elangelo commented on code in PR #4023:
URL: https://github.com/apache/solr/pull/4023#discussion_r3158799456


##########
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:
   I landed on 8 as that was where I in my tests had the biggest throughput... 
Higher and I would see too many retries to the S3 buckets, lower and I was not 
using the full capacity of target solrcloud cluster. Mind you the 8 is really 
great for disaster recovery scenario's... if you want to restore to a live 
solrcloud cluster I would not use 8, as 8 does cause a significant load on the 
solrcloud cluster.... (especially IOPS and bandwidth on the disks).
   
   So I can document all of this of course but I never saw any kind of 
'operating sanity defaults' anywhere else in the solrcloud documentation, so I 
didn't.



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