Dell Unity: How Asynchronous Replication and Space Reclamation Affect Array Performance
Summary: This article explores the impact of asynchronous replication on array performance during space reclamation from large virtual machines (VM).
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Symptoms
Massive latency throughout ALL storage resources (entire array) can be seen when reproducing the following steps:
1. Virtual Machines (VM) on Host have no usage, and are offline - no I/O - fully idle - (meant to be deleted).
2. VM is being deleted from Host (Space Reclamation).
3. Everything is working fine. All LUNs and Hosts in the Array have expected Response Times.
4. [Trigger] Replication starts either manually or by the Recover Point Objective (RPO). Latency increases in ALL LUNs and Hosts - Data became Unavailable.
2. VM is being deleted from Host (Space Reclamation).
3. Everything is working fine. All LUNs and Hosts in the Array have expected Response Times.
4. [Trigger] Replication starts either manually or by the Recover Point Objective (RPO). Latency increases in ALL LUNs and Hosts - Data became Unavailable.
Cause
Asynchronous Replication:
- When the Replication Session is paused, the Reclamation Rate does not stop nor slow down.
- While the Replication Session is paused, the work accumulates on the array.
- Once the Replication Session is resumed (either manually or automatically), this pushes the accumulated work, from when it was paused.
- Depending on the amount of work accumulated, this might cause latency, in this case, severe.
- This is Function As Design.
Space Reclamation:
- As per Best Practices, 64 TB it is not recommended for ESXi block LUNs, specially combined with Asynchronous Replication.
- When the Array queries through 40 TB of empty space to find blocks with Variable-length map-for-read, this is a lot of no-value-add work.
- While that happens, the load increases and impacts both LUN and Array (all resources).
- When the Replication Session is paused, the Reclamation Rate does not stop nor slow down.
- While the Replication Session is paused, the work accumulates on the array.
- Once the Replication Session is resumed (either manually or automatically), this pushes the accumulated work, from when it was paused.
- Depending on the amount of work accumulated, this might cause latency, in this case, severe.
- This is Function As Design.
Space Reclamation:
- As per Best Practices, 64 TB it is not recommended for ESXi block LUNs, specially combined with Asynchronous Replication.
- When the Array queries through 40 TB of empty space to find blocks with Variable-length map-for-read, this is a lot of no-value-add work.
- While that happens, the load increases and impacts both LUN and Array (all resources).
Resolution
To avoid massive latency when these conditions are met, it is advised to:
1. Stop and Delete the Replication Session.
2. Purge the unused thin and uncommitted space (with data migration) to a smaller LUN.
3. Create a new Replication session from the smaller LUN, instead.
2. Purge the unused thin and uncommitted space (with data migration) to a smaller LUN.
3. Create a new Replication session from the smaller LUN, instead.
Alternatively, if the Migration Datastore is obsolete, and it is meant to be removed, our recommendations are:
1. Delete the Replication Session.
2. Delete affected LUN.
3. Create a smaller LUN in the future, following Best Practices.
2. Delete affected LUN.
3. Create a smaller LUN in the future, following Best Practices.
Further considerations:
- Misconfiguring LUNs, ESXi hosts/VMs, and replication can have unclear consequences, even with best practices in place.
- For this case in particular, this is what was causing latency at this massive scale.
- According to Development, this is an expected behavior, and Unity is Functioning As Designed.
Reference Documentation:
- Dell EMC Unity: VMware vSphere Best Practices
- vSphere Storage Appliance (Community - External Link)
Additional Information
To confirm this is the same behavior, verify that Replication is or was being triggered, look at the following files live on the array:
Live on Array:
- /EMC/backend/log/shared/EMCSystemLogFile.log
- /EMC/C4Core/log/c4_safe_ktrace.log
Manually triggered by the Admin user:
14:10:21 service@spb:~/user# tailf 00_emc_backend_log_shared/EMCSystemLogFile.log | grep -i "refresh the replication"
B 05/04/21 14:10:48.405 ReplicPlugin 610021 [INFO] Audit: User Admin started to refresh the replication session rep_sess_res_xx_res_xx_CKM00xxxxxxxxx_CKM00xxxxxxxxx.Automatically, by Recovery Point Objective (RPO):
B 05/04/21 14:45:33.687 std ddf1a70d c4_safe_ktrace SASPMC 0 (BE99) API INFO set_dynamic_int_coalescing: nr_busy_ports=1 ict=100 icc=16
B 05/04/21 14:45:33.708 sade 9f3f70f c4_safe_ktrace DIC: 6:[SVDM_B] Handle request /dic_s/DicXmlAsyncMsgService/async=4=302002=503370607
B 05/04/21 14:45:33.708 sade 9f3f70f c4_safe_ktrace DIC: 6:[SVDM_B] Send back /dic_s/DicXmlAsyncMsgService/async=4=302002=503370607 response (200 OK)
B 05/04/21 14:45:33.708 sade 9f3f70f c4_safe_ktrace DIC: 6:[SVDM_B] Response for /dic_s/DicXmlAsyncMsgService/async=4=302002=503370607 sent
B 05/04/21 14:45:33.708 sade e5243708 c4_safe_ktrace XML: 6:[core]
B 05/04/21 14:52:07.605 mlu 16d001c [INFO] System: Snapshot 81604378643_CKM00201901039_0000.ckpt000_3275157210146496.2_684 has been created for LUN 23.
B 05/04/21 14:52:07.710 mlu 12d0003 [INFO] System: Operation Create Replica started by 900002a4a on 9000000fd.
B 05/04/21 14:52:07.724 mlu 16d0027 [INFO] System: The new name for snapshot being destroyed is Destroying_20210106115207.694+00-000.
B 05/04/21 14:52:07.738 mlu 16d0027 [INFO] System: The new name for snapshot being destroyed is Destroying_20210106115207.694+00-001.Note: Every time the RPO is reached or a manual update is started, a snapshot refresh occurs. A single snapshot refresh can be considered a destroy or re-create.
Affected Products
Dell EMC Unity, Dell EMC Unity Family |Dell EMC Unity All Flash, Dell EMC Unity HybridArticle Properties
Article Number: 000184863
Article Type: Solution
Last Modified: 21 Oct 2025
Version: 5
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