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Linux on IBM Z

Linux on IBM Z refers to the implementation of operating systems on mainframe computers, which employ the s390x architecture to enable high-performance, secure, and scalable enterprise computing for workloads such as , data analytics, and cloud-native applications. This platform integrates the open-source flexibility of with the mainframe's inherent strengths in reliability, , and cryptographic capabilities, allowing organizations to run thousands of virtual machines or containers on a single system while achieving up to 99.999% uptime. The history of Linux on IBM Z began in late 1999, when IBM released kernel patches for 2.2.13 to run on the , marking the first steps toward bringing to mainframe environments. In February 1999, IBM announced a partnership with to support , followed by the announcement of full production support for on S/390 mainframes in May 2000, transitioning it from experimental to a fully supported enterprise option. This was followed by the launch of the first commercial distribution, Server for S/390 mainframes, on October 31, 2000. IBM invested $1 billion in development in 2001, fostering ecosystem growth with additional distributions like and community variants such as . Key milestones include the 2015 introduction of LinuxONE, the first Linux-only mainframe servers based on the z13 , which expanded access to workloads in hybrid cloud environments; the addition of KVM support; and the formation of the Open Mainframe Project to promote open-source innovation. Subsequent releases, such as the z14 in 2017 and z15 in 2019, further integrated with features like air-cooled designs for standard data centers and enhanced security tools. By 2020, over 35% of 's installed capacity ran , reflecting a 55% year-over-year growth and underscoring its role in modern computing. As of 2025, Linux on supports a range of certified distributions, including (versions 8.10 and higher, 9.4 and higher, 10.0), SUSE Linux Enterprise Server (version 15 SP6 and higher), and (22.04 LTS and later, 24.04 LTS), all tested on recent hardware like the z17 and LinuxONE 5. Recent models like the z17 (announced April 2025) and LinuxONE 5 (May 2025) enhance integration and security. As of Q1 2025, 96 of the top 100 enterprises run workloads. options such as Logical Partitions (LPARs), , and KVM enable efficient resource sharing, while the Integrated Facility for Linux (IFL) processors optimize costs by dedicating hardware to without full mainframe licensing fees. Notable features include exceptional security through capable of processing up to 19 billion transactions daily, performance enhancements that reduce by up to 4.7 times via workload colocation, and by consolidating up to 2,000 x86 cores into one system, thereby lowering operational costs and supporting sustainability goals. These attributes make Linux on IBM Z ideal for industries like and healthcare, where it powers mission-critical applications with high throughput, resiliency, and compliance adherence, often integrated with platforms like for containerized deployments.

Overview

Definition and Scope

Linux on IBM Z refers to the port and implementation of the operating system specifically compiled and optimized for mainframe systems, which utilize the . This port enables the execution of Linux distributions on the s390x hardware platform, a 64-bit big-endian designed for enterprise-scale computing. Since the release of version 4.1 in June 2015, has been exclusive to the 64-bit s390x variant, with the removal of 31-bit to streamline and leverage modern hardware capabilities. The scope of Linux on IBM Z encompasses native execution on dedicated hardware resources, particularly the Integrated Facility for Linux (IFL) processors, which are specialized central processing units optimized for workloads and priced separately from general-purpose processors. These IFL engines allow instances to run concurrently with traditional IBM Z operating systems like on the same physical machine, sharing infrastructure such as I/O channels and memory while maintaining isolation for security and resource allocation. This setup supports a where can handle distributed applications alongside mainframe-specific tasks, benefiting from the platform's inherent high reliability and scalability features. The nomenclature for this Linux implementation has evolved in tandem with IBM's mainframe branding. Initially termed Linux/390 when ported to the S/390 architecture in the late , it progressed to Linux for zSeries with the introduction of 64-bit in 2000, then to Linux on System z, Linux for z Systems around 2015, and finally to the current designation of Linux on following the 2017 rebranding of the hardware family. This evolution reflects ongoing adaptations to advancing hardware generations, including the IBM LinuxONE servers, which extend the same Linux capabilities to a Linux-optimized mainframe variant. In distinction from other Linux ports, such as those for x86 or architectures, Linux on Z is confined to the s390x ecosystem and emphasizes mainframe-specific optimizations like enhanced I/O throughput via channel-attached devices and integrated cryptographic accelerators, rather than general-purpose commodity hardware adaptations. This focus ensures compatibility with Z's logical partitions (LPARs) and virtualization layers, without support for non-mainframe instruction sets.

Advantages and Use Cases

Linux on offers exceptional reliability, achieving 99.999% uptime, which supports continuous operation for critical business applications. This stems from the platform's robust hardware and features, enabling fault-tolerant environments that minimize . Additionally, the system provides massive , supporting thousands of virtual machines on a single through technology. High I/O throughput further enhances performance for data-intensive workloads, leveraging optimized networking and storage integrations. is bolstered by pervasive , which protects and in transit across the entire stack without significant performance overhead. Cost efficiencies are realized through Integrated Facility for Linux (IFL) processors, which are dedicated to Linux workloads and can reduce per-processor software licensing costs by up to 97%. This specialization avoids full mainframe pricing for Linux-only environments, allowing of hundreds or thousands of distributed servers onto one platform, thereby lowering energy, cooling, and space requirements. Common use cases include mission-critical enterprise applications, such as banking , where the platform handles billions of daily transactions with fraud detection. cloud integrations enable seamless extension of core mainframe data and services to distributed environments via tools like . and workloads on LinuxONE leverage collocated processing for efficient model training and inference on large-scale data. Server consolidation from x86 architectures to reduces operational complexity while maintaining performance for legacy and modern applications. As of early 2025, 96 of the top 100 enterprises are running workloads, reflecting widespread adoption driven by its integration with containerized applications through . This trend underscores the platform's role in hybrid cloud strategies and AI-infused operations.

History

Origins and Early Development

The development of Linux on IBM Z traces its origins to , when independent developer Linas Vepstas initiated the "" project to the to the and S/390 mainframe architectures. This volunteer-driven effort successfully adapted 2.2.1, along with tools like , , and , achieving a bootable system on emulators by late 1999, though it faced unresolved issues in stability and hardware integration. Concurrently, engineers at the Boeblingen laboratory began a parallel in to create an official , culminating in the release of kernel patches in mid-December 1999 that enabled to run natively on S/390 hardware. These early patches targeted distributions such as TurboLinux and , supporting initial hardware platforms including the System/390 G5 and G6 models as well as the Multiprise 3000 enterprise server. Key technical challenges included adapting the —originally designed for little-endian, PC-style s—to the big-endian, 31-bit addressing mode of the ESA/390 , which limited to approximately 2 per process. Additionally, integrating support for channel-attached I/O devices required new drivers to handle subchannels and channel command words (CCWs), diverging from standard or interfaces; early implementations mapped up to 65,536 subchannels to interrupt requests for devices like network adapters. The shift to the 64-bit , introduced with the zSeries in 2000, further necessitated kernel modifications for expanded addressing and compatibility modes, though initial focus remained on 31-bit compatibility. The first commercial distribution, Server for S/390 (version 7.0), arrived in October 2000, providing over 700 enterprise packages optimized for mainframe workloads and marking the transition from experimental ports to production-ready software. followed in 2002 with support for zSeries in 7.2, expanding options for enterprise deployment on 64-bit hardware. These releases laid the groundwork for broader adoption, emphasizing Linux's ability to leverage mainframe strengths like reliability and scalability.

Key Milestones and Adoption Trends

The launch of the zSeries 900 in October 2000 marked a pivotal milestone, introducing native support for as a production workload on the , enabling direct execution without emulation. This built upon early porting efforts from the late 1990s, allowing to leverage the full capabilities of the introduced with zSeries. The subsequent release of 2.4 in January 2001 further optimized support for , incorporating 64-bit addressing and enhanced I/O handling tailored to mainframe environments. By 2006, adoption had accelerated significantly, with over 1,700 mainframe customers running on , reflecting growing enterprise interest in consolidating workloads onto the platform. A key technical advancement came in 2015 with 4.1, which shifted exclusively to 64-bit support for , dropping 31-bit compatibility to streamline development and improve performance for modern applications. Recent developments continued to bolster Linux on IBM Z, building on the September 2024 release of 7.4, which enhanced for guests with improved resource management and support for hybrid cloud environments hosting thousands of virtual machines. In May 2025, announced a tech preview of 4.18 on , enabling seamless integration of virtual machines and containers within the same cluster, with general availability following in August 2025. In November 2025, 4.20 became generally available on , further enabling organizations to run and manage VMs alongside containers. Adoption trends demonstrate robust growth, with comprising approximately 19% of System z capacity by 2011 and expanding to power workloads in 96 of the top 100 enterprises by the first quarter of 2025. Key drivers include cloud migration strategies that utilize 's reliability for mission-critical applications and ongoing validation efforts, such as IBM's monthly reports confirming compatibility for tools like projects and in 2025.

Architecture

Hardware Platforms

Linux on IBM Z is supported on the latest enterprise-class mainframe platforms, including the (machine type 9175, introduced in 2025) and the (machine type 9175-ML1), both powered by the . The , fabricated on a , features eight per chip running at up to 5.5 GHz, with configurations supporting up to 208 active across the system through dual-chip modules (DCMs) organized in central complex () drawers. These platforms are designed for , emphasizing reliability, availability, and scalability for workloads. Processor units on systems are specialized to optimize for different operating environments. serve as general-purpose engines capable of handling mixed workloads, including those from , , and . In contrast, Integrated Facility for Linux (IFL) processors are dedicated exclusively to workloads, offering a cost-optimized alternative by exempting them from software licensing fees while delivering equivalent performance to . IFLs can only be allocated in -only or logical partitions (LPARs), enabling efficient resource dedication for deployments. Memory capacity on these platforms reaches up to 64 TB of real memory per system, with a maximum of 64 TB allocatable per LPAR, supporting large-scale Linux instances and data-intensive applications. connectivity is provided through FICON (Fibre Connection) channels with features like FICON Express16S supporting up to 16 Gbps link rates and enhanced density via an on-chip (DPU), facilitating high-speed access to storage devices and improved I/O performance for enterprise systems. These hardware capabilities enable support for up to tens of thousands of virtual machines per system when using virtualization layers like . Backward compatibility ensures Linux on IBM Z can operate on hardware from the IBM zEnterprise EC12 (zEC12, introduced in 2012) and subsequent models, including all z13, z14, z15, z16, and z17 series systems. However, support for 31-bit addressing modes was discontinued starting with version 4.1 in 2015, requiring 64-bit kernels on all supported platforms thereafter. This shift aligns with the architecture's evolution toward 64-bit operations while maintaining compatibility for 31-bit applications running under 64-bit kernels.

Core System Features

Linux on IBM Z leverages the , a 64-bit that supports extensive virtual addressing, enabling distributions to utilize up to 16 exabytes of per process, which facilitates handling large-scale data processing and virtualization workloads on mainframe hardware. This architecture includes millicode, an internal layer of microcode that executes certain privileged instructions and handles fast interrupts, reducing latency for system calls and improving overall efficiency in Linux environments by offloading complex operations from the . Additionally, hardware-assisted through the zEnterprise Data Compression (zEDC) accelerator, integrated into processors starting with the IBM z13, allows Linux applications to perform and /decompression at near-line speeds, minimizing CPU overhead for data-intensive tasks like backups and network transfers. The platform's fault tolerance is enhanced by (RAS) features inherited from the underlying hardware, which Linux on IBM Z exploits through kernel integrations. Predictive Failure Analysis (PFA) monitors hardware components such as processors and memory in real time, using algorithms to detect anomalies and predict potential failures before they impact operations, thereby enabling proactive maintenance without interrupting Linux workloads. Dynamic reconfiguration capabilities further support by allowing online replacement or addition of resources like I/O adapters and cryptographic cards, ensuring continuous operation of Linux instances even during hardware upgrades or repairs. Performance optimizations in Linux on IBM Z include the Vector Facility, introduced with the z14 processor in 2017, which provides SIMD instructions for vector processing that accelerate and workloads by enabling parallel computations on large datasets directly in the and user-space applications. More recent advancements in the Telum II processor, powering IBM z17 and later systems as of 2025, incorporate an on-chip (24 TOPS) for enhanced inferencing and quantum-safe hardware supporting algorithms like CRYSTALS-Kyber and CRYSTALS-Dilithium to protect Linux-based transactions and data against future threats without software overhead. The integrated (DPU) further optimizes I/O processing for Linux workloads. Integration with the broader mainframe ecosystem allows on IBM Z to coexist seamlessly with on the same physical machine, sharing I/O resources such as channels and storage subsystems to optimize resource utilization across operating systems. Linux-specific modules, including those for subchannel access and extended control instructions, enable direct exploitation of features like the Integrated Facility for Linux (IFL) processors, which are dedicated to Linux workloads and provide cost-effective scaling.

Virtualization

Partitioning and Hypervisors

Logical partitioning on systems is facilitated by the Processor Resource/Systems Manager (PR/SM), a type-1 integrated into the hardware that divides a single physical machine into multiple independent logical partitions (LPARs). Each LPAR operates as a self-contained environment, capable of running natively or hosting additional layers, with resources such as processors, memory, and I/O devices allocated statically or dynamically. Recent models, such as the z16 and z17, support up to 85 LPARs per central processor complex (CPC), enabling fine-grained isolation for workloads while maintaining high security through hardware-enforced boundaries that prevent interference between partitions. This partitioning approach leverages the underlying to provide robust fault isolation and resource dedication, essential for mission-critical deployments. The z/VM hypervisor extends virtualization capabilities on IBM Z as a full type-1 hypervisor, allowing multiple virtual machines (VMs) to run concurrently within an LPAR by emulating a complete mainframe environment for each guest. z/VM supports large-scale deployments, capable of running thousands of Linux instances, depending on configuration and resource availability. A key feature is the Single System Image (SSI), which clusters up to four z/VM instances into a unified logical system, facilitating seamless workload balancing, live guest relocation, and shared resource management across nodes without disrupting operations. This clustering enhances availability and scalability for Linux on IBM Z, supporting features like automated failover and centralized administration through a single control point. Kernel-based Virtual Machine (KVM) provides an open-source option on , integrated directly into the since its upstream support for the s390x , with comprehensive documentation and tools available from 2017 onward. Designed for lightweight guest OSes, KVM enables efficient sharing of CPU, memory, and I/O resources among multiple Linux VMs within an LPAR, using for device emulation and libvirt for management. It is natively supported in major distributions such as , , and SUSE Linux Enterprise Server, allowing straightforward deployment of virtualized Linux environments with features like and secure execution on newer hardware. Resource allocation in these virtualization environments is enhanced by the IBM z Unified Resource Manager (zManager), which automates dynamic reassignment of CPU and memory across LPARs and based on policy-defined goals. zManager integrates with the Hardware Management Console to monitor performance and adjust resources in real-time, optimizing utilization for workloads without manual intervention. This capability ensures elastic scaling, where excess capacity from one can be reallocated to others, improving overall efficiency for virtualized operations.

Internal Networking and I/O Optimization

HiperSockets provides high-speed, low-latency internal virtual Ethernet connectivity for intra-system communication among partitions on , enabling TCP/IP traffic without physical network hardware by leveraging shared memory and internal queued direct I/O (iQDIO). This technology supports up to 32 independent virtual LANs per central processing complex, with configurable maximum transmission units (MTUs) up to 56 KB to optimize throughput for workloads like streaming transfers. Latency is effectively zero, as demonstrated by tests showing 0.000 seconds, due to memory-based movement that bypasses external and protocols. For external networking, Linux on IBM Z utilizes OSA-Express and Network Express adapters in QDIO mode for standard Ethernet connectivity to LANs, supporting speeds up to 25 GbE and enhanced performance on recent models like OSA-Express7S and Network Express (introduced with z17). Complementing this, Network Express adapters (on z17 and later) enable capabilities, allowing low-latency, high-throughput between systems while reducing CPU overhead for I/O-intensive applications, similar to RoCE on earlier models. Network Express supports both IP-based and RDMA-based connections on two-port Ethernet adapters, with Linux drivers handling concurrent TCP/IP and RDMA traffic over a single card. I/O optimization in Linux on IBM Z environments relies on channel I/O architecture, where Fibre Channel Protocol (FCP) facilitates SCSI-based storage access over Fibre Channel fabrics, enabling direct attachment to SANs for block-level devices. This supports multipathing and N_Port ID virtualization for resilient, high-availability storage configurations. For cache-efficient data transfer, zHyperWrite enhances synchronous write operations in zHyperLink setups, allowing immediate cache acknowledgment to the host before full disk commitment, thereby reducing latency in replication scenarios. System-wide, these optimizations enable up to 1.5 million I/O operations per second (IOPS) across FICON and FCP channels, scaling with hardware like FICON Express16S+ for large-block transfers. Security integrations include support for Encrypted HiperSockets through encapsulation, ensuring protected intra-system traffic via isolation and policy-based . Additionally, hardware-accelerated on uses the Central Processor Assist for Cryptographic Function (CPACF) co-processor for efficient in-kernel and decryption of network packets in , offloading operations from general-purpose CPUs to achieve near-line-rate performance without specialized adapters.

Software Ecosystem

Supported Distributions

Several major Linux distributions are certified for use on and LinuxONE platforms, providing robust support for the . As of November 2025, (RHEL) versions 10.0, 9.4, and 8.10 are tested and supported on the latest hardware such as the IBM z17 and LinuxONE Emperor 5, with earlier versions available for previous generations like z16 and z15. SUSE Linux Enterprise Server (SLES) 15 SP6 is certified for z17 and Emperor 5, while SLES 15 SP3 and older service packs support z16 and earlier systems. LTS releases, including 24.04 and 22.04.1, are validated for z17, with 24.04 extending compatibility back to z14. The certification process involves rigorous testing by and distribution partners to ensure compatibility with , including specialized patches that enable features like Integrated Facility for Linux (IFL) processors and virtualization under . These patches, such as 6.12.0-55.30.1.el10_0 for RHEL 10.0, address mainframe-specific optimizations for , , and exploitation. Certified distributions meet minimum version requirements, with higher minor updates generally supported to maintain ongoing compatibility. Specialized variants enhance these distributions for enterprise workloads on . Ubuntu Server for and LinuxONE includes s390x architecture support and integrates filesystem capabilities for advanced storage management in mainframe environments. RHEL for Applications on and LinuxONE, available in versions 9 and 8, is optimized for deployments, providing certified integration with LinuxONE hardware for high-performance database operations. Certifications are updated regularly to incorporate new hardware and software advancements, with a particular emphasis in 2025 on orchestration support, including tools like Podman and integrated into RHEL and SLES for streamlined deployment on . This ongoing validation process supports the growing adoption of Linux on for mission-critical applications.

Applications and Middleware

Linux on IBM Z supports a robust open-source , with numerous packages validated for and performance on the s390x architecture. For instance, , a distributed database, has been deployed on for , enabling scalable data storage in containerized environments like clusters. , an automation tool for , is certified for through 's content collections, facilitating infrastructure orchestration across hybrid environments. integration is achieved via Container Platform version 4.15, certified for and LinuxONE in 2025, supporting containerized workloads with bridging traditional VMs and modern applications. Enterprise applications are well-established on for , leveraging the platform's reliability for mission-critical workloads. , a management system, runs natively on for , supporting features like pureScale clustering for high availability and scalability across zSystems hardware. operates on for , with specific cryptographic setups enabling secure SSL support and integration with System z hardware accelerators. SAP workloads, including NetWeaver and S/4HANA, utilize high-availability clustering on for , often managed through IBM Z System Automation to automate and for reduced . Middleware components are optimized for Linux on , enhancing application development and efficiency. provides a full-featured port for s390x, including just-in-time () compilation tailored to Z's architecture for improved performance in enterprise applications. , via the IBM Open Enterprise SDK, runs on Linux for , offering a standalone for connecting applications to resources and supporting scalable services. serves as a key language for workloads, with the AI Toolkit for providing open-source libraries like and , adapted for s390x to enable inference and training directly on the platform. publishes monthly open-source software validation reports, such as the April 2025 edition confirming compatibility of and on Linux for . Porting applications from x86 to s390x is facilitated by tools like Chiphopper, an program that automates the recompilation and adaptation of applications for platforms, enabling independent software vendors to create multi-architecture binaries efficiently.

Development and Tools

Programming Resources

Developers targeting on have access to specialized kits (SDKs) and trial environments provided by to facilitate application development and testing. The SDK, Java Technology Edition, supports on platforms, offering runtime environments and tools for -based applications with optimizations for the s390x architecture. Additionally, compilers are fully supported on distributions for , enabling standard open-source builds with architecture-specific tuning for 64-bit s390x systems. These tools allow developers to compile and optimize code that leverages 's unique capabilities, including enhanced instruction sets in recent processors like the z17. As of 2025, and the SDK have been updated to support features of the z17 processor, such as Telum II enhancements for workloads and improved cryptographic instructions. IBM also offers cloud-based trial environments to enable hands-on experimentation without dedicated hardware. The LinuxONE Community Cloud provides free access to Linux on IBM Z instances, allowing developers to provision virtual machines for testing applications in a production-like setting, typically for periods up to 30 days. This service supports major distributions such as and SUSE Linux Enterprise Server, integrated with infrastructure for seamless scalability and networking. Key APIs and libraries extend Linux functionality to exploit IBM Z hardware accelerations. The libica library serves as a C API for accessing cryptographic operations via the Central Processor Assist for Cryptographic Functions (CPACF) and coprocessors, supporting symmetric encryption, hashing, and random number generation with minimal overhead. It integrates with for broader compatibility, enabling FIPS-certified modes when the kernel is configured accordingly. For , the hardware-accelerated zlib implementation utilizes the Frame Compression (DFLTCC) on z15 and later processors, providing up to 10x faster deflate/inflate operations compared to software-only methods by offloading to dedicated hardware units. This extension is available across supported distributions and can be invoked transparently through standard zlib calls when the accelerator is present. Comprehensive documentation aids kernel-level and on for . IBM Redbooks publications, such as "Linux on IBM System z: Performance Measurement and Tuning," detail kernel configuration, driver development, and optimization strategies for s390x, including tuning for high-availability setups and I/O subsystems. These guides cover practical aspects like and integration with . For open-source contributions, the s390 architecture tree at hosts the upstream code for , where engineers maintain device drivers, architecture-specific patches, and features like protected-key support. Developers can submit patches via the standard process, with 's Linux Technology Center actively contributing enhancements for hardware features in each kernel release. The Linux Foundation plays a central role in fostering community-driven development for Linux on IBM Z through initiatives like the Open Mainframe Project, which coordinates open-source tools, APIs, and best practices for mainframe workloads. This involvement ensures interoperability and innovation, with ongoing collaborations on security, AI integration, and hybrid cloud extensions as of 2025.

Emulation and Testing Environments

The Hercules emulator is an open-source software implementation of the IBM System/370, ESA/390, and z/Architecture mainframe architectures, designed to run on x86-based host systems such as Linux, Windows, or macOS. It enables developers to simulate IBM Z hardware environments for testing and porting Linux distributions without requiring physical mainframes, supporting full execution of the Linux kernel and user-space applications on the s390x architecture. For instance, users can install and boot Ubuntu or other s390x-compatible Linux images on virtual disks within Hercules, facilitating kernel-level debugging and software compatibility verification. Many developers leverage Hercules specifically for porting open-source software to Linux on IBM Z, as it provides a cost-effective alternative for initial validation before hardware deployment. IBM's Z Development and Test Environment (ZD&T) offers a licensed, commercial emulation solution that runs on Intel-based workstations, creating a simulated system complete with processor —including the Integrated Facility for (IFL)—and I/O peripherals like FICON channels and OSA adapters. This environment supports the development and testing of on applications by allowing users to boot s390x guests alongside emulated instances, enabling hybrid workload simulations for integration and . ZD&T's containerized options, such as the wazi-sandbox image, further simplify setup on modern distributions like RHEL or , providing isolated virtual machines for iterative coding and regression testing. Cloud-based testing environments extend these capabilities by hosting emulated IBM Z setups on public infrastructures, such as deploying ZD&T instances on (AWS) via CloudFormation templates for on-demand Linux on experimentation. In 2025, transitioned away from certain on-premises PC-based emulators toward cloud-hosted alternatives from independent software vendors (ISVs), enhancing scalability for Linux testing without local hardware investments. These options integrate with tools like Test Accelerator for Z, which provisions virtualized or emulated environments for of hybrid applications including Linux workloads. Despite their utility, emulation environments like and ZD&T cannot fully replicate the performance characteristics of native hardware, such as the high-throughput I/O and multi-core scaling of Telum processors, due to the overhead of instruction-set translation on x86 hosts. As a result, they are primarily intended for code porting, functional debugging, and educational purposes rather than production-scale benchmarking or high-volume .

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