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224 lines
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==========================
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Hardware Recommendations
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==========================
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Ceph was designed to run on commodity hardware, which makes building and
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maintaining petabyte-scale data clusters economically feasible.
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When planning out your cluster hardware, you will need to balance a number
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of considerations, including failure domains and potential performance
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issues. Hardware planning should include distributing Ceph daemons and
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other processes that use Ceph across many hosts. Generally, we recommend
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running Ceph daemons of a specific type on a host configured for that type
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of daemon. We recommend using other hosts for processes that utilize your
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data cluster (e.g., OpenStack, CloudStack, etc).
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`Inktank`_ provides excellent premium support for hardware planning.
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.. _Inktank: http://www.inktank.com
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CPU
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===
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Ceph metadata servers dynamically redistribute their load, which is CPU
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intensive. So your metadata servers should have significant processing power
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(e.g., quad core or better CPUs). Ceph OSDs run the RADOS service, calculate
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data placement with CRUSH, replicate data, and maintain their own copy of the
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cluster map. Therefore, OSDs should have a reasonable amount of processing power
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(e.g., dual-core processors). Monitors simply maintain a master copy of the
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cluster map, so they are not CPU intensive. You must also consider whether the
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host machine will run CPU-intensive processes in addition to Ceph daemons. For
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example, if your hosts will run computing VMs (e.g., OpenStack Nova), you will
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need to ensure that these other processes leave sufficient processing power for
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Ceph daemons. We recommend running additional CPU-intensive processes on
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separate hosts.
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RAM
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===
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Metadata servers and monitors must be capable of serving their data quickly, so
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they should have plenty of RAM (e.g., 1GB of RAM per daemon instance). OSDs do
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not require as much RAM (e.g., 500MB of RAM per daemon instance). Generally,
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more RAM is better.
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Data Storage
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============
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Plan your data storage configuration carefully, because there are significant
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opportunities for performance improvement by incurring the added cost of using
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solid state drives (SSDs), and there are significant cost-per-gigabyte
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considerations with hard disk drives. Metadata servers and monitors don't use a
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lot of storage space. A metadata server requires approximately 1MB of storage
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space per daemon instance. A monitor requires approximately 10GB of storage
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space per daemon instance. One opportunity for performance improvement is to use
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solid-state drives to reduce random access time and read latency while
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accelerating throughput. Solid state drives cost more than 10x as much per
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gigabyte when compared to a hard disk, but they often exhibit access times that
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are at least 100x faster than a hard disk drive. Since the storage requirements
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for metadata servers and monitors are so low, solid state drives may provide an
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economical opportunity to improve performance.
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.. important:: We recommend exploring the use of SSDs to improve performance.
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However, before making a significant investment in SSDs, we **strongly
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recommend** both reviewing the performance metrics of an SSD and testing the
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SSD in a test configuration to gauge performance. SSD write latency may
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**NOT** improve performance compared to a high performance hard disk.
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Inexpensive SSDs may introduce write latency even as they accelerate
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access time, because sometimes hard drives will write faster than SSDs!
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OSDs should have plenty of disk space. We recommend a minimum disk size of 1
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terabyte. We recommend dividing the price of the hard disk drive by the number
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of gigabytes to arrive at a cost per gigabyte, because larger drives may have a
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significant impact on the cost-per-gigabyte. For example, a 1 terabyte hard disk
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priced at $75.00 has a cost of $0.07 per gigabyte (i.e., $75 / 1024 = 0.0732).
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By contrast, a 3 terabyte hard disk priced at $150.00 has a cost of $0.05 per
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gigabyte (i.e., $150 / 3072 = 0.0488). In the foregoing example, using the 1
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terabyte disks would generally increase the cost per gigabyte by 40%--rendering
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your cluster substantially less cost efficient. For OSD hosts, we recommend
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using an OS disk for the operating system and software, and one disk for each
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OSD daemon you run on the host. While solid state drives are cost prohibitive
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for object storage, OSDs may see a performance improvement by storing an OSD's
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journal on a solid state drive and the OSD's object data on a hard disk drive.
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You may run multiple OSDs per host, but you should ensure that the sum of the
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total throughput of your OSD hard disks doesn't exceed the network bandwidth
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required to service a client's need to read or write data. You should also
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consider what percentage of the cluster's data storage is on each host. If the
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percentage is large and the host fails, it can lead to problems such as
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exceeding the ``full ratio``, which causes Ceph to halt operations as a safety
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precaution that prevents data loss.
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Networks
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========
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We recommend that each host have at least two 1Gbps network interface
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controllers (NICs). Since most commodity hard disk drives have a throughput of
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approximately 100MB/second, your NICs should be able to handle the traffic for
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the OSD disks on your host. We recommend a minimum of two NICs to account for a
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public (front-side) network and a cluster (back-side) network. A cluster network
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(preferably not connected to the internet) handles the additional load for data
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replication and helps stop denial of service attacks that prevent the cluster
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from achieving ``active + clean`` states for placement groups as OSDs replicate
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data across the cluster. Consider starting with a 10Gbps network in your racks.
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Replicating 1TB of data across a 1Gbps network takes 3 hours, and 3TBs (a
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typical drive configuration) takes 9 hours. By contrast, with a 10Gbps network,
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the replication times would be 20 minutes and 1 hour respectively. In a
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petabyte-scale cluster, failure of an OSD disk should be an expectation, not an
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exception. System administrators will appreciate PGs recovering from a
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``degraded`` state to an ``active + clean`` state as rapidly as possible, with
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price / performance tradeoffs taken into consideration. Additionally, some
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deployment tools (e.g., Dell's Crowbar) deploy with five different networks,
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but employ VLANs to make hardware and network cabling more manageable. VLANs
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using 802.1q protocol require VLAN-capable NICs and Switches. The added hardware
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expense may be offset by the operational cost savings for network setup and
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maintenance. When using VLANs to handle VM traffic between between the cluster
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and compute stacks (e.g., OpenStack, CloudStack, etc.), it is also worth
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considering using 10G Ethernet. Top-of-rack routers for each network also need
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to be able to communicate with spine routers that have even faster
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throughput--e.g., 40Gbps to 100Gbps.
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Your server hardware should have a Baseboard Management Controller (BMC).
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Administration and deployment tools may also use BMCs extensively, so consider
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the cost/benefit tradeoff of an out-of-band network for administration.
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Hypervisor SSH access, VM image uploads, OS image installs, management sockets,
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etc. can impose significant loads on a network. Running three networks may seem
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like overkill, but each traffic path represents a potential capacity, throughput
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and/or performance bottleneck that you should carefully consider before
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deploying a large scale data cluster.
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Failure Domains
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===============
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A failure domain is any failure that prevents access to one or more OSDs. That
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could be a stopped daemon on a host; a hard disk failure, an OS crash, a
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malfunctioning NIC, a failed power supply, a network outage, a power outage, and
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so forth. When planning out your hardware needs, you must balance the
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temptation to reduce costs by placing too many responsibilities into too few
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failure domains, and the added costs of isolating every potential failure
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domain.
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Minimum Hardware Recommendations
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================================
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Ceph can run on inexpensive commodity hardware. Small production clusters
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and development clusters can run successfully with modest hardware.
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+--------------+----------------+------------------------------------+
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| Process | Criteria | Minimum Recommended |
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+==============+================+====================================+
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| ``ceph-osd`` | Processor | 1x 64-bit AMD-64/i386 dual-core |
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| +----------------+------------------------------------+
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| | RAM | 500 MB per daemon |
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| +----------------+------------------------------------+
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| | Volume Storage | 1x Disk per daemon |
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| +----------------+------------------------------------+
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| | Network | 2x 1GB Ethernet NICs |
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+--------------+----------------+------------------------------------+
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| ``ceph-mon`` | Processor | 1x 64-bit AMD-64/i386 |
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| +----------------+------------------------------------+
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| | RAM | 1 GB per daemon |
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| +----------------+------------------------------------+
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| | Disk Space | 10 GB per daemon |
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| +----------------+------------------------------------+
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| | Network | 2x 1GB Ethernet NICs |
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+--------------+----------------+------------------------------------+
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| ``ceph-mds`` | Processor | 1x 64-bit AMD-64/i386 quad-core |
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| +----------------+------------------------------------+
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| | RAM | 1 GB minimum per daemon |
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| +----------------+------------------------------------+
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| | Disk Space | 1 MB per daemon |
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| +----------------+------------------------------------+
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| | Network | 2x 1GB Ethernet NICs |
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+--------------+----------------+------------------------------------+
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.. tip:: If you are running an OSD with a single disk, create a
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partition for your volume storage that is separate from the partition
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containing the OS. Generally, we recommend separate disks for the
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OS and the volume storage.
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Production Cluster Example
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==========================
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Production clusters for petabyte scale data storage may also use commodity
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hardware, but should have considerably more memory, processing power and data
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storage to account for heavy traffic loads.
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A recent (2012) Ceph cluster project is using two fairly robust hardware
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configurations for Ceph OSDs, and a lighter configuration for monitors.
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+----------------+----------------+------------------------------------+
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| Configuration | Criteria | Minimum Recommended |
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+================+================+====================================+
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| Dell PE R510 | Processor | 2x 64-bit quad-core Xeon CPUs |
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| | RAM | 16 GB |
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| | Volume Storage | 8x 2TB drives. 1 OS, 7 Storage |
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| | Client Network | 2x 1GB Ethernet NICs |
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| | OSD Network | 2x 1GB Ethernet NICs |
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| | Mgmt. Network | 2x 1GB Ethernet NICs |
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+----------------+----------------+------------------------------------+
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| Dell PE R515 | Processor | 1x hex-core Opteron CPU |
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| | RAM | 16 GB |
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| | Volume Storage | 12x 3TB drives. Storage |
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| | OS Storage | 1x 500GB drive. Operating System. |
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| | Client Network | 2x 1GB Ethernet NICs |
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| | OSD Network | 2x 1GB Ethernet NICs |
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| | Mgmt. Network | 2x 1GB Ethernet NICs |
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+----------------+----------------+------------------------------------+
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