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Chapter 2. General principles for selecting hardware
As a storage administrator, you must select the appropriate hardware for running a production Red Hat Ceph Storage cluster. When selecting hardware for Red Hat Ceph Storage, review these following general principles. These principles will help save time, avoid common mistakes, save money and achieve a more effective solution.
Prerequisites
- A planned use for Red Hat Ceph Storage.
- Linux System Administration Advance level with Red Hat Enterprise Linux certification.
- Storage administrator with Ceph Certification.
2.1. Identify performance use case
One of the most important steps in a successful Ceph deployment is identifying a price-to-performance profile suitable for the cluster’s use case and workload. It is important to choose the right hardware for the use case. For example, choosing IOPS-optimized hardware for a cloud storage application increases hardware costs unnecessarily. Whereas, choosing capacity-optimized hardware for its more attractive price point in an IOPS-intensive workload will likely lead to unhappy users complaining about slow performance.
The primary use cases for Ceph are:
- IOPS optimized: IOPS optimized deployments are suitable for cloud computing operations, such as running MYSQL or MariaDB instances as virtual machines on OpenStack. IOPS optimized deployments require higher performance storage such as 15k RPM SAS drives and separate SSD journals to handle frequent write operations. Some high IOPS scenarios use all flash storage to improve IOPS and total throughput.
- Throughput optimized: Throughput-optimized deployments are suitable for serving up significant amounts of data, such as graphic, audio and video content. Throughput-optimized deployments require networking hardware, controllers and hard disk drives with acceptable total throughput characteristics. In cases where write performance is a requirement, SSD journals will substantially improve write performance.
- Capacity optimized: Capacity-optimized deployments are suitable for storing significant amounts of data as inexpensively as possible. Capacity-optimized deployments typically trade performance for a more attractive price point. For example, capacity-optimized deployments often use slower and less expensive SATA drives and co-locate journals rather than using SSDs for journaling.
This document provides examples of Red Hat tested hardware suitable for these use cases.
2.2. Consider storage density
Hardware planning should include distributing Ceph daemons and other processes that use Ceph across many hosts to maintain high availability in the event of hardware faults. Balance storage density considerations with the need to rebalance the cluster in the event of hardware faults. A common hardware selection mistake is to use very high storage density in small clusters, which can overload networking during backfill and recovery operations.
2.3. Identical hardware configuration
Create pools and define CRUSH hierarchies such that the OSD hardware within the pool is identical.
- Same controller.
- Same drive size.
- Same RPMs.
- Same seek times.
- Same I/O.
- Same network throughput.
- Same journal configuration.
Using the same hardware within a pool provides a consistent performance profile, simplifies provisioning and streamlines troubleshooting.
2.4. Network considerations for Red Hat Ceph Storage
An important aspect of a cloud storage solution is that storage clusters can run out of IOPS due to network latency, and other factors. Also, the storage cluster can run out of throughput due to bandwidth constraints long before the storage clusters run out of storage capacity. This means that the network hardware configuration must support the chosen workloads to meet price versus performance requirements.
Storage administrators prefer that a storage cluster recovers as quickly as possible. Carefully consider bandwidth requirements for the storage cluster network, be mindful of network link oversubscription, and segregate the intra-cluster traffic from the client-to-cluster traffic. Also consider that network performance is increasingly important when considering the use of Solid State Disks (SSD), flash, NVMe, and other high performing storage devices.
Ceph supports a public network and a storage cluster network. The public network handles client traffic and communication with Ceph Monitors. The storage cluster network handles Ceph OSD heartbeats, replication, backfilling, and recovery traffic. At a minimum, a single 10 GB Ethernet link should be used for storage hardware, and you can add additional 10 GB Ethernet links for connectivity and throughput.
Red Hat recommends allocating bandwidth to the storage cluster network, such that it is a multiple of the public network using the osd_pool_default_size as the basis for the multiple on replicated pools. Red Hat also recommends running the public and storage cluster networks on separate network cards.
Red Hat recommends using 10 GB Ethernet for Red Hat Ceph Storage deployments in production. A 1 GB Ethernet network is not suitable for production storage clusters.
In the case of a drive failure, replicating 1 TB of data across a 1 GB Ethernet network takes 3 hours, and 3 TB takes 9 hours. Using 3 TB is the typical drive configuration. By contrast, with a 10 GB Ethernet network, the replication times would be 20 minutes and 1 hour. Remember that when a Ceph OSD fails, the storage cluster will recover by replicating the data it contained to other Ceph OSDs within the pool.
The failure of a larger domain such as a rack means that the storage cluster utilizes considerably more bandwidth. When building a storage cluster consisting of multiple racks, which is common for large storage implementations, consider utilizing as much network bandwidth between switches in a "fat tree" design for optimal performance. A typical 10 GB Ethernet switch has 48 10 GB ports and four 40 GB ports. Use the 40 GB ports on the spine for maximum throughput. Alternatively, consider aggregating unused 10 GB ports with QSFP+ and SFP+ cables into more 40 GB ports to connect to other rack and spine routers. Also, consider using LACP mode 4 to bond network interfaces. Additionally, use jumbo frames, with a maximum transmission unit (MTU) of 9000, especially on the backend or cluster network.
Before installing and testing a Red Hat Ceph Storage cluster, verify the network throughput. Most performance-related problems in Ceph usually begin with a networking issue. Simple network issues like a kinked or bent Cat-6 cable could result in degraded bandwidth. Use a minimum of 10 GB ethernet for the front side network. For large clusters, consider using 40 GB ethernet for the backend or cluster network.
For network optimization, Red Hat recommends using jumbo frames for a better CPU per bandwidth ratio, and a non-blocking network switch back-plane. Red Hat Ceph Storage requires the same MTU value throughout all networking devices in the communication path, end-to-end for both public and cluster networks. Verify that the MTU value is the same on all hosts and networking equipment in the environment before using a Red Hat Ceph Storage cluster in production.
2.5. Avoid using RAID solutions
Ceph can replicate or erasure code objects. RAID duplicates this functionality on the block level and reduces available capacity. Consequently, RAID is an unnecessary expense. Additionally, a degraded RAID will have a negative impact on performance.
Red Hat recommends that each hard drive be exported separately from the RAID controller as a single volume with write-back caching enabled.
This requires a battery-backed, or a non-volatile flash memory device on the storage controller. It is important to make sure the battery is working, as most controllers will disable write-back caching if the memory on the controller can be lost as a result of a power failure. Periodically check the batteries and replace them if necessary, as they do degrade over time. See the storage controller vendor’s documentation for details. Typically, the storage controller vendor provides storage management utilities to monitor and adjust the storage controller configuration without any downtime.
Using Just a Bunch of Drives (JBOD) in independent drive mode with Ceph is supported when using all Solid State Drives (SSDs), or for configurations with high numbers of drives per controller. For example, 60 drives attached to one controller. In this scenario, the write-back caching can become a source of I/O contention. Since JBOD disables write-back caching, it is ideal in this scenario. One advantage of using JBOD mode is the ease of adding or replacing drives and then exposing the drive to the operating system immediately after it is physically plugged in.
2.6. Summary of common mistakes when selecting hardware
- Repurposing underpowered legacy hardware for use with Ceph.
- Using dissimilar hardware in the same pool.
- Using 1Gbps networks instead of 10Gbps or greater.
- Neglecting to setup both public and cluster networks.
- Using RAID instead of JBOD.
- Selecting drives on a price basis without regard to performance or throughput.
- Journaling on OSD data drives when the use case calls for an SSD journal.
- Having a disk controller with insufficient throughput characteristics.
Use the examples in this document of Red Hat tested configurations for different workloads to avoid some of the foregoing hardware selection mistakes.
Additional Resources
- Supported configurations article on the Red Hat Customer Portal.
