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Hi, this is Shobana Balakrishnan here to talk about staging folders in DFS Replication. Staging folders are used to (a) isolate the files from the changes on the file system, and (b) amortize the cost of compression and computing RDC hashes across multiple partners. As the heuristic is set, staging is beneficial for reasonably large files greater than 64 KB. If DFS Replication didn’t stage the file downstream then (a) it would not be able to use cross-file RDC since cross-file RDC prefers files in staging whose hashes are pre-computed, and (b) DFS Replication will need to do in-memory de-compression, which does not scale with large files.
Here is some background on current staging space management. There are three values that are important to staging space management.
DFS Replication will do roughly the following when trying to stage a file:
In summary, the configured staging size and the currently used/reserved staging space are used as a gate to allow or deny new staging requests. The watermarks are used for staging cleanup.
Note the following:
It is not easy to define an optimal size for staging because staging is a trade-off between disk usage and performance. Generally you want enough space to hold at a minimum several of the files that will be replicated at the same time. So it you are replicating all 1-GB files, then you want staging to be at least 8-10 GB. On the other hand, if you are replicating mostly 100-KB files with an occasional 1-GB file, then a staging space of 1.5 – 2 GB would be OK for a minimum. As you give more space to staging, then RDC will be more efficient because RDC is only done on the staging files, so if the previous version of the file is still in the staging folder on the receiving member, then that member will not need to restage the file before comparing RDC hashes on the file. Also in this case when the file is restaged on the receiving server, the RDC hashes will need to be calculated again on the receiving member, which will delay replication and use CPU cycles.
There are several factors that affect the size of staging. Without going into theories, here are some rules of thumb:
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