Reducing dump size

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Methods exist for handling memory dumps that are especially large (greater than 10 GB in size).

Before you begin

The preferred method for handling dumps of large production systems is to use kdump. With kdump, you do not need to set up a dedicated dump device with a dump tool for each individual system. Instead, set aside storage space to receive any dumps from across your installation. When you use kdump, the information here applies if you want to set up a backup dump method for a critical system with a large memory.

About this task

Large dumps present a challenge as they:
  • Take up a large amount of disk space
  • Take a long time when dumping
  • Use considerable network bandwidth when being sent to the support organization.
Note: Sometimes you can re-create the problem on a test system with less memory, which makes the dump handling much easier. Take this option into account before creating a large dump.

Procedure

  1. Choose a dump device.
    If you want to dump a system with a large memory footprint, you have to prepare a dump device that is large enough. You can use the following dump devices for large dumps:
    Single-volume DASD
    Depending on the disk size that is configured on the storage server you can use, for example:
    • 3390 DASD with 45 GB
    • 3390 DASD with 1 TB
    Multi-volume DASD
    Can be up to the combined size of 32 ECKD DASD partitions. With the example DASD sizes of 45 GB and 1 TB:
    • 32 x 45 GB ≅ 1.4 TB
    • 32 x 1 TB = 32 TB
    z/VM® emulated FBA device that represents a real SCSI device
    FBA disks can be defined with the CP command SET EDEVICE. These disks can be used as single-volume DASD dump disks. The SCSI disk size depends on your storage server setup.
    SCSI disk
    The SCSI disk size depends on your storage server setup. For SCSI dump partitions greater than 2 TB, you must use the GPT disk layout.
    NVMe disk
    For NVMe dump partitions greater than 2 TB, you must use the GPT disk layout.
    Dump on 3592 channel-attached tape drive
    Cartridges with up to 10 TB capacity.
    Do not use VMDUMP for large systems, because this dump method is very slow.
  2. Estimate the dump time.
    The dump speed depends on your environment, for example your SAN setup and your storage server. Assuming about 100 MB per second dump speed on DASDs or SCSI disks and a system with 50 GB memory, the dump takes about eight minutes. Do a test dump on your system to determine the dump speed for it. Then, you have an indication of how long a dump takes in case of emergency.
  3. Reduce the dump size.
    For transferring dumps in a short amount of time to a support organization, it is often useful to reduce the dump size or split the dump into several parts for easier and faster transmission. To reduce the dump, choose one of these methods:
  4. Send the dump.