Configuring a balanced storage system

The attachment of a storage system to a SAN Volume Controller requires that specific settings are applied to the device.

There are two major steps to attaching a storage system to a SAN Volume Controller:

Setting the characteristics of the SAN Volume Controller to storage connections
Mapping logical units to these storage connections that allow the SAN Volume Controller to access the logical units

The virtualization features of the SAN Volume Controller enable you to choose how your storage is divided and presented to hosts. While virtualization provides you with a great deal of flexibility, it also offers the potential to set up an overloaded storage system. A storage system is overloaded if the quantity of I/O transactions that are issued by the host systems exceeds the capability of the storage to process those transactions. If a storage system is overloaded, it causes delays in the host systems and might cause I/O transactions to time out in the host. If I/O transactions time out, the host logs errors and I/Os fail to the applications.

Scenario: You have an overloaded storage system

Under one scenario, you have used the SAN Volume Controller to virtualize a single RAID array and to divide the storage across 64 host systems. If all host systems attempt to access the storage at the same time, the single RAID array is overloaded.

Perform the following steps to configure a balanced storage system:

Use Table 1 to calculate the I/O rate for each RAID array in the storage system.

Note: The actual number of I/O operations per second that can be processed depends on the location and length of each I/O, whether the I/O is a read or a write operation and on the specifications of the component disks of the RAID array. For example, a RAID-5 array with eight component disks has an approximate I/O rate of 150×7=1050.

Table 1. Calculate the I/O rate
Type of RAID array	Number of component disks in the RAID array	Approximate I/O rate per second
RAID-1 (mirrored) arrays	2	300
RAID-3, RAID-4, RAID-5 (striped + parity) arrays	N+1 parity	150×N
RAID-10, RAID 0+1, RAID 1+0 (striped + mirrored) arrays	N	150×N

Calculate the I/O rate for a managed disk (MDisk).
- If there is a one-to-one relationship between backend arrays and MDisks, the I/O rate for an MDisk is the same as the I/O rate of the corresponding array.
- If an array is divided into multiple MDisks, the I/O rate per MDisk is the I/O rate of the array divided by the number of MDisks that are using the array.
Calculate the I/O rate for an MDisk group. The I/O rate for an MDisk group is the sum of the I/O rates of the MDisk that is in the MDisk group. For example, an MDisk group contains eight MDisks and each MDisk corresponds to a RAID-1 array. Using Table 1, the I/O rate for each MDisk is calculated as 300. The I/O rate for the MDisk group is 300×8 = 2400.

Use Table 2 to calculate the impact of FlashCopy® mappings. If you are using the FlashCopy feature that is provided by the SAN Volume Controller, you must consider the additional amount of I/O that FlashCopy operations generate because it reduces the rate at which I/O from host systems can be processed. When a FlashCopy mapping copies write I/Os from the host systems to areas of the source or target virtual disk (VDisk) that are not yet copied, the SAN Volume Controller generates extra I/Os to copy the data before the write I/O is performed. The effect of using the FlashCopy feature depends on the type of I/O workload that is generated by an application.

Table 2. Calculate the impact of FlashCopy mappings
Type of application	Impact to I/O rate	Additional weighting for FlashCopy
Application is not performing I/O	Insignificant impact	0
Application is only reading data	Insignificant impact	0
Application is only issuing random writes	Up to 50 times as much I/O	49
Application is issuing random reads and writes	Up to 15 times as much I/O	14
Application is issuing sequential reads or writes	Up to 2 times as much I/O	1

For each VDisk that is the source or target of an active FlashCopy mapping, consider the type of application that you want to use the VDisk and record the additional weighting for the VDisk.

Example

For example, a FlashCopy mapping is used to provide point-in-time backups. During the FlashCopy process, a host application generates an I/O workload of random read and write operations to the source VDisk. A second host application reads the target VDisk and writes the data to tape to create a backup. The additional weighting for the source VDisk is 14. The additional weighting for the target VDisk is 0.

Calculate the I/O rate for VDisks in an MDisk group by performing the following steps:
1. Calculate the number of VDisks in the MDisk group.
2. Add the additional weighting for each VDisk that is the source or target of an active FlashCopy mapping.
3. Divide the I/O rate of the MDisk group by this number to calculate the I/O rate per VDisk.
Example 1

An MDisk group has an I/O rate of 2400 and contains 20 VDisks. There are no FlashCopy mappings. The I/O rate per VDisk is 2400 / 20 = 120.

Example 2

An MDisk group has an I/O rate of 5000 and contains 20 VDisks. There are two active FlashCopy mappings that have source VDisks in the MDisk group. Both source VDisks are accessed by applications that issue random read and write operations. As a result, the additional weighting for each VDisk is 14. The I/O rate per VDisk is 5000 / ( 20 + 14 + 14 ) = 104.

Determine if the storage system is overloaded. The figure that was determined in step 4 provides some indication of how many I/O operations per second can be processed by each VDisk in the MDisk group.

If you know how many I/O operations per second that your host applications generate, you can compare these figures to determine if the system is overloaded.
If you do not know how many I/O operations per second that your host applications generate, you can use the I/O statistics facilities that are provided by the SAN Volume Controller to measure the I/O rate of your virtual disks, or you can use Table 3 as a guideline.

Table 3. Determine if the storage system is overloaded
Type of Application	I/O rate per VDisk
Applications that generate a high I/O workload	200
Applications that generate a medium I/O workload	80
Applications that generate a low I/O workload	10

Interpret the result. If the I/O rate that is generated by the application exceeds the I/O rate per VDisk that you calculated, you might be overloading your storage system. You must carefully monitor the storage system to determine if the backend storage limits the overall performance of the storage system. It is also possible that the previous calculation is too simplistic to model your storage use after. For example, the calculation assumes that your applications generate the same I/O workload to all VDisks, which might not be the case.
You can use the I/O statistics facilities that are provided by the SAN Volume Controller to measure the I/O rate of your MDisks. You can also use the performance and I/O statistics facilities that are provided by your storage systems.

If your storage system is overloaded there are several actions that you can take to resolve the problem:

Add more backend storage to the system to increase the quantity of I/O that can be processed by the storage system. The SAN Volume Controller provides virtualization and data migration facilities to redistribute the I/O workload of VDisks across a greater number of MDisks without having to take the storage offline.
Stop unnecessary FlashCopy mappings to reduce the amount of I/O operations that are submitted to the backend storage. If you perform FlashCopy operations in parallel, consider reducing the amount of FlashCopy mappings that start in parallel.
Adjust the queue depth to limit the I/O workload that is generated by a host. Depending on the type of host and type of host bus adapters (HBAs), it might be possible to limit the queue depth per VDisk or limit the queue depth per HBA, or both. The SAN Volume Controller also provides I/O governing features that can limit the I/O workload that is generated by hosts.

Note: Although these actions can be used to avoid I/O time-outs, performance of your storage system is still limited by the amount of storage that you have.