SSDs always provide a significantly higher performance than classical hard disks. To what extent this is also evident in everyday use depends on the specific application in the case of storage systems. A few aspects which can play a role here are focused on below. The aim is to create an understanding for the situations in which SSDs can be used effectively in order to derive the greatest benefit from their enormous performance potential. At the end of the section, a brief summary of the resulting recommendations is provided.
Background information on the behavior of the cache in storage systems is provided in section "Cache behavior with different load types".
Influence of the cache hit rate
Classically, high performance is achieved by a high cache rate. The performance of the hard disks only becomes directly visible with cache misses. If a very high cache hit rate is already achieved consistently with a storage system or on a RAID group, only a small proportion of the IOs is actually accelerated by the SSDs.
When planning the use of SSDs in an existing storage system, it is therefore advisable to ascertain the hit rates and IO times of the various RAID groups in advance over a meaningful period.
Advantages of SSDs with various loads
SSDs are superior in performance to classical hard disks in almost all areas, but some loads profit from them more than others:
In the case of sequential, throughput-oriented loads, classical HDDs do offer lower raw performance, but this is nevertheless generally sufficient. In conjunction with the read-ahead and write-back processes of a storage system, such loads can only be processed via the cache and thus with extremely high performance.
Classical hard disks come closest to their limits in the case of random IOs. Here SSDs offer an IO performance which is many times higher. Above all, SSDs can also maintain their very good IO times at close to the maximum possible IO rate, whereas with HDDs the IO times increase sharply in the high-load range. The classical example of this is provided by database files, which are ideal candidates for migration to SSDs.
In practice, mixed loads frequently occur. The monitored variables cache hit rate, IO time, and utilization of the disks are indicators for estimating whether it is worthwhile changing a particular RAID group of classical hard disks to SSDs.
Write accesses
Write accesses are always handled as cache hits. The data is initially buffered in the cache, and then written asynchronously to the disks. Here SSDs initially provide no direct benefit.
One possible scenario is provided by high write loads which surpass the raw performance of a RAID group. On the one hand, these accumulate in the cache, which can negatively influence the hit rate of all other jobs; on the other hand, in such an overload situation parallel read IOs to the same RAID groups are enormously hampered when HDDs are employed. SSDs can handle such an overload better and significantly defuse the situation.
If such write bottlenecks only occur occasionally or on different RAID groups, under certain circumstances it may make more sense to integrate the SSDs as an extreme cache pool (see below) instead of replacing individual RAID groups. This enables them to deal with the write bursts as an expanded cache, but otherwise also serve to optimize the performance of other RAID groups.
Conclusion
The use of SSDs makes sense primarily for dedicated RAID groups:
RAID groups with low cache hit rates on account of the load
RAID groups with high random IO loads (databases)
RAID groups whose utilization is already very high
Extreme cache pool as an alternative
In some storage systems, a small number of SSDs can be installed as an "extreme cache pool". They are then used as an extension to the cache, but offer a lower performance than a "genuine" expansion of the cache. Use of the extreme cache pool can be enabled or disabled individually for each volume. This feature is should not be confused with the more powerful "extreme cache” with which flash memory is installed directly in the storage system controller and serves as a global enhancement of the cache area.
For further information on whether and under what circumstances using SSDs as an extreme storage pool is possible and makes sense, please refer to your storage system’s manual or talk to your customer consultant.