vol_fmr_logsz

The maximum size in kilobytes of the bitmap that Non-Persistent FastResync uses to track changed blocks in a volume. The number of blocks in a volume that are mapped to each bit in the bitmap depends on the size of the volume, and this value changes if the size of the volume is changed.

For example, if the volume size is 1 gigabyte and the system block size is 512 bytes, a value for this tunable of 4 yields a map that contains 16,384 bits, each bit representing one region of 128 blocks.

The larger the bitmap size, the fewer the number of blocks that are mapped to each bit. This can reduce the amount of reading and writing required on resynchronization, at the expense of requiring more non-pageable kernel memory for the bitmap. Additionally, on clustered systems, a larger bitmap size increases the latency in I/O performance, and it also increases the load on the private network between the cluster members. This is because every other member of the cluster must be informed each time a bit in the map is marked.

Since the region size must be the same on all nodes in a cluster for a shared volume, the value of this tunable on the master node overrides the tunable values on the slave nodes, if these values are different. Because the value of a shared volume can change, the value of this tunable is retained for the life of the volume.

In configurations which have thousands of mirrors with attached snapshot plexes, the total memory overhead can represent a significantly higher overhead in memory consumption than is usual for VxVM.

The default value is 4 KB. The maximum and minimum permitted values are 1 KB and 8 KB.

Specify a value to vxtune in kilobytes.

voldrl_dirty_regions

This parameter applies to enhanced DCO layout (version 30) only.

Represents the number of dirty regions to cache before another write to the same region causes a DRL update. A smaller number results in more frequent updates to the DRL, which decreases performance. A larger number results in better I/O performance, but requires that the DRL uses more memory.

The default value is 1024.

voldrl_max_drtregs

The maximum number of dirty regions that can exist on the system for non-sequential DRL on volumes. A larger value may result in improved system performance at the expense of recovery time. This tunable can be used to regulate the worse-case recovery time for the system following a failure.

The default value is 2048.

voldrl_max_seq_dirty

The maximum number of dirty regions allowed for sequential DRL. This is useful for volumes that are usually written to sequentially, such as database logs. Limiting the number of dirty regions allows for faster recovery if a crash occurs.

The default value is 3.

voldrl_min_regionsz

The minimum number of sectors for a dirty region logging (DRL) volume region. With DRL, VxVM logically divides a volume into a set of consecutive regions. Larger region sizes tend to cause the cache hit-ratio for regions to improve. This improves the write performance, but it also prolongs the recovery time.

The default value is 512 sectors.

If DRL sequential logging is configured, the value of voldrl_min_regionsz must be set to at least half the value of vol_maxio.

Specify the value in sectors.

voldrl_volumemax_drtregs

Maximum per-volume limit on dirty regions for a mirrored volume using traditional DRL. For heavily-used volumes, increase the value of this parameter to improve performance.

The default value is 256.

voldrl_volumemax_drtregs_20

Maximum per-volume limit on dirty regions for a mirrored volume using version 20 DCO. For heavily-used volumes, increase the value of this parameter to improve performance. The default value is 1024.

volpagemod_max_memsz

The amount of memory that is allocated for caching FastResync and cache object metadata. The memory allocated for this cache is exclusively dedicated and is not available for other processes or applications.

The default value is 6144KB (6MB).

If cache objects or volumes that are prepared for instant snapshot operations are present on the system, setting the value below 512KB fails. If you do not use the FastResync or DRL features that are implemented using a version 20 DCO volume, you can set the value to 0. If you subsequently decide to enable these features, change the value to an appropriate one.

Specify the value in kilobytes. The new value is page-aligned automatically; however the actual value specified is made persistent.

Determine the value based on the region size and the number of volumes for which instant snapshots are taken. The paging module size must be at least twice the size required for the largest size volume, as calculated with the following formula:

size_in_KB = 6 * (total_volume_size_in_GB) * (64/region_size_in_KB)

For example, a single 1TB volume requires around 6MB of paging memory if the region size is 64KB. The minimum value for the tunable parameter is at least twice that, or 12 MB.

If there are multiple volumes, all volumes share the same paging module. The maximum requirement is calculated by multiplying the above formula by the number of volumes. However, a more reasonable value depends on the average load to each volume. For example, if only 20% of the data on each volume is updated, the paging module size can be reduced proportionally without compromising the performance. The minimum requirement for the largest volume still must be met. For example, if there are 10 volumes of 1TB each, the initial calculation is 60MB of paging memory. If only 20% of the data is updated, calculate the revised value as 12MB.