Hybrid Aggregates: Combining SSDs and HDDs in a single storage pool

摘要

Relative to traditional hard disk drives (HDDs), solid state drives (SSDs) provide a very large number of I/Os per second, but they have limited capacity. From a cost-effectiveness perspective, SSDs provide significantly better random I/O throughput per dollar than a typical disk, but the capacity provided per dollar spent on SSDs limits them to the most demanding of datasets. Traditionally, Data ONTAP? storage aggregates have been provisioned using a single type of disk. This restriction limits the cost-effectiveness of the storage pool to that of the underlying disks. The Hybrid Aggregates project within the Advanced Technology Group (ATG) explored the potential to combine multiple disk types within a single aggregate. One of the primary goals of the project was to determine whether a hybrid aggregate, composed of SSDs (for their cost-effective performance) and Serial-ATA (SATA) disks (for their cost-effective capacity), could simultaneously provide better cost/performance and cost/throughput ratios than an all Fibre-Channel (FC) solution. The project has taken a two-pronged approach to building a prototype system capable of supporting hybrid aggregates. The first part of the project investigated the changes necessary for Data ONTAP RAID and WAFL? layers to support a hybrid aggregate. This included propagating disk-type information to WAFL, modifying WAFL to support the allocation of blocks from a particular storage class (i.e., disk type), and repurposing the existing write-after-read and segment-cleaning infrastructure to support the movement of data between storage classes. The second part of the project examined potential policies for allocating and moving data between storage classes within a hybrid aggregate. Through proper policies, it is possible to automatically segregate the data within the aggregate such that the SSD-backed portion of the aggregate absorbs a large fraction of the I/O requests, leaving the SATA disks to contribute capacity for colder data. This paper describes the implementation of the Hybrid Aggregates prototype and the policies for automatic data placement and movement that have been evaluated. It also presents some performance results from the prototype system.