IBM Support

Compressed Volumes Are Limited to 96TiB

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Abstract

When creating a compressed volume, the size of the compressed volume should never be greater than 96TiB of user data. Any compressed volume that is larger than 96 TiB may experience a loss of access to the volume. A future release of software will prevent compressed volumes and compressed volume copies that are larger than 96TiB from being created.

This limitation applies to compressed volumes in regular pools only. Compressed volumes in data reduction pools are not affected.

This limitation does not affect data integrity.

Content

Any compressed volume that is larger than 96 TiB may experience a loss of access.
Most volumes that are greater than 96 TiB will not experience this loss of access. Nevertheless, IBM strongly recommends that customers stop using these volumes as soon as possible. If a volume experiences this issue, the volume may be taken offline, causing the application to lose access to the data. In some rare scenarios, some data may become unreadable.

Customers should not create any new compressed volumes that are larger than 96 TiB. A future release of software will not allow any compressed volumes or compressed volume copies to be created that are larger than 96 TiB


What to do if you have compressed volumes with a capacity of greater than 96 TiB

To retain the benefits of compression for this data, create multiple smaller compressed volumes and use host techniques to migrate the data from the current volume onto the new volumes.

Alternatively you can convert the large compressed volume into a thin provisioned volume by adding a thin provisioned copy to the volume and then deleting the compressed copy once the two copies are in sync.


How to check the capacity for your volumes

Using the GUI

In the volumes panel of the GUI, right click on the heading bar at the top of the table and add the Compressed column to the table

Then you can sort by the capacity column to find the largest volumes in the system, and check which of them have a compressed copy.

Using the CLI

The following command will list all compressed volume copies in your system
    lssevdiskcopy -filtervalue compressed_copy=yes -delim :

The output will look like this. The capacity, compressed_copy columns have been highlighted
    vdisk_id:vdisk_name:copy_id:mdisk_grp_id:mdisk_grp_name:capacity:used_capacity:real
    _capacity:free_capacity:overallocation:autoexpand:warning:grainsize:se_copy:compressed_copy
    :uncompressed_used_capacity
    0:vv1:0:0:ppp:16.00GB:2.00GB:2.01GB:6.00GB:796:off:20:32:no:yes:3.27GB
    1:se1:1:0:ppp:16.00GB:2.00GB:2.01GB:14.00GB:796:off:45:256:no:yes:4.46GB


Internal Use Only

OLD: Any compressed volume which has a used capacity of greater than 128 TiB will experience a loss of access to the volume or IO errors. To avoid a compressed volume ever exceeding this threshold, a compressed volume should never be greater than 96TiB. A future release of software will prevent compressed volumes and compressed volume copies that are larger than 96TiB from being created.

Note: In the following document I refer to two different types of capacity.

  • Virtual Capacity is the size of the volume as presented to the host. This is normally referred to as simply capacity, but virtual capacity is being used in this document for clarity
  • Used Capacity is the amount of data that is being stored in the storage pool. For a compressed volume the used capacity is a combination of the compressed data plus the compression metadata.


This issue does not affect Thin Provisioned (also known as Space Efficient) volumes, or Fully Allocated volumes. This issue does not cause data integrity .

An issue in the software which manages the compressed volume means that it is not able to correctly manage the volume when there is more than 128 TiB of data being stored in the storage pool. That 128 TiB of data includes both the compressed data and the compression metadata.

Users should not create any new compressed volumes that are larger than 96 TiB. A future release of software will not allow any compressed volumes or compressed volume copies to be created that are larger than 96 TiB

The reason that compression volume size will be limited to 96 TiB rather than 128 TiB is to ensure that no compressed volume can ever exceed the 128 TiB used capacity limit. If a compressed volume was being used to store completely un-compressible content (such as data that was encrypted by the server before being stored in the compressed volume), then a 96 TiB volume could use 128 TiB of used capacity. This is because the 96TiB of compressed data will also need to store 32 TiB of metadata as well as the volume data.




What to do if you have compressed volumes that have volumes with a virtual capacity of greater than 96 TiB

If you need to keep your volumes compressed due to availability of space in your storage pool then you will need to create multiple smaller compressed volumes and use host techniques to migrate the data from the current volume onto the new volumes.

Alternatively you can convert the large compressed volume into a thin provisioned volume by adding a thin provisioned copy to the volume and then deleting the compressed copy once the two copies are in sync

What to do if you have a compressed volume with a used capacity of greater than 128TiB

Please contact your IBM support representative as soon as possible.


How to check the virtual capacity and used capacity for your volumes

Using the GUI

In the volumes panel of the GUI, you can right click on the heading bar at the top of the table and add the following additional columns to the table
  • Compressed
  • Used Capacity

Then you can sort by the different columns to help you identify if any volumes are exposed

Using the CLI

The following command will list all compressed volume copies in your system

lssevdiskcopy -filtervalue compressed_copy=yes -delim :

The output will look like this. The capacity, used_capacity and compressed_copy columns have been highlighted

vdisk_id:vdisk_name:copy_id:mdisk_grp_id:mdisk_grp_name:capacity:used_capacity:real
_capacity:free_capacity:overallocation:autoexpand:warning:grainsize:se_copy:compressed_copy
:uncompressed_used_capacity
0:vv1:0:0:ppp:16.00GB:2.00GB:2.01GB:6.00GB:796:off:20:32:no:yes:3.27GB
1:se1:1:0:ppp:16.00GB:2.00GB:2.01GB:14.00GB:796:off:45:256:no:yes:4.46GB

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Document Information

Modified date:
22 June 2018

UID

ssg1S1005731