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is it possible to serve blocks of storage to multiple hosts using iSCSI protocol?

each block is reserved to one writer (host)

is it only limited by available NIC ports?

update: shared storage solution based on ZFS. Primary purpose is to serve this to multiple VMs - iSCSI is considered. My question: is iSCSI LUN 1-to-1, meaning only 1 host can write?

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I think my answer addresses your question about iSCSI and multiple hosts. In particular: sure, you can share an iSCSI LUN to multiple hosts, but they need to be running some sort of cluster filesystem to coordinate their access. – larsks Feb 12 '11 at 3:32
thanks for quick response; let me correct my understaning: you mean a Linux host (not VM-underlying VMFS) running a cluster FS like GFS can share the iSCSI LUN. – John-ZFS Feb 12 '11 at 3:36
correct. The fact that the iSCSI LUN might be backed by a ZPool doesn't change how the clients see it (it's still just block storage to them). The file system the clients put on the block storage (accessible through iSCSI) needs to be able to coordinate access. – Chris S♦ Feb 12 '11 at 14:34
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3 Answers

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I'm not sure I completely understand your question. A Solaris system running ZFS can provide iSCSI storage to other hosts backed by ZFS volumes ("zvols"). FreeBSD can do this too, although the iSCSI implementation is neither as nice nor as integrated with ZFS.

An iSCSI LUN can be shared by multiple writers -- provided that they're running the appropriate cluster-aware filesystem to support the necessary coordination. Examples of cluster filesystems are VMFS (used by VMware), GFS, GPFS, and so forth.

An iSCSI LUN can easily be shared by multiple hosts if only one is using it at a given time (for example, in a failover environment where one host will take over if the primary host fails).

The limit on how many hosts you can serve is not available NIC ports...it is (a) your available bandwidth and (b) your IO requirements. If you're not doing intensive io, you can happily support many machines over a single gigabit NIC.

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do you mean the clients(writers) need to be running a cluster FS and not the storage server (which in this case is ZFS)? – John-ZFS Feb 12 '11 at 3:31
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Yes, in a nutshell. The iSCSI server doesn't particularly care how the blocks are being used; it's up to the writers to coordinate their access. – larsks Feb 12 '11 at 3:35
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up vote 2 down vote

Yes, a ZFS NAS can preset iSCSI LUNS or zvols to multiple servers. This is commonly used as a way to provide storage to multiple VMWare hosts. If you're not talking about a virtualization solution, this is still possible. Number of NIC interfaces shouldn't matter. Ideally, you'll have two or more. What exactly are you looking to do?

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up vote 1 down vote

Block storage doesn't have anything to do with ZFS (the ZFS file system, can be shared over a NAS protocol like CIFS or NFS - ZFS does not work on shared block storage as of writing this). Portions of ZPools can be shared by iscsi, this is part of ZFS, but I don't think it's what you're looking for.

iSCSI (a SAN protocol, not a NAS protocol) can serve many clients (initiators), it's primarily limited by the clients' ability to coordinate, which is most often done through SCSI3-PR (the iSCSI Target must support this for it to work).

The number of NICs and/or Ports (which isn't necessarily the same thing) does not factor into any of the above (except that you need at least one obviously).

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That's not entirely true. ZFS can provision block storage locally via the zfs create -V command (which creates a block device allocated from ZFS storage). And under Solaris, ZFS integrates tightly with the iSCSI target driver, so you can simply run zfs set shareiscsi=on to provision a LUN. – larsks Feb 12 '11 at 3:20
@larsks, corrected my answer. I'm thinking he mean the ZFS file system, not the volume manager (unfortunately most people don't realize there's a difference in the first place). Thank you! – Chris S♦ Feb 12 '11 at 3:22
thanks for your response. – John-ZFS Feb 12 '11 at 3:44
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