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Object Storage for IBM zSystems

By Offer Baruch posted Thu May 19, 2022 04:03 AM


Introducing object storage terminology and concepts – and how to leverage cost-effective cloud data management for zSystems.

Object storage is coming to IBM zSystems.  It’s the optimal platform for demanding backup, archive, DR, and big-data analytics operations, allowing zSystems data centers to leverage scalable, cost-effective cloud infrastructures.

For zSystems personnel, object storage is a new language to speak.  It’s not complex, just a few new buzzwords to learn.  This paper was written to introduce you to object storage, and to assist in learning the relevant terminology.  Each term is compared to familiar mainframe concepts.  Let’s go!

What is Object Storage?

Object storage is a computer data architecture in which data is stored in object form – as compared to DASD, file/NAS storage and block storage.  Object storage is a cost-effective technology that makes data easily accessible for large-scale operations, such as backup, archive, DR, and big-data analytics and BI applications. 

IT departments with zSystems can use object storage to modernize their ecosystems and reduce dependence on legacy storage systems.

Basic Terms

Let’s take a look at some basic object storage terminology (and compare it to mainframe lingo):

  • Objects.  Object storage contains objects, which are also known as blobs.  These are analogous to zSystems data sets.
  • Buckets.  A bucket is a container that hosts zero or more objects.  In the zSystems realm, data sets are hosted on a volume – such as a tape or DASD device.

Data Sets vs. Objects – a Closer Look

As with data sets, objects contain both data and some basic metadata describing the object’s properties, such as creation date and object size.  Here is a table with a detailed comparison between data set and object attributes:


zSystems Data Sets

Object Storage 

File system structure

Non-hierarchical, no directory structure.

Governed by catalogs and VTOCs.

Non-hierarchical, no directory structure.  Delimiter characters can be used to emulate a directory structure.


Limited by the size of the physical media (tape, DASD) used and the DSNTYPE of the data set.

Maximum object size: 5 TB


Storage capacity is limited by the size of the physical media (tape, DASD) used.

The number of data sets is also limited by VTOC, VVDS and catalog size. Larger sizes cause degradation in performance.

“Directories” are virtual, and span multiple hardware devices. Therefore there are no physical or performance limits on the number of objects stored.  


Data set names are all caps, limited to 44 characters.
Example object name:

Object names are case sensitive, limited to 1024 characters.  The object name is also known as the object key.

Example object name:

Name delimiters

‘.’ (dot) is used to delimit name qualifiers

Any character can be used as a name qualifier.  Usually, ‘/’ is used to simulate the Unix directory structure


Standard metadata - VTOC / VVDS / Catalog has to be carefully managed to be kept in sync among different systems

Standard metadata, plus:
Custom metadata in keyvalue format, example: DocumentAuthor”John Doe”  


Data sets are indexed in VTOCs and catalogs, which are managed by the MF storage administrator.

Indexing is not managed by the storage administrator.


Data sets are sorted/filtered by prefix.

Objects are sorted/filtered by prefix.


The content of data sets can be modified multiple times.

Objects are not modifiable, they can be created, replaced, and deleted.  Versioning is a feature used to maintain older versions and deleted versions of the object.


Data sets can be accessed natively by z/OS using the proprietary Ficon protocol.

Objects can be accessed using the Open S3 protocol over TCP/IP.  

z/OS requires an additional software layer to access the object storage (this is provided by Model9).


Volumes vs. Buckets – a Closer Look

Buckets, which are analogous to mainframe volumes, are unlimited in size.  Separate buckets are often deployed for security reasons, and not because of performance limitations.  A bucket can be assigned a life cycle policy that includes automatic tiering, data protection, replication, and automatic at-rest encryption.


zSystems Volume

Object Storage Bucket


Up to 6 characters.

Up to 63 characters. 

On cloud platforms the bucket name must be unique globally. In order to avoid collisions, it is common to add the organization name as a prefix.

Example bucket name:


Limited by the size of the physical media volume (tape, DASD) used. The limitation is partially overcome using storage groups and multi-volume data sets.

A volume has a Volume Table Of Contents (VTOC) and a VTOC Index that is managed by the storage administrator.  The size of the VTOC determines the number of data sets and extents that can be created on the volume.

Buckets can span multiple hardware devices, and can be thought of as an unlimited version of the mainframe storage group.

  • Bucket size is unlimited.
  • Buckets can contain an unlimited number of objects.


The number of volumes is limited by a maximum number of devices - 64K (including secondary and flashcopy devices).

An unlimited number of buckets can be used.


  • On-premise tapes
  • DASD HDD device types and RPMs
  • Multiple cloud-storage accessibility levels also known as storage classes

Security Considerations

In the z/OS domain, a SAF user and password are required, as well as the necessary authorization level for the volume and data set.  For example, users with ALTER access to a data set can perform any action – read/write/create/delete.

In object storage, users are defined in the storage system.  Each user is granted access to specific buckets, prefixes, patterns, objects, and separate permissions are defined for each action, for example:

  • PutObject
  • DeleteObject
  • ListBucket
  • DeleteObjectVersion

In addition, each user can be associated with a programmatic API key and API secret in order to access the bucket and the object storage via a TCP/IP-based API. When accessing data in the cloud, HTTPS is used to encrypt the in-transit stream.  When accessing data on-premises, HTTP can be used to avoid encryption overhead. Object storage platforms are usually configured to perform data-at-rest encryption by default. If not, data-at-rest encryption can be enabled manually using customer managed keys. 

Disaster Recovery Considerations

While traditional zSystems storage platforms such as tape and DASD rely on full storage replication, object storage supports both replication and erasure coding.  Erasure coding provides significant savings in storage space, as the data can be spread over multiple geographical locations. For example, on IBM Cloud Object Storage, data is automatically spread across a minimum of 3 geographical locations, thus providing multi-site redundancy and disaster recovery from anywhere in the world. Erasure-coded buckets can also be fully replicated to another region, as is practiced with traditional storage. Most object storage platforms support both synchronous and asynchronous replication.

Model9 – Connecting Object Storage to the IBM zSystems platform

Model9 Cloud Data Platform is a software-only platform that leverages powerful, scalable cloud-based object storage capabilities for data centers that operate zSystems.

The platform runs on the mainframe’s zIIP processors, providing cost-efficient storage, backup, archive, and recovery functionalities with an easy-to-use interface that requires no object-storage knowledge or skills.