An initial thought on Designing SLO for Cloud Storage

1. Service Level Objective (SLO) for
Cloud Storage
An Initial thought on deriving SLO for Data Center Cloud

2. Controls available for storage
• The following knobs are available
• IOPS throttling
• Bandwidth throttling
• SLO must be defined in a way that can be mapped internally to IOPS
or Bandwidth.

3. SLO definition options
• Storage slo can be defined in terms of
• Storage Bandwidth
• IOPS
• Latency

4. Limiting using IOPS as SLO
• For a given workload, sequential access IOPS is typically higher than
random access IOPS
• Random access can be limited by IOPS throttling
• Sequential access can be limited by bandwidth throttling instead
• Given a fixed block size, we have a relationship between Sequential
access bandwidth and sequential access IOPS.
• IOPS = bandwidth/blocksize.
• Caveat: Libvirt considers 512KB as block size. Therefore, 512KB/s = 1
IOP
• Read, write IOPS can be separately tuned.
• Also supports Total IOPS

5. Minimum Guaranteed IOPS
• Since Random IOPS reported are smaller than Sequential IOPS for a
given workload, to commit a minimum guaranteed iops value we
should consider random iops of the given storage.
• random iops can be calculated theoretically using Avg seek time and Avg
latency time.
• Value of random iops can also be extracted from the disk specification.
• Calculation of random iops for the storage cluster where several disks with
different specifications are present is required.

6. Effect of Network Bandwidth on IOPS
• Using network bandwidth throttling , a minimum and maximum network
bandwidth can be ensured on a per VM basis.
• If network bandwidth supported is greater or equal to the IO bandwidth(i.e
iops supported by cluster) then we will get the performance as of a local disk.
• To ensure a minimum iops to a VM we also have to ensure a minimum
network bandwidth for that VM.

7. Defining SLO in the unit of IOPS
Min iops = random iops (calculated for the cluster) / no. of VMs
running
Max iops = iops with throttling limit.
SLO unit = Min iops <= iops_value <= Max iops
Note- for both Min iops and Max iops calculatiions , we have the limitations on
a. Network bandwidth
b. Block (request) size – Our iops calculation is based on the consideration , 1BS=512KB