Dell PS Series: Space Borrowing for Snapshots and Replicas Dell PS Series firmware v8.
Revisions Date Description September 2012 Initial release May 2015 Updates for Dell PS Series firmware v8.0, including Space Borrowing for Replication November 2019 vVols branding update THIS WHITE PAPER IS FOR INFORMATIONAL PURPOSES ONLY, AND MAY CONTAIN TYPOGRAPHICAL ERRORS AND TECHNICAL INACCURACIES. THE CONTENT IS PROVIDED AS IS, WITHOUT EXPRESS OR IMPLIED WARRANTIES OF ANY KIND. © 2015–2019 Dell Inc. All rights reserved. Dell, the DELL logo, and the DELL badge are trademarks of Dell Inc.
Table of contents Revisions ............................................................................................................................................................................................... 2 Audience............................................................................................................................................................................................... 4 Objective ..............................................................................
Audience The information in this guide is intended for technology professionals interested in using Dell PS Series storage to protect and recover data using PS Series snapshots and replicas. Objective This guide details the space borrowing features in Dell PS Series firmware v8.0 and later for snapshots and replicas. Introduction Storage plays a critical role in business operations. With the ever-growing presence of new applications and data, storage demands continue to grow.
1 Space borrowing In a Dell PS Series group, snapshots and replicas are space-efficient, point-in-time copies of a volume that can be used to protect data and are often used to recover data lost by human error and data corruption. Snapshots and replicas require available disk space to operate successfully. This disk space is referred to as snapshot reserve and replica reserve.
1.1 Snapshots In a PS Series group, Snapshots are space-efficient, point-in-time copies of a volume that can be used to protect data and are often used to recover data lost by human error and data corruption. Multiple snapshots of a volume can be retained for data protection and are accessible to other hosts. When a Snapshot of a volume is created, it does not initially consume any space (as the snapshot shares all data with the volume), but instead is a set of pointers to the data in the base volume.
1.1.1.1 Snapshot reserve settings Depending on the number of snapshots and the amount of changes made to the base volume, snapshot reserve can become entirely consumed. If this happens, the default action is to delete the oldest snapshot, providing the necessary free space to create a new snapshot. However, there are various other configurable defaults for snapshot space. Default Snapshot reserve settings can be managed at the PS Series group level, and at the individual volume level.
5. 6. 7. Expand Volumes, right-click the volume and select Modify snapshot policy. Select a snapshot space recovery option and click OK. To enable space borrowing for snapshots, check Borrow snapshot space as needed. For additional information on snapshots, refer to the following documents. • • 1.2 Dell EqualLogic Group Manager Administrator’s Manual on eqlsupport.dell.
1.2.1 Replication Space Volume replication between partners requires space on both the primary group (the volume location) and the secondary group (the replica location). These space requirements are classified as follows: Delegated space on the secondary group, is the amount of space dedicated to storing replicas from the primary group. All replica reserves are confined to exist within this configured limit.
2 Best practices for space borrowing For a system to gain the optimum benefit of space borrowing, apply the following best practices when planning and configuring snapshot and replication storage space. 2.1 Planning space requirements Space borrowing is intended to help during peaks of activity when additional space is needed temporarily. It does not take the place of carefully provisioning reserves.
Also, if the replica reserve for a volume is configured with a very low value, such as the minimum 105%, the system can potentially require manual administrative intervention to increase the reserve percentage so that an in-progress replica can continue. In-progress replicas are not eligible to borrow space. See section 5.1.4 for details. Note: To use space borrowing for replicas, all members in the secondary group must be running Dell PS Series firmware version 8.0 or later.
3 Space borrowing with thin provisioned volumes and SCSI UNMAP Snapshot reserve is defined as a percentage of volume reserve. If the volume size changes, the snapshot reserve also adjusts automatically. With thin provisioned volumes using SCSI UNMAP (with Microsoft Windows, VMware ESXi, or Red Hat Linux) where the PS Series volume size shrinks as a result of data being deleted on the volume from the operating system, snapshot reserve will also shrink.
4 Displaying borrowed space information To view borrowed space information in the PS Series group: 1. In Group Manager, click Borrowed Space in the left pane. You can also view space-borrowing statistics through several CLI commands. For additional information, refer to the Dell EqualLogic Group Manager CLI Reference Guide at eqlsupport.dell.com (login required).
1 1. 2. 14 2 Local Space tab: Displays space borrowing information for the local PS Series group (section 4.1.1). Remote Delegated Space tab: Displays space borrowing information for remote PS Series groups that are replication partners (section 4.1.2).
4.1.1 Local Space tab The Local Space tab displays space borrowing information for the local PS Series Group. A 3 B 3 Callout 15 C D E F F Description A Borrowed space by storage pool: This section summarizes borrowed space for the selected pool or all pools. B Storage pool and resource column: • Free space is the amount of unused space still available in the selected storage pool.
G 3 H 3 Callout 16 I 3 J K 3 Description G Object types in that can borrow space section: A summary of the space usage of objects that can borrow space in the selected pool.
L 3 M 3 Callout 17 N Q 3 P 3 Description L Objects in that can borrow space section: A list of objects that can borrow space in the selected pool, including: • Snapshots • Replica sets (inbound replicas in delegated space) • Local replicas (outbound) • Deleted volumes (in the Volume recovery bin) M Total reserves column: The amount of space reserved for each object N Total size column: The amount of space being used for each object O Reserves used column: The amount of reserve
4.1.2 Remote Delegated Space tab The Remote Delegated Space tab displays space borrowing information for secondary groups that are replication partners and summarizes the primary group delegated space usage at each replication partner. The data is automatically updated each time the group communicates using a replication operation (such as enabling replication on a volume or creating, modifying, or deleting a replica) with a replication partner.
H J K L M N O I 19 Callout Description H Replica sets on section: Displays information, supplied by the selected replication partner, about the local group replica sets that exist on the selected replication partner I Volume column: The name of the replica set on the selected partner J Number of replicas column: The number of replicas in the replica set K Total reserves column: The amount of space reserved for replicas on the volume L Total size column: The amount of sp
5 Space borrowing examples 5.1.1 Space borrowing with snapshots In the following example, a 10 GB thick volume is configured with 100% (10 GB) of snapshot reserve. As snapshot borrowing is enabled on the volume, it is possible for the snapshot space to exceed the configured 10 GB snapshot reserve for the volume. In the example below, the full 10 GB of configured snapshot reserve space is being used, two additional GB are borrowed from the storage pool. A total of 12 GB of snapshot space is used.
5.1.2 Space borrowing with snapshots and replication schedules Space borrowing is also useful as it relates to snapshot and replication schedules. Because older snapshots and replicas can borrow space and remain available even if the remaining reserve space runs low. In this example, a volume is configured with 200% replica reserve.
Below, you can see that there are four existing replicas for the volume with a total a size of 19.82 GB, which is closely approaching the configured 20 GB of replica reserve. With space borrowing, the replication operation for the fifth replica succeeds and maintains all five required replicas by enabling the four existing replicas to borrow 10.62 GB.
5.1.3 Space borrowing with low delegated space on the secondary group (replication) Remote replicas can borrow beyond their total replica reserve, but the total amount of configured reserve space must still fit within the delegated space. In this example, a new 100 GB thick volume on the primary group is configured for replication with the default total replica reserve, 200%.
The secondary group is configured with 95 GB of delegated space for the primary group. 40 GB of data are written to the volume. An attempt to replicate the volume fails with a critical alert, “Volume replication failed because the replica reserve could not increase automatically due to lack of free delegated space. Contact the replication partner administrator and request an increase in delegated space.
Writing 40 GB of data to the volume and creating a replica changes the current replica volume reserve to 50 GB. (40 GB of data + 10 GB current replica volume reserve = 50 GB). Total replica reserve is a percentage of current replica volume reserve. As the total replica reserve is 200%, and the current replica volume reserve is now 50 GB, 200% of 50 GB results in 100 GB of total replica reserve.
5.1.4 Space borrowing with low total replica reserve (replication) Even with plenty of delegated space and borrowable space, if there is insufficient replica reserve on the secondary group, the system will require manual administrative intervention to increase the amount of total replica reserve space to complete an active replication operation. In this example, a new 100 GB thick volume on the primary group is configured for replication with the minimum total replica reserve, 105%.
The secondary group is configured with 1 TB of delegated space for the primary group. 50 GB of data are written to the volume.
A replica of the volume is created. An additional 10 GB of data are written to the volume. There are now 60 GB of data in use. The next attempt to replicate the volume is automatically paused with a critical alert, “Volume replication paused because the replica reserve was exceeded. You must increase the replica reserve to resume replication.
Writing 60 GB of data to the volume and creating replica changes the current replica volume reserve to 70 GB (60 GB of data + 10 GB of current replica volume reserve = 70 GB). Total replica reserve is a percentage of current replica volume reserve. As the total replica reserve is 105%, and the current replica volume reserve is 70 GB, 105% of 70 GB results approximately in a 74 GB Total Replica Reserve.
A Additional resources Dell.com/support is focused on meeting customer needs with proven services and support. Storage technical documents and videos provide expertise that helps to ensure customer success on Dell EMC storage platforms. A.1 Related documentation See the following referenced or recommended resources related to this document. • • • • 30 Dell EqualLogic Group Manager Administrator’s Manual at eqlsupport.dell.
