Recommendations

This topic describes recommendations on what block sizes to be used with each erasure code and how many node failures can be tolerated based on the recovery group size.

The following table shows how many node and disk failures can be tolerated with different RAID protections and node numbers.

There are limits on what block sizes can be used with each RAID Code. The following table provides information about the limits:

Even though the number of failures that can be tolerated in a smaller recovery group is the same as the number of failures in a larger recovery group, the amount of critical data that must be rebuilt for each failure is less for a larger recovery group. For example, with an 8+3P array on an 11-node recovery group, three-node failures would impact all of the data in the file system. On a 30-node recovery group, three node failures would impact only about 10% of the data on the file system (assuming all disks are the same size). The critical rebuild would complete more quickly because the rebuild work is distributed across a larger number of remaining nodes.

When you plan the erasure code type, also consider future expansion of the cluster and storage utilization. Erasure codes for vdisks cannot be changed after the vdisk is created, and larger stripe widths have better storage utilization. A 4+3P code uses 57% of total capacity for usable data, while an 8+3P code uses 73% of the total capacity for usable data. So, rather than creating a 9-node cluster with 4+3P and expanding it in the future, a 12 or more-node cluster by using 8+3P might be more cost-effective. In some cases, using a non-recommended erasure code might be tolerable if there are plans to increase the cluster size.