clarifying slow transfer of many small files

Author: khdownie

includes/storage-files-migration-robocopy-optimize.md

@@ -5,7 +5,7 @@ services: storage
 author: khdownie
 ms.service: storage
 ms.topic: include
-ms.date: 4/05/2021
+ms.date: 06/22/2022
 ms.author: kendownie
 ms.custom: include file
 ---
@@ -16,34 +16,35 @@ Speed and success rate of a given RoboCopy run will depend on several factors:
 * the available network bandwidth between source and target
 * the ability to quickly process files and folders in a namespace
 * the number of changes between RoboCopy runs
+* the size and number of files you need to copy
 
 
 ### IOPS and bandwidth considerations
 
 In this category, you need to consider abilities of the **source storage**, the **target storage**, and the **network** connecting them. The maximum possible throughput is determined by the slowest of these three components. Make sure your network infrastructure is configured to support optimal transfer speeds to its best abilities.
 
 > [!CAUTION]
-> While copying as fast as possible is often most desireable, consider the utilization of your local network and NAS appliance for other, often business critical tasks.
+> While copying as fast as possible is often most desireable, consider the utilization of your local network and NAS appliance for other, often business-critical tasks.
 
 Copying as fast as possible might not be desirable when there's a risk that the migration could monopolize available resources.
 
 * Consider when it's best in your environment to run migrations: during the day, off-hours, or during weekends.
 * Also consider networking QoS on a Windows Server to throttle the RoboCopy speed.
 * Avoid unnecessary work for the migration tools.
 
-RobCopy can insert inter-packet delays by specifying the `/IPG:n` switch where `n` is measured in milliseconds between RoboCopy packets. Using this switch can help avoid monopolization of resources on both IO constrained devices, and crowded network links.
+RoboCopy can insert inter-packet delays by specifying the `/IPG:n` switch where `n` is measured in milliseconds between RoboCopy packets. Using this switch can help avoid monopolization of resources on both IO constrained devices, and crowded network links.
 
-`/IPG:n` cannot be used for precise network throttling to a certain Mbps. Use Windows Server Network QoS instead. RoboCopy entirely relies on the SMB protocol for all networking needs. Using SMB is the reason why RoboCopy can't influence the network throughput itself, but it can slow down its use. 
+`/IPG:n` can't be used for precise network throttling to a certain Mbps. Use Windows Server Network QoS instead. RoboCopy entirely relies on the SMB protocol for all networking needs. Using SMB is the reason why RoboCopy can't influence the network throughput itself, but it can slow down its use.
 
 A similar line of thought applies to the IOPS observed on the NAS. The cluster size on the NAS volume, packet sizes, and an array of other factors influence the observed IOPS. Introducing inter-packet delay is often the easiest way to control the load on the NAS. Test multiple values, for instance from about 20 milliseconds (n=20) to multiples of that number. Once you introduce a delay, you can evaluate if your other apps can now work as expected. This optimization strategy will allow you to find the optimal RoboCopy speed in your environment.
 
 ### Processing speed
 
 RoboCopy will traverse the namespace it's pointed to and evaluate each file and folder for copy. Every file will be evaluated during an initial copy and during catch-up copies. For example, repeated runs of RoboCopy /MIR against the same source and target storage locations. These repeated runs are useful to minimize downtime for users and apps, and to improve the overall success rate of files migrated.
 
-We often default to considering bandwidth as the most limiting factor in a migration - and that can be true. But the ability to enumerate a namespace can influence the total time to copy even more for larger namespaces with smaller files. Consider that copying 1 TiB of small files will take considerably longer than copying 1 TiB of fewer but larger files. Assuming that all other variables remain the same.
+We often default to considering bandwidth as the most limiting factor in a migration - and that can be true. But the ability to enumerate a namespace can influence the total time to copy even more for larger namespaces with smaller files. Consider that copying 1 TiB of small files will take considerably longer than copying 1 TiB of fewer but larger files, assuming that all other variables remain the same. Therefore, you may experience slow transfer if you're migrating a large number of small files. This is an expected behavior.
 
-The cause for this difference is the processing power needed to walk through a namespace. RoboCopy supports multi-threaded copies through the `/MT:n` parameter where n stands for the number of threads to be used. So when provisioning a machine specifically for RoboCopy, consider the number of processor cores and their relationship to the thread count they provide. Most common are two threads per core. The core and thread count of a machine is an important data point to decide what multi-thread values `/MT:n` you should specify. Also consider how many RoboCopy jobs you plan to run in parallel on a given machine.
+The cause for this difference is the processing power needed to walk through a namespace. RoboCopy supports multi-threaded copies through the `/MT:n` parameter where **n** stands for the number of threads to be used. So when provisioning a machine specifically for RoboCopy, consider the number of processor cores and their relationship to the thread count they provide. Most common are two threads per core. The core and thread count of a machine is an important data point to decide what multi-thread values `/MT:n` you should specify. Also consider how many RoboCopy jobs you plan to run in parallel on a given machine.
 
 More threads will copy our 1-TiB example of small files considerably faster than fewer threads. At the same time, the extra resource investment on our 1 TiB of larger files may not yield proportional benefits. A high thread count will attempt to copy more of the large files over the network simultaneously. This extra network activity increases the probability of getting constrained by throughput or storage IOPS.