration dependent, i.e. the administrator will be able to partition the storage area to his likings and export certain mount points to the user.

 

[unhappy_mage]

1) Does that mean the SAN is seen as just another hard drive to the system connected to the switch fabric?

More or less. It may have additional capabilities, but it generally acts like Just Another Block Device (tm) in terms of how it's accessed by a single system.

2) Can multiple systems see the same drive? Or is the relationship 1-to-1?

It depends, really. GFS (not to be confused with GFS ) lets you connect multiple machines to the same physical storage device, and they all access it at the block level. But a more usual scenario is a 1:1 relationship. At my college, there is a set of servers, each connected to its own SAN storage, and joined together with AFS.

 

[klbennet]

1) Does that mean the SAN is seen as just another hard drive to the system connected to the switch fabric?
Yes, once you have configured the SAN connection and allocated a block of storage to the system, the SAN space will appear as just another drive. You will need to partition and format it just like any other drive.

2) Can multiple systems see the same drive? Or is the relationship 1-to-1?
System to LUN (Logical Unit, the space you allocated from the SAN for the system to use) is 1:N (one system accessing many LUN). A SAN can have multiple LUN, and each LUN can be allocated to a different system. You do not want more than one system accessing a single LUN at the same time, unless you are using some sort of clustering solution (GFS, Microsoft, Oracle RAC). The standard Windows and Linux filesystems are not cluster aware and will result in corruption of you share LUNs without a clustering solution.

If you want to get your feet wet, you can mess with iSCSI under Linux fairly cheaply. You need two machines, two networks cards (GigE preferred), a software initiator (available for Linux [openiscsi] and Windows [from MS] for free), a copy of Linux and one of the iSCSI targets for Linux (I use iscsitarget from sourceforge). If you look around you can also find some trial versions of Windows iSCSI targets.

My SAN at home consists of the following:
3 Servers (File Server, VM Server, Desktop) --> GigE Switch --> Linux iSCSI Target (iSCSI, LVM, RAID) --> PATA & Firewire Disks

The disks are carved up into about a dozen LUNs.

I use Linux’s LVM and RAID capabilities to manage my space. This gives me the ability to expand my storage and extend my LUNs as my servers require more space. Minus some of the management and monitoring tools, I have similar functionality comparable to a mid-range commercial SAN. Total SAN space is ~800Gigabytes. I hope to expand to 1TB soon.

My SAN at work consists of:
4 Servers (VM Servers, File Server, Tape Server) --> Fiber Channel --> EMC Target --> SCSI Disks

Don’t ask me how the storage is allocated. I know we have RAID and LVM abilities. Total SAN space is 2+Terabytes.