HP 3PAR StoreServ 7450 4-node HP 3PAR StoreServ Storage Concepts Guide (OS 3.1 - Page 44

TPVV Warnings and Limits, for Remote Copy

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allocation limit, the system allows allocation of an additional 128 MB per node beyond these limits in order to ensure that the exported TPVV address space is usable. CAUTION: Use of allocation limits is recommended to prevent consumption of physical raw capacity beyond a tolerable limit. However, you should exercise caution when setting the value of the allocation limit. Upon reaching the allocation limit, any new writes to TPVVs will fail and/or snapshot volumes associated with the CPG may become invalid. Under this condition, some host applications do not handle write failures gracefully and may produce unexpected failures. CAUTION: Do not allow the volumes that draw from a CPG to exceed the CPG's growth limit. Doing so can invalidate snapshot volumes. Refer to "Common Provisioning Groups" (page 39) for additional cautions and recommendations. TPVV Warnings and Limits The TPVV volume size limit is 16 TB. When creating a TPVV, you have the option to set an allocation warning threshold and an allocation limit threshold. • allocation warning threshold: For volumes capable of allocating space on demand, the user-defined threshold at which the system generates an alert. This threshold is a percentage of the volume's virtual size, the size that the volume presents to the host. • allocation limit threshold: For volumes capable of allocating space on demand, the user-defined threshold at which writes fail, preventing the volume from consuming additional resources. This threshold is a percentage of the volume's virtual size, the size that the volume presents to the host. When setting TPVV allocation warnings and allocation limits, you must take into account the space to be consumed by both the volume's user data and the snapshot data. The total amount of snapshot space consumed by a TPVV and its snapshots includes the data written to the base volume and the data written to the snapshots. The size of the data written to the snapshots equals the total writes to the base volume since the oldest existing read-only (RO) snapshot was created. When deciding on the allocation warning and allocation limit thresholds for a TPVV, you can use an estimate of the maximum write rate to compute the snapshot data growth rate. • If there are no RO snapshots, and the volume is not a physical copy or used for Remote Copy, use the maximum write rate as the growth rate. • If there are RO snapshots, or if the volume is not a physical copy or used for Remote Copy, use twice the maximum write rate as the growth rate. • Set the allocation warning and limit thresholds based on the growth rate and how much advance warning you require before the volume reaches its limit and writes fail. Use the following formula to generate the allocation warning threshold: where the value of n is as follows: • for a TPVV without read-only snapshots, and when that TPVV is not a physical copy or used for Remote Copy, n=1. • for a TPVV with read-only snapshots, or when that TPVV is a physical copy or used for Remote Copy, n=2. 44 Virtual Volumes

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allocation limit, the system allows allocation of an additional 128 MB per node beyond these limits
in order to ensure that the exported TPVV address space is usable.
CAUTION:
Use of allocation limits is recommended to prevent consumption of physical raw
capacity beyond a tolerable limit. However, you should exercise caution when setting the value
of the allocation limit. Upon reaching the allocation limit, any new writes to TPVVs will fail and/or
snapshot volumes associated with the CPG may become invalid. Under this condition, some host
applications do not handle write failures gracefully and may produce unexpected failures.
CAUTION:
Do not allow the volumes that draw from a CPG to exceed the CPG’s growth limit.
Doing so can invalidate snapshot volumes. Refer to
“Common Provisioning Groups” (page 39)
for
additional cautions and recommendations.
TPVV Warnings and Limits
The TPVV volume size limit is 16 TB. When creating a TPVV, you have the option to set an allocation
warning threshold and an allocation limit threshold.
•
allocation warning
threshold: For volumes capable of allocating space on demand, the
user-defined threshold at which the system generates an alert. This threshold is a percentage
of the volume's virtual size, the size that the volume presents to the host.
•
allocation limit
threshold: For volumes capable of allocating space on demand, the user-defined
threshold at which writes fail, preventing the volume from consuming additional resources.
This threshold is a percentage of the volume's virtual size, the size that the volume presents
to the host.
When setting TPVV allocation warnings and allocation limits, you must take into account the space
to be consumed by both the volume’s user data and the snapshot data.
The total amount of snapshot space consumed by a TPVV and its snapshots includes the data written
to the base volume and the data written to the snapshots. The size of the data written to the snapshots
equals the total writes to the base volume since the oldest existing read-only (RO) snapshot was
created.
When deciding on the allocation warning and allocation limit thresholds for a TPVV, you can use
an estimate of the maximum write rate to compute the snapshot data growth rate.
•
If there are no RO snapshots, and the volume is not a physical copy or used for Remote Copy,
use the maximum write rate as the growth rate.
•
If there are RO snapshots, or if the volume is not a physical copy or used for Remote Copy,
use twice the maximum write rate as the growth rate.
•
Set the allocation warning and limit thresholds based on the growth rate and how much
advance warning you require before the volume reaches its limit and writes fail.
Use the following formula to generate the allocation warning threshold:
where the value of
n
is as follows:
•
for a TPVV without read-only snapshots, and when that TPVV is not a physical copy or used
for Remote Copy,
n
=1.
•
for a TPVV with read-only snapshots, or when that TPVV is a physical copy or used for Remote
Copy,
n
=2.
44
Virtual Volumes