There are six mainframe suppliers of z/OS virtual tape. The three traditional suppliers are IBM with its Virtual Tape System (VTS), SUN with its Virtual Tape Storage System (VTSS) and Fujitsu-Siemens with CentricStor. Fujitsu market their product as Eternus VT600 in Asia. The fundamental difference between the VTS, CentricStor and the VTSS, is that the intelligence for the VTS and CentricStor runs on a processor within the Virtual Tape System, while the intelligence for the VTSS is held outboard, on an z/OS processor.
IBM virtual tape
The IBM mainframe offering is the TS7700. This comprises a virtual tape grid system with synchronous replication and automated fail-over. The cluster currently has three nodes, but the architecture can support up to eight. The management is an integral part of the VTS and the intelligence is outboard of the mainframe servers. This means that the location and number of virtual volumes are virtualised, which will assist with content-based access. It is possible that IBM will add tape data indexing to the tape grid in future, outboard from the mainframe.
The TS7700 uses IP links for replication, which means the replication is cheaper than a FICON or ESCON solution. It also has separate channels for the front end host communications and backend tape drive communications.
SUN virtual tape
The SUN / StorageTek offering is the VSM-5. They have adopted a modular approach to mainframe tape virtualisation, the four modules being: tape libraries, tape drives, virtual tape disk buffers, and software. This means that most of the logic and intelligence is still held on the mainframe. The VSM uses an SVA for the disk cache and just comes as a 2 node cluster. It uses ESCON or FICON for replication channels with just one set of channels to talk to both hosts and drives.
The controlling software is z/OS host-based, and SUN claims that this gives them enormous scalability, with up to 256 VSM systems as a single image.
The VSM now supports both synchronous and asynchronous replication of virtual tape volumes within a clustered VSM environment with VSM4 and upwards. Synchronous replication performs the VTV replication before the virtual tape drive completes the rewind/unload command at the sending VTSS.
Both Asynchronous and Synchronous replication modes can be used simultaneously in the same VTSS by assigning tapes to different SMS management classes.
Fujitsu virtual tape.
The Fujitsu offering is the CentricStor. While the IBM and SUN solutions both lock you into that supplier's tape drives and tape silos, the CentricStor solution supports both SUN and IBM tape libraries, and others. This makes it independent of the backend tape drives, and so it is less of a lockin solution. CentricStor can also be attached to Open Systems and supports heterogeneous platforms at the same time (Mainframe, UNIX, Windows).
The CentricStor consists of a number of standard modules like RAID devices and Standard Processors that are all internally connected with a fibre-channel SAN. The high end CentricStor devices have specialised processors for Host data management, Device data management to the tape drives and control processors.
EMC, HDS and Bus-Tech
HDS EMC and Bus-tech all offer disk only virtual tape solutions. They all work on the same set of principles:
Software that emulate real tape drives
Large amounts of disk storage
An outboard processor to manage them all
Just for completeness, some vendors offer a software only solution. An example is CA with its Brightstor product.
The following table describes the SUN VSM 5 system, CentricStor 5000, an IBM TS7700 an HDS VTF, an EMC DLM and a Bus-tech MDL 6000. The table includes the terms used by each supplier to describe the components.
CentricStor VTA 5000
SUN VSM 5
IBM TS7700
HDS VTFM
EMC 4VTE DLM
Bus-Tech MDL 6000
Maximum number of virtual drives
32-512 with 3595-VDR expansion. (emulated as 3490E or generic SCSI-II, Exabyte Mammoth-2, 3590)
256/512
256-512-768 (these number triples refer to 1, 2 or 3 node clusters)
256
1024
78-1536
Maximum number of virtual volumes in one virtual tape system
1,500,000
100,000 active in the VTSS cache, no effective limit for VTVs migrated to MVCs; Virtual Tape Volume (VTV)
1,000,000; Virtual Volumes
unlimited
unlimited
unlimited
Back end storage
12 drives of type LTO (Gen1/2), IBM Magstar or STK 9840/9940
32 drives, called Real tape Drives (RTD) of 9840, 9940 or Timberline; Tapes are called Multi-Volume Cartridge
(MVC)
16-32-48 tape drives of 3590B or E, 3592, TS1200. Tapes are called "Stacked Cartridge"
SATA disk based - no data given on size
1TB SATAII disk drives to a maximum of 190 TB
External disk is connected by FC channels, no real limits.
Server Support
z/OS mainframe and variants, Windows versions, Unix versions, Linux. See Fujitsu for full list.
z/OS variants
z/OS variants
z/OS variants
z/OS variants
z/OS variants
Library Support
SUN libraries, ADIC libraries, IBM 3494 libraries Fujitsu libraries
SUN libraries
IBM libraries
N/A
N/A
N/A
Maximum disk cache capacity
230 TB native (TVC)
7 TB native; Virtual Tape Subsystem Buffer (VTSB)
6-12-18; Tape Volume Cache
N/A
N/A
N/A
Controlling Software
Based in the CentricStor appliance
HSC software runs on z/OS Host
AIX based Virtual Tape Controller software, within the VTS.
Vtape emulation controller
Virtuant software
Connectivity
32 ESCON or 16 Fibre channel or mixed from Host to
CentricStor. 16 SCSI or FC to RTDs. FICON is now available
4- 16FICON. The VSM 4 devices are also ESCON capable.
4-8-12 4 Gb/s FICON
to RTDs.
No Data
8 * 4Gb FICON or 4 ESCON
6-12 FICON
Scalabilty
Very scalable, as CentricStor is independent of the
tape silos.
As the controlling software and database is in the
Host, up to 256 VSMs can be clustered together, and this will appear
to be seamless to the z/OS operating system. The backend tape
drives are located in an STK siloplex, which has almost unlimited
capacity
Two VTS systems can be defined to one TS3500 tape library.
No data
Limited to 190TB disk store
Appears to have no limit, but check with vendor
Mirroring capability
Uses 'dual save function' to save data to two physical
cartridges, which can be held in two different libraries. Mirroring
between 2 CentricStor systems is planned
Immediate copy (i.e. after tape rewind command issued,
this is not synchronous) or deferred copy, can be selected by file
name
Either all synchronous (tape rewind not issued until
both virtual tape copies are written) or all asynchronous.
Can use Truecopy replication to mirror between two VLTs
