About this task
To attach a storage system to Storwize V3700, consider the following two major
steps: - Setting the characteristics of the system to storage connections
- Mapping logical units to these storage connections that allow the system to access the logical
units
You can use the virtualization features
of the system 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
this scenario, you used the Storwize V3700 system
to virtualize a single 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 array is overloaded.
To configure a balanced storage system that is not overloaded,
follow these steps:
- Use Table 1 to
calculate the I/O rate for each RAID 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. It also depends on whether the I/O is a read or a write operation and on the
specifications of the component disks of the 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 rateType of array |
Number of component disks in the 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 you have 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 that is divided by the number
of MDisks that are using the array.
- Calculate the I/O rate for a storage
pool. The I/O rate for a storage
pool is the sum of the I/O rates of the MDisk that is in
the storage pool. For
example, a storage pool 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 storage pool is 300×8 = 2400.
- Use Table 2 to calculate the
impact of FlashCopy® mappings. If you are using the FlashCopy function that is provided by the system, you must
consider the additional amount of I/O that FlashCopy
operations generate. The amount of I/O can reduce 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 volume
that are not yet copied, the system generates extra I/Os to copy the data before the write I/O is
done. The effect of using the FlashCopy function depends
on the type of I/O workload that is generated by an application.
Table 2. Calculate the impact of FlashCopy
mappingsType of application |
Impact to I/O rate |
Additional weighting for FlashCopy |
Application is not doing I/O |
Insignificant impact |
0 |
Application is reading only data |
Insignificant impact |
0 |
Application is issuing only random write operations |
Up to 50 times as much I/O |
49 |
Application is issuing random read operations and write operations |
Up to 15 times as much I/O |
14 |
Application is issuing sequential read operations or write operations |
Up to 2 times as much I/O |
1 |
For each volume that is the source or target of an active
FlashCopy mapping, consider the type of application that
you want to use the volume. Also, record the additional
weighting for the volume.
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
volume. A second host application reads the target volume and writes the data to tape to create a backup. The additional
weighting for the source volume is 14. The additional weighting
for the target volume is 0.
- Calculate the I/O rate for volumes in
a storage pool by completing
the following steps:
- Calculate the number of volumes in
the storage pool.
- Add the additional weighting for each volume that is the source or target of
an active FlashCopy mapping.
- Divide the I/O rate of the storage
pool by this number to calculate the I/O rate per volume.
Example 1:
A storage pool has an I/O rate of 2400 and contains 20
volumes. No FlashCopy mappings exist. The I/O rate per volume is 2400 / 20 = 120.
Example 2:
A storage pool has an I/O rate of 5000 and contains 20
volumes. Two active FlashCopy mappings have source volumes in the storage pool. Both source volumes
are accessed by applications that issue random read and write operations. As a result, the
additional weighting for each volume is 14. The I/O rate per
volume is 5000 / (20 + 14 + 14) = 104.
- Determine whether 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 volume in the
storage pool.
- If you know how many I/O operations per second that your host applications generate, you can
compare these figures to determine whether the system is overloaded.
- If you do not know how many I/O operations per second that your host applications generate,
use the I/O statistics facilities that are provided by Storwize V3700 to measure the I/O rate of your
volumes. You can also use Table 3 as a
guideline.
Table 3. Determine whether the storage system is overloadedType of application |
I/O rate per volume |
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 volume that you calculated, you might be overloading your
storage system. You must carefully
monitor the storage system to determine
whether 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 volumes, which might not be the case.
You can use the I/O statistics facilities that are provided by Storwize V3700 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.
What to do next
If your
storage system is
overloaded, you can take several possible actions 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 system provides
virtualization and data migration facilities to redistribute the I/O workload of volumes across a greater number of MDisks without having to take the
storage offline.
- Stop unnecessary FlashCopy mappings to reduce the
number of I/O operations that are submitted to the backend storage. If you process 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
volume, limit the queue depth per HBA, or both. The system 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 timeouts, performance of your storage system is still limited by the
amount of storage that you have.