Technically, with NDMP, NetBackup is not doing any of the reading, the filer is. This is also file level and will cause rehydration if the volume has any sort of compression or deduplication.
There are a five passes.
Pass I is where the filer maps the files.
Pass II is where the filer maps the directories.
Pass III dumps the directories (backs up to the storage unit)
Pass IV dumps the regular files
Pass V dumps the ACLs
Pass 1 and 2 can take a while and potentially reach NetBackup's NDMP Progress Timeout (Can be changed, Default is 8 hours, max 1 week)
Things that can slow this portion down do include a large number of files and/or fragmentation. (As the inodes are being mapped)
Second bottle neck can be the storage location. The filer can dump to direct attached tape, in which case we are looking at tape drives speed. The faster the better.
3-Way NDMP invokes LAN traffic over to another filer, in which case the LAN or the other filer's tape drives can be a bottle neck.
Lastly with a remote NDMP, the traffic is once again across the LAN to the NetBackup server and its storage unit. Standard tuning for buffers apply here.
In addition to a large number of inodes slowing down Pass I and/or II, this also can affect the performance in another way. The filer will send this file history information through the media server to the master server. This meta informational data is the same size regardless of the size of the file. So millions of small files will increase the amount of meta data. What I have seen here is that by making use of 7.5.0.7 or 7.6.0.2 and the MAX_FILES_PER_ADD touch file set to 100k (max of 50k in previous versions, default 5k) actually helps with the load when passing this information to the Master server's processes.
Beyond this, any other issues that arrise generally become one-off situations that result in a support call.
P.S. An additional "performance" enhancer when running local ndmp backups, is to utilize the max number of streams the filer can support, break the selections up into smaller ammounts by utilizing "allow multiple data streams" with the NEW_STREAM directive and to have enough drives available locally to the filer to support the max number of streams. (As filer hardware is capable; memory and such items.)
