Log vdisks

IBM Storage Scale RAID uses log vdisks to store such internal information as event log entries, updates to vdisk configuration data, and certain data write operations quickly. There are four types of log vdisks, as follows. Among them, they can be created and destroyed in any order.
log home vdisk
Every recovery group requires one log home vdisk to function. The log home vdisk must be created before any other non-log vdisks in the recovery group, and it can only be deleted after all other non-log vdisks in the recovery group have been deleted. The log home vdisk is divided into four sublogs: long-term event log, short-term event log, metadata log, and fast-write log to log small write operations.
log tip vdisk
The log tip vdisk (appropriate for certain environments, but not required for all) is a vdisk to which log records are initially written, then migrated to the log home vdisk. The intent is to use a small, high-performance NVRAM device for the log tip, and a larger vdisk on conventional spinning disks for the log home vdisk. The fast writes to the log tip hide the latency of the spinning disks used for the main body of the log.
log tip backup vdisk
The log tip backup vdisk (appropriate for certain environments, but not required for all) is used as an additional replica of the log tip vdisk when the log tip vdisk is two-way replicated on nonvolatile RAM disks. Ideally, the log tip backup vdisk provides a level of performance between that of NVRAM disks and that of spinning disks.
log reserved vdisk
Log reserved vdisks are optional vdisks that are used when the log home disk is not allocated in its own declustered array. Log reserved vdisks have the same size as the log home vdisk and are used to equalize the space consumption on the data declustered arrays, but they are otherwise unused.

Typical configurations

The following are descriptions of typical vdisk configurations in various recovery group environments:
IBM Storage Scale System 3200 without NVRAM disks
In this configuration, a three-way replicated log tip vdisk is allocated on a declustered array made up of three SSDs. A four-way replicated log home vdisk is allocated in the first declustered array of HDDs.
IBM Storage Scale System 3200 with NVRAM disks
In this configuration, a two-way replicated log tip vdisk is allocated on NVRAM disks, one from each of the servers.

A log tip backup vdisk is allocated on a declustered array of one or more SSDs. This provides an additional copy of the log tip data when one of the NVRAM disks is unavailable. If only one SSD is used, then the log tip backup uses a RAID code of Unreplicated.

A four-way replicated log home vdisk is allocated in the first declustered array of HDDs. A four-way replicated log reserved vdisk for each of the data declustered arrays that do not contain the log home vdisk.

IBM Storage Scale System 3200 with NVRAM disks, using SSDs for data
In this configuration, a two-way replicated log tip vdisk is allocated on NVRAM disks, one from each of the servers.

All SSDs for a recovery group form a single declustered array, containing the log home vdisk and user data vdisks. No log tip backup disk is used.

Power® 775 configuration
In this configuration, the log home vdisk is allocated on a declustered array made up of four SSDs. Only three-way and four-way replication codes are supported for the log home vdisk. In the typical system with four SSDs and with spare space equal to the size of one disk, the three-way replication code would be used for the log home vdisk