Object Storage Daemons (OSDs) are the Ceph stack's workhorses for data storage. They're significantly smarter than many of their counterparts in distributed block-storage solutions (open source or not), and their design is instrumental in securing the stack's reliability and scalability.

Among other things, OSDs are responsible for the decentralized replication — which is highly configurable — of objects in the store. They do so in a primary-copy fashion: every Ceph object (more precisely, the Placement Group it is a part of) is written to the primary OSD first, and from there replicates to one or several replica OSDs to ensure redundancy. This replication is synchronous, such that a new or updated object guarantees its availability (in the way configured by the cluster administrator) before an application is notified that the write has completed.

More specifically, in order for an OSD to acknowledge a write as completed, the new object must have been written to the OSD's journal. OSDs use a write-ahead mode for local operations: a write hits the journal first, and from there is then being copied into the backing filestore. (Note: if your filestore is using btrfs, the journal is applied in parallel with the filestore write instead. Btrfs still being experimental, however, this is not a configuration often used in production.) Thus, for best cluster performance it is crucial that the journal is fast, whereas the filestore can be comparatively slow.

This, in turn, leads to a common design principle for Ceph clusters that are both fast and cost-effective:

  • Put your filestores on slow, cheap drives (such as SATA spinners),
  • put your journals on fast drives (SSDs, Fusion-IO cards, whatever you can afford).

Another common design principle is that you create one OSD per spinning disk that you have in the system. Many contemporary systems come with only two SSD slots, and then as many spinners as you want. That is not a problem for journal capacity — a single OSD's journal is usually no larger than about 6 GB, so even for a 16-spinner system (approx. 96GB journal space) appropriate SSDs are available at reasonable expense.

Many operators are scared of an SSD suddenly dying a horrible death, so they put their SSDs in a RAID-1. Many are also tempted to put their OSD journal partitions onto the same RAID. Another option is to use, say, one partition on each of your SSD in a RAID for the operating system installation, and then chop up the rest of your SSDs as non-RAIDed Ceph OSD journals.

This creates an interesting situation when you get to more than about 10-or-so OSDs (the exact number is hard to give). Now you have your OS and several OSD journals on the same physical SSD. SSDs are much faster than spinners, but they have neither infinite throughput nor zero latency. Eventually, you might hit your SSD's physical limits for random I/O all over the place. For example, if one of your hosts dies and the rest now reshuffles data to restore the desired level of redundancy, you may see relatively intensive I/O all over the other OSDs — this is exacerbated in a system where you have few OSD hosts which host many OSD disks.

Putting your journal SSDs in a RAID set looks like a good idea at first. Specifically, Ceph OSDs currently cannot recover from a broken SSD journal without reinitializing and recovering the entire filestore. This means that as soon as SSD acting as journal backing storage burns up, you've effectively lost those OSDs completely and need to recover them from scratch.1

Put them in a RAID-1, problem solved? Well, not quite, because you've now duplicated all of your journal writes and you're hitting two SSDs all over the place. Thus it's generally a much better idea to put half of your journals on one SSD, and half on the other. If one of your SSDs burns up you'll still lose the OSDs whose journals it hosts — but it'll only be half of the OSDs hosted on that node altogether.

Any such performance issues get worse if some of your OSDs are also MONs: your OSD journals now compete with your operating system and your MONs for I/O on the same SSDs. Once your SSDs get hit so hard that your MONs can't do I/O, those MONs eventually die. This might not harm your operations if you have sufficient backup MONs available, and everything will be fine again once your recovery is complete, but it's still a nuisance. This is remarkably common specifically in POCs, by the way, where people often try to repurpose three of their old, two-SSDs-plus-dozens-of-disks storage servers for a 3-node Ceph cluster.

So, as you are considering your OSD journal and filestore layout, take note of the following general guidelines:

  • By and large, try to go for a relatively small number of OSDs per node, ideally not more than 8. This combined with SSD journals is likely to give you the best overall performance.

  • If you do go with OSD nodes with a very high number of disks, consider dropping the idea of an SSD-based journal. Yes, in this kind of setup you might actually do better with journals on the spinners.

  • Alternatively in the same scenario, consider putting your operating system install on one or a couple of the spinners (presumably smaller ones than the others), and use the (un-RAIDed) SSDs for OSD journals exclusively.

  • Consider having a few dedicated MONs (MONs that are not also OSDs).

Note on ceph-osd --mkjournal

This article originally appeared on the hastexo.com website (now defunct).

  1. Since this article was originally published, a --mkjournal option was added to the ceph-osd command, allowing you to recreate a journal for an existing OSD. This mitigates the issue in that you don't need to recreate OSDs from scratch when a journal device breaks — but the OSDs will still be temporarily unavailable.