Home > Storage > PowerScale (Isilon) > Product Documentation > Storage (general) > Storage Quota Management and Provisioning with Dell PowerScale SmartQuotas > Constituents and quota account blocks
Quota account blocks (QABs) enable high-performance accounting using transaction deltas. Because the quota usage info is stale when viewed anyway, locking is simplified by using an exclusive domain lock for coherent reads of usage.
Quota accounts also have a mechanism to avoid hot spots on the nodes storing QABs. It is easy to imagine all nodes in a cluster producing accounting changes in the same domain, for example when an ALL domain is configured on a top-level directory. This problem is solved using quota account constituents, which parallelize the accounting to include additional QABs. Quota performance tuning provides more information.