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A Lustre filesystem can have one or more OSS. An OSS typically has between two and eight OSTs attached. To increase the storage of the Lustre filesystem, additional OSTs can be attached. To increase the bandwidth of the Lustre filesystem, additional OSS can be attached . Provided – provided the connection to the Client is not saturated.
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If your environment is biased toward heavy write, or fast write operations, 16GB is recommenced per Object Storage Server (OSS). If reading is of primary concern, increasing OSS memroy to 48GB will enable significant read caching and greatly increase performance.
For the OSS servers, prior to Lustre 1.8 which introduces OSS read caching, 16 GB or 8GB per core was usually more than sufficient for an OSS server. So, if your environment is primarily a write heavy environment, or if all you care about is the write speed, then 16 GB of memory per OSS server should still be fine. On the other hand, if you think your environment will be able to make use of the OSS read caching, then bumping the OSS server memory up to 48 GB can greatly improve your read performance.
For the MDS servers you would like to have as much memory as you can afford so that you can cache as much as possible in the MDS server. But, the cost to jump to higher GB DIMMs can be prohibitive. We routinely use 48 GB, or 24GB per core for MDS servers and this combination works very well. If you can afford higher amounts of memory then that should be even better.
Further reading
A detailed discussion of Lustre design considerations can be found in the whitepaper LUSTRE WHITEPAPER DOWNLOAD http://wiki.whamcloud.com/download/attachments/425994/Lustre+OSS+and+MDS+server+node+Requirements+white+paper.docx
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