iSCSI Multiple Connections to 1 LUN Read/write

[Photubias]

Hello,

We have several LUNs with each 1 target per LUN.
But we want 2 servers (initiators) to have access to the same LUN.
One of them needs to read what the other one writes.

But when ServerA writes to LUNA, it takes an hour for the file to be visible on ServerB.
Is this normal?

Greetings,

Photubias

[Ahead-IT]

Hello,

We have several LUNs with each 1 target per LUN.
But we want 2 servers (initiators) to have access to the same LUN.
One of them needs to read what the other one writes.

But when ServerA writes to LUNA, it takes an hour for the file to be visible on ServerB.
Is this normal?

Greetings,

Photubias

Same problem here

[To-L]

That seems normal since ISCSI targets are block based and not file based. Changes made by one person are not seen by another person working on the same LUN.
One way to see the changes quicker is to quit you ISCSI session and then reconnect to see the changes.

[Photubias]

OK, I want a centralized system where One Server writes to shared storage and other servers (at block level) kan read from that storage?

- Samba is not possible, because the ColdFusion servers need block access.
- iSCSI is not possible, because the changes are not (immediatly) visible in shared storage?

[Photubias]

That seems normal since ISCSI targets are block based and not file based. Changes made by one person are not seen by another person working on the same LUN.
One way to see the changes quicker is to quit you ISCSI session and then reconnect to see the changes.

OK, but when I want to run a Database on the iSCSI LUN?
When changes are made to that database, other servers can't see them immediately?

[vkeven]

OK, but when I want to run a Database on the iSCSI LUN?
When changes are made to that database, other servers can't see them immediately?

I never worked with cold fusion database but the thing you describe is probably support with some extension to coldfusion database and generally speaking when you want to access data on the same filesystem from more than 1 computer at the same time you need a network filesystem like Redhat GFS or something similar , doing this is not gonna be a simple one click task.

[To-M]

vkeven is correct.

To control this you need so called SAN File System programs. SAN file system solutions are very expensive.

Examples are listed below:
MelioFS:
http://www.sanbolic.com/melio.htm
IBM:
http://www-306.ibm.com/software/tivo...ducts/sanergy/
http://www.ibm.qassociates.co.uk/sto...rage-san-files
yste
m.htm
StorNext:
http://www.adic.com/ibeCCtpItmDspRte...24&item=121889
SGI:
http://www.sgi.com/products/storage/...e_systems.html
PolyServe
http://www.polyserve.com/
DataPlow SFS
http://www.dataplow.com

[miguel.jamous@iway.be]

Hello all,

ISCSI is not made for sharing files or folders !

You can use the same iscsi target from different initiators as long you use clustering software to access it. otherwise you will corrupt the data.

Miguel Jamous
IT Administrator


This article said it all and explain it very clear.

'''Internet SCSI''' ('''iSCSI''') is a [[Protocol (computing)|network protocol]] standard, officially ratified on [[2003-02-11]] by the [[Internet Engineering Task Force]], that allows the use of the [[SCSI]] protocol over [[TCP/IP]] [[computer network|networks]]. iSCSI is a transport layer protocol in the [[SCSI]]-3 specifications framework. Other protocols in the transport layer include [[SCSI Parallel Interface]] (SPI), [[Serial Attached SCSI]] (SAS) and [[Fibre Channel]].

Acceptance of iSCSI in corporate production environments has accelerated now that [[gigabit ethernet]] is becoming more common. Building iSCSI-based [[storage area network|storage area networks (SAN)]] has become a less costly but worthy alternative to creating Fibre Channel-based SANs.

==Functionality==
The iSCSI ([[International Phonetic Alphabet|IPA pronunciation]]: {{IPA|[aɪskʌzi]}}) protocol uses TCP/IP for its data transfer. Unlike other network storage protocols, such as [[Fibre Channel]] (which is the foundation of most SANs), it requires only the simple and ubiquitous [[ethernet]] interface (or any other TCP/IP-capable network) to operate. This enables low-cost centralization of storage without all of the usual expense normally associated with Fibre Channel [[storage area network]]s.

Critics of iSCSI expect worse performance than Fibre Channel due to the overhead added by the TCP/IP protocol to the communication between client and storage. However new techniques like [[TCP Offload Engine]] (ToE) help in reducing this overhead. Tests have shown excellent performance of iSCSI SANs, whether ToEs or plain [[gigabit ethernet]] [[network card]]s were used. The iSCSI market is growing steadily, and should improve in performance and usability as more organizations deploy Gigabit and 10 Gigabit networks, and manufacturers integrate iSCSI support into their operating systems, SAN products and storage subsystems. iSCSI becomes even more appealing as [[ethernet]] can now support higher speeds than [[Fibre Channel]].

==Storage Devices==
In the context of [[computer storage]], iSCSI allows a machine to use an iSCSI [[SCSI initiator|initiator]] (client) to connect to remote [[SCSI target|target]]s (servers) such as [[hard disk|disks]] and [[tape drive]]s on an IP network for [[block storage|block level I/O]]. From the point of view of the class drivers and application software, the devices appear as locally attached SCSI devices.

iSCSI devices should not be confused with [[network-attached storage|network-attached storage (NAS)]] devices which include server software to handle the arbitration of simultaneous access requests from different hosts. Allowing multiple hosts to have simultaneous access to a single device is a difficult task common to iSCSI and FC devices. Without host-to-host communication, each host is unaware of the state and intentions of the other hosts. This condition can lead to data corruption and race conditions. In the realm of disk storage, [[Computer cluster|cluster software]] solves this issue.

==iSCSI terminology and how iSCSI can be used by an OS==
An ''iSCSI initiator'', in client/server terminology, is akin to a client device that connects to some service offered by the server. An ''iSCSI target'' is akin to a server in that it provides block level access to its storage media. This storage medium is usually a hard drive, but can be other types of SCSI devices if the iSCSI target software supports them. The primary difference between a regular client/server system and an iSCSI initiator/target system is that while many clients can simultaneously access the same files served by a single server, iSCSI requires complex coordination between initiators to synchronize accesses to the same files.

iSCSI can be used to give an OS block-level access to a remote device (one attached to another device likely on the same [[LAN]]). Depending on the types of devices supported by the iSCSI target, this block-level access can be used to build [[RAID]] volumes using remote disks, to connect and use remote DVD/CD-writers and [[scanners]], or to simply access additional storage space. iSCSI (like all [[SCSI]] protocols) does not mandate how devices are shared amongst multiple computers at the same time. Instead, the operating system in control of the iSCSI device is responsible for controlling remote access to the device. The initiator and the operating system that it is running on will determine how the iSCSI device appears to the operating system's end user.

==Industry support==

==== OS Support Overview ====
{| class="wikitable"
|-
! OS
! First release date
! Version
! Features
! Limitations
|-
! [[AIX operating system|AIX]]
| 2002-10
| AIX 5.2
| Initiator
|
|-
! [[Microsoft Windows|Windows]]
| 2003-06
| 2000, XP Pro, 2003, Vista, 2008
| Initiator, Target, Multipath
| Limited command data length (256k)
|-
! [[Novell NetWare|NetWare]]
| 2003-08
| NetWare 5.1, 6.5, & OES
| Initiator, Target
|
|-
! [[HP-UX]]
| 2003-10
| HP 11i v1, HP 11i v2
| Initiator
|
|-
! [[Solaris Operating System|Solaris]]
| 2005-02
| Solaris 10
| Initiator, (OpenSolaris and Solaris 8/07 only)Target
|
|-
! [[Linux kernel]]
| 2005-06
| 2.6.12
| Initiator, Target, [[ISCSI_Extensions_for_RDMA|iSER]]
|
|-
! [[Mac OS X]]
| 2007-10
| 10.5 (Leopard)
| Initiator
|
|}

=== Host Bus Adapters ===
iSCSI ''host bus adapters'' (HBAs) are [[Network card|network interface controllers]] that incorporate a [[TCP Offload Engine]] with onboard iSCSI processing. iSCSI HBAs are usually abstracted purely as SCSI controllers by the operating system driver system. In this case, the HBA is not a part of the system network stack. Recently there has been considerable effort in providing standard vendor-independent iSCSI management facilities via operating system driver interfaces. These standard interfaces also work with software initiators.

Since iSCSI HBAs offload considerable processing in the iSCSI stack, [[interrupt]] generation is less than comparable software initiator setups. Applications leaving few spare processing cycles should benefit from the use of a dedicated HBA.

iSCSI HBAs have the advantage of including [[PCI]] [[option ROM]]s to allow booting from iSCSI targets. Alternative iSCSI boot methods with software initiators would require substantial work.

=== Targets ===

Most industry focus has been placed on creating iSCSI disk targets, though iSCSI tape and medium changer targets are popular as well. So far, physical devices have not featured native iSCSI interfaces on a component level. Instead, devices with [[SCSI Parallel Interface]] or [[Fibre Channel]] interfaces are bridged by using iSCSI target software, external bridges, or controllers internal to the device enclosure.

Alternatively, disk and tape targets can be virtualizations. Rather than representing an actual physical device, an emulated virtual device is presented. The underlying implementation can deviate drastically from the presented target as is done with [[Virtual Tape Library]] (VTL) solutions. VTLs use disk storage for storing data written to virtual tapes. As with actual physical devices, virtual targets are presented by using iSCSI target software, external bridges, or controllers internal to the device enclosure.

In the security products industry, some manufacturers use an iSCSI RAID as a target, with the initiator being either an IP-enabled encoder or camera.

=== Converters/Bridges ===
There are multiple systems which allow Fibre Channel, SCSI & SAS devices to be attached to an IP network for use via iSCSI. They can be used to allow migration from older storage technologies, access to SANs from remote servers and the linking of SANs over IP networks.

== Addressing and Name formats ==

iSCSI provides three name formats for different types of naming authorities:

* iSCSI Qualified Name (IQN), Format: iqn.yyyy-mm.{reversed domain name} (e.g. <tt>iqn.2001-04.com.acme:storage.tape.sys1.xyz</tt&gt
* Extended Unique Identifier (EUI), Format: eui.{EUI-64 bit address} (e.g. <tt>eui.02004567A425678D</tt&gt
* T11 Network Address Authority (NAA), Format: naa.{NAA 64 or 128 bit identifier} (e.g. <tt>naa.52004567BA64678D</tt&gt

In the case of IQN the address is qualified by a date (yyyy-mm) since a domain name can expire or be acquired by another entity. EUI is provided by the IEEE Registration authority in accordance with EUI-64 standard. NAA is part OUI which is provided by the IEEE Registration Authority.

NAA name formats were added to iSCSI in RFC 3980, to provide compatibility with naming conventions used in [[Fibre Channel]] and [[Serial Attached SCSI|SAS]] storage technologies.

Each target is defined by three or four fields:

HostNameOrAddress PortNumber iSCSIName CHAPSecret (optional)

Example:

iscsi.example.com 3260 iqn.2003-01.com.ibm:00.fcd0ab21.shark128 "123ismysecretpassword"

[SuperSAN]

Miguel thanks for the good article...

I used to have the same question until open-e clarify it for me.. As somebody said in other posts, think about iSCSI connection as an IDE cable. You can't connect 2 cables tot he same drive at the same time....

[Robotbeat]

You can, of course, use a snapshot and have the read-only computer mount the snapshot. You'll only be able to see changes when the snapshot is refreshed, however.