The most important components of the storage systems which are relevant to performance are:
Cache
The component of a storage system which most influences the performance is the cache. Each input/output request is subject to caching. The hardware service time (I/O time) depends largely on whether the data is located in the cache or must first be read from or transferred to the disk.
If the data block which is being searched for is not contained in the cache, it must be read from the disks (read miss). Such a read miss takes considerably more time than a read hit, in the case of which the data is already in the cache. When sequential processing is recognized, the data is read ahead in order to prevent a read miss.
Since the cache of the storage systems is protected against a power failure by batteries, write requests are regarded as completed when they have been written to the cache and verified (write hit rate = 100 %). A write I/O operation therefore takes almost as long as a read hit. Writing to disk takes place asynchronously to the I/O operation. If not enough space is available in the cache, so-called “delayed fast writes” occur, i.e. the cache contents must first be saved before new data can be read into the cache (write hit rate < 100 %). This typically occurs when large quantities of data are reconstructed.
The hit rate is the share of cache hits in the total number of inputs/outputs to/from the storage system. Sometimes a distinction is also made between the read hit rate and the total hit rate. The total hit rate contains the cache write hits. Only the read hit rates are examined in this manual.
Vendors of external storage systems often specify the “total hit rate”, which also contains the cache write hits and is therefore higher than the read hit rate examined here.
However, the read hit rate can be determined using the ratio of write and read IOs, provided there is no bottleneck and a 100% write hit rate can be assumed. The write IO times provide the starting point for this calculation.
A high hit rate (> 90 %) can also be achieved for read requests by using large caches. A read hit rate of 80 % or higher is normally regarded as good performance for a storage system.
Large cache configurations in the ETERNUS DX/AF and Symmetrix storage systems are urgently recommended for performance-critical applications.
Physical disks
The following are known for physical disks:
storage type (HDD/SSD)
storage capacity (in Gbyte, e.g. 300 Gbyte)
rotational speed (in “rotations per minute”, e.g. 15,000 rpm).
The following are also decisive for the performance of a disk:
internal connection with connection type (e.g. Fibre Channel, SAS, Serial ATA (SATA)), nearline SAS (NL-SAS)) and data transfer rate (e.g. 8 Gbit/s)
cache on the drive (drive cache)
Generally the user only sees the performance of the physical disks in the case of a read miss.
From the BS2000 viewpoint no restrictions exist for using the disk sizes offered today.
Controllers
The controllers of the storage systems process the server’s I/O requests, control cache access and handle access to the physical disk drives.
The number of controllers and the number of paths to the servers which are controlled and to the internal disks depend on the model.