NVRAM cards as OSD journals

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Hi,

 

I am contemplating using a NVRAM card for OSD journals in place of SSD drives in our ceph cluster.   

 

Configuration:

·         4 Ceph servers

·         Each server has 24 OSDs (each OSD is a 1TB SAS drive)

·         1 PCIe NVRAM card of 16GB capacity per ceph server

·         Both Client & cluster network is 10Gbps

 

As per ceph documents:

The expected throughput number should include the expected disk throughput (i.e., sustained data transfer rate), and network throughput. For example, a 7200 RPM disk will likely have approximately 100 MB/s. Taking the min() of the disk and network throughput should provide a reasonable expected throughput. Some users just start off with a 10GB journal size. For example:

osd journal size = 10000

Given that I have a single 16GB card per server that has to be carved among all 24OSDs, I will have to configure each OSD journal to be much smaller around 600MB, i.e., 16GB/24 drives.  This value is much smaller than 10GB/OSD journal that is generally used.  So, I am wondering if this configuration and journal size is valid.  Is there a performance benefit of having a journal that is this small?  Also, do I have to reduce the default “filestore maxsync interval” from 5 seconds to a smaller value say 2 seconds to match the smaller journal size?

 

Have people used NVRAM cards in the Ceph clusters as journals?  What is their experience?

 

Any thoughts?

 

 


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