Sequential Storage (Random Write Accelerator)

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About Sequential Storage

Applications are limited by write performance. The performance of some storage devices can improve drastically when writes are stored sequentially. The Sequential Storage feature, also referred to as Random Write Accelerator (RWA), optimizes the data layout of write operations for a virtual disk by storing them sequentially (instead of randomly) which may improve performance for certain storage configurations. Sequential storage improves the speed of random writes by converting them into sequential writes. This feature is set per virtual disk or virtual disk group.

Performance improvements will vary based on storage configurations in the server group. Significant improvements have been observed running RAID5 storage configurations; I/O operations have increased significantly. No improvement has been observed with single physical disks. The overall load on the pool needs to be taken into account when considering the effectiveness of the feature. For example, enabling this feature on only one virtual disk in a pool supporting many other virtual disks may not significantly improve performance for the storage devices in that pool. DataCore Software does not guarantee performance improvements. Limitations due to network latency and throughput will still exist.
Converting random writes into sequential writes requires additional allocation from the disk pool, so the total amount of allocated space for the virtual disk will be larger than expected. Space is reserved in the pool for each virtual disk and used to convert writes to sequential order.
- In the case of very large virtual disks, should the pool space used to convert writes grow to 900 GB, the Sequential Storage feature will be automatically disabled for the virtual disk and the additional allocation used to convert writes will be returned to the pool.
Additional memory is required for converting random writes. If available memory reserves get low, reduce the cache size.
The best practice is to enable this feature when the virtual disk is new or the allocation is relatively low.
This feature is supported for single and mirrored virtual disks that have all storage sources created from disk pools. The feature is not supported for dual virtual disk and virtual disks created from pass-through disks.
The feature can be enabled or disabled for eligible members in virtual disk groups. All group members must be eligible for sequential storage in order to enable the feature at the group level; otherwise, members must be enabled individually.
This feature is not supported for use with these features and commands:
- The Sequential Storage feature is not compatible with the Continuous Data Protection (CDP) feature. Virtual disks can only be enabled for one of these features.
- Pool repair (Purge Disk operation) Move operations.
Do not reserve disk pool space for virtual disks using sequential storage because the space will be reserved in the pool, but will not be used.
The sequential storage setting will not be retained if the virtual disk is used as a template to create other virtual disks. See Create Another.

Enabling/disabling Sequential Storage

Disabling sequential storage can require a considerable amount of time. Progress can be monitored in the Virtual Disk Details page>Info tab. In the Sequential storage field, the status will change to Disabling... and a progress bar will be displayed.
After disabling the feature is complete, allocated space in the pool used to convert writes from random to sequential will be returned as free space in the pool.

To enable or disable sequential storage:

In the Virtual Disk Details page>Settings tab, expand Advanced Options.
The feature can also be enabled from the server panel or virtual disk list.
Select or clear the Sequential Storage Enabled check box as appropriate.
Click Apply.

Status

The sequential storage status is displayed in:

The details page of the virtual disk in the Info tab (see the Data protection field). See the Sequential storage field.
The Virtual Disks List in the Virtual Disk Summary area (see the Data protection field).

Possible status:

Status	Description
Broken	Indicates that sequential storage is permanently broken and the problem cannot be resolved. This may be due to a configuration problem. See the Event Log or System Health for additional information.
Disabled	This momentary status indicates the completion of disabling sequential storage on the virtual disk.
Disabling...	Sequential storage is being disabled on the virtual disk.
Enabled	Sequential storage is enabled on the virtual disk and is in normal working condition.
Failed	Indicates that there is a problem with sequential storage on the virtual disk. It may be possible for this problem to be resolved. See the Event Log or System Health for additional information.
Unknown	The sequential storage status is unknown. See the Event Log or System Health for additional information.