IN THIS ARTICLE
Provides an overview of Qumulo's SSD caching in versions 2.8.4 and below of Qumulo Core
- Cluster running Qumulo Core 2.8.4 and below
NOTE: If running Qumulo Core 2.8.5 or above, check out the SSD Caching with 2.8.5 and above article for details on Qumulo's improved FIFO based SSD caching feature available with later versions.
Qumulo Core makes use of SSDs in 2 ways.
- 80% of SSD capacity serves as a naive read cache
- Currently, 80% of the usable SSD capacity (~2.6TB on a 4-node QC24; ~4.2TB on a 4-node QC208) is reserved as a read cache. As of the current release, the heuristics for determining what data is kept in this read cache is naive and unpredictable. It is based on a combination of what data has been written most recently and what data has been expired from SSDs to HDDs.
- Read performance is therefore determined by whether the data is in the cache. This is why we supply both cached and uncached read performance measurements.
- 20% of SSD capacity serves as a buffer to satisfy a burst write workload
- Assuming you have already written enough data to fill the 80% of SSD capacity reserved for the read cache (~2.6TB on a QC24), a helpful analogy for the remaining 20% of SSD capacity is a sink and drain:
Data being written to the cluster comes in through the faucet at a fast speed known as “Burst” write speed. At the same time, data is expired or ‘drained’ to HDDs at a slower rate (determined in large part by the speed of HDDs). If data continues to be written to the cluster at a rate that the drain can’t keep up and the sink fills, further writes will be limited to the speed of the drain. This is known a the “Sustained” Write speed or in our case, the act of the faucet being turned down so that the sink does not overflow. As soon as the cluster stops receiving writes (the faucet turns off or down), expiration (draining) will continue until the sink is empty (20% of SSD capacity is available again as a cushion for burst writes). Workloads that involve deleting or overwriting data quickly after it is written will enjoy a longer Burst period or a higher Sustained write speed because the cluster will not have to expire (or 'drain') this data.
You should now have an overall understanding of Qumulo's SSD caching in versions 2.8.4 and below of Qumulo Core
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