IOPS Computing Guide: Performance Evaluation of Hard Disks, RAID Groups, and Ceph

Mechanical hard drive

7200 hard disk IOPS = 1000/(3 1000*(7200/60)/2) = 140
10k hard disk IOPS = 1000/(3 60000/10000/2) = 167
15k hard disk IOPS = 1000/(3 60000/15000/2) = 200

3 is the seek delay, 7200/10k/15k is the rotation speed (rpm), 1000*(7200/60)/2 is the rotation delay (the rotation delay is generally expressed as 1/2 of the time required for one revolution) , the result is a theoretical peak value. In fact, there will be system delays that cause the measured IOPS to be generally lower than this value.

RAID Group

Since the RAID group needs to be verified to provide recovery function, there will be a certain write penalty (one business write operation corresponds to the I/O operation of the actual hard disk, please refer to https://community.emc.com/docs/DOC- 26624), this coefficient is as follows:
RAID0: 1
RAID1: 2
RAID5: 4
RAID6: 6
RAID1-0: 2

So RAID group IOPS = hard disk write IOPS*number of hard disks*percentage of write operations/write penalty coefficient Hard disk read IOPS*number of hard disks*percentage of read operations.

Take 4 SAS hard disk groups with 180 IOPS RAID 6 and then perform 100% random write operations as an example:

IOPS = 180*4/6 = 120

Ceph’s IOPS empirical formula

Since the Ceph storage structure is different from the physical hardware, the factors that affect its IOPS mainly include the network, the number of copies, logs, the number of OSD (hard disks), the number of OSD servers, OSD IOPS, etc. Here is an empirical formula from Mirantis ：

IOPS = hard disk IOPS * number of hard disks * 0.88 / number of copies

Among them, 0.88 is the proportion of 4-8k random read operations (88%). If the OSD is not a hard disk but a RAID group, replace the corresponding parameters in the formula.

The IOPS calculation of Ceph is for reference only, and the calculation results may deviate greatly from the actual measurement of the physical environment.

Author introduction: Cloud technology community expert Jiang Di (WeChat public account: Cloud Technology Practice)

Jiang Di, senior virtualization infrastructure engineer, author of "KVM Private Cloud Architecture Design and Practice", cloud technology community expert, good at KVM cloud platform architecture analysis and virtualization POC, has front-line development and delivery experience.

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