mlbiscoc commented on code in PR #4023:
URL: https://github.com/apache/solr/pull/4023#discussion_r3163999494


##########
solr/solr-ref-guide/modules/deployment-guide/pages/backup-restore.adoc:
##########
@@ -396,6 +396,39 @@ Any children under the `<repository>` tag are passed as 
additional configuration
 
 Information on each of the repository implementations provided with Solr is 
provided below.
 
+=== Parallel File Transfers
+
+Backup and restore operations can transfer multiple index files in parallel to 
improve throughput, especially when using cloud storage repositories like S3 or 
GCS where latency is higher.
+The parallelism is controlled via system properties or environment variables:
+
+`solr.backup.maxparalleluploads`::
++
+[%autowidth,frame=none]
+|===
+|Optional |Default: `1`
+|===
++
+Maximum number of index files to upload in parallel during backup operations.
+Can also be set via the `SOLR_BACKUP_MAXPARALLELUPLOADS` environment variable.
+For cloud storage repositories (S3, GCS), consider setting this to `8` or 
higher to improve backup performance.

Review Comment:
   Got it, glad the 8 worked but I still personally don't think we should put 8 
here. You also just said `don't use 8 on a live cluster as 8 does cause a 
significant load` which is counters your # recommendation. It is a tuning 
parameter so the the user should figure out for their cloud and hardware not 
the number we placed on a ref-guide without context of their setup. I think if 
you want to place a recommendation, it is completely fair to say something 
along the lines of "Increasing this value can improve backup/restore throughput 
but too high can result in xyz problems. Recommend to start small and increase 
based on your Solr clouds observed throughput."



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