Ceph use cases

Before jumping into a specific use case, let's cover some key points that should be understood and considered before thinking about deploying a Ceph cluster.

Replacing your storage array with Ceph

Ceph should not be compared with a traditional scale-up storage array. It is fundamentally different, and trying to shoehorn Ceph into that role using existing knowledge, infrastructure, and expectation will lead to disappointment. Ceph is Software Defined Storage (SDS) whose internal data movements operate over TCP/IP networking. This introduces several extra layers of technology and complexity compared with a SAS cable at the rear of a traditional storage array.

Performance

Due to Ceph's distributed approach, it can offer unrestrained performance compared with scale-up storage arrays, which typically have to funnel all I/O through a pair of controller heads. Although technology has constantly been providing new faster CPUs and faster network speeds, there is still a limit to the performance you can expect to achieve with just a pair of controllers. With recent advances in flash technology combined with new interfaces such as Non-volatile Memory Express (NVMe), the scale-out nature of Ceph provides a linear increase in CPU and network resource with every added OSD node.

Let us also consider where Ceph is not a good fit for performance, and this is mainly around use cases where extremely low latency is desired. For the very reason that enables Ceph to become a scale-out solution, it also means that low latency performance will suffer. The overhead of software and additional network hops means that latency will tend to be about double that of a traditional storage array and 10 times that of local storage. A thought should be given to selecting the best technology for given performance requirements. That said, a well-designed and tuned Ceph cluster should be able to meet performance requirements in all but the most extreme cases.

Reliability

Ceph is designed to provide a highly fault-tolerant storage system by the scale-out nature of its components. Although no individual component is highly available, when clustered together any component should be able to fail without causing an inability to service client requests. In fact, as your Ceph cluster grows, the failure of individual components should be expected and become part of normal operating conditions. However, Ceph's ability to provide a resilient cluster should not be an invitation to compromise on hardware or design choice. Doing so will likely lead to failure. There are several factors that Ceph assumes your hardware will meet, which are covered later in this chapter.

Unlike RAID where disk rebuilds with larger disks can now stretch into time periods measured in weeks, Ceph will often recover from single disk failures in a matter of hours. With the increasing trend of larger capacity disks, Ceph offers numerous advantages to both the reliability and degraded performance when compared with a traditional storage array.

The use of commodity hardware

Ceph is designed to be run on commodity hardware, which gives the ability to be able to design and build a cluster without the premium demanded by traditional tier 1 storage and server vendors. This can be both a blessing and a curse. Be able to choose your own hardware that allows you to build your Ceph infrastructure to exactly match your requirements. However, one thing branded hardware does offer is compatibility testing; it's not unknown for strange exotic firmware bugs to be discovered, which can cause very confusing symptoms. A thought should be applied to whether your IT teams have the time and skills to cope with any obscure issues that may crop up with untested hardware solutions.

The use of commodity hardware also protects against the traditional fork lift upgrade model, where the upgrade of a single component often requires the complete replacement of the whole storage array. With Ceph you can replace individual components in a very granular nature, and with automatic data balancing, lengthy data migration periods are avoided. The distributed nature of Ceph means that hardware replacement or upgrades can be done during working hours without effecting service availability.