Local disks vs Storage server

A strategy that is commonly employed at present and which often seems strange on first sight is the use of storage servers. This is common, and is often justified for a host of reasons centring around the management of the data on the disks. It is strange, however, that it is also claimed to improve performance, rather than being implemented at the expense of performance. It common to assume that to improve performance you should remove client-server network hops, and network storage goes against this. Can it be true, or is this a myth?

The quick answer is that “yes it can be true”. A little more analysis would be needed to be absolutely sure it is true for a particular implementation. This can be validated by considering what happens for a disk read: The OS will request a data block from the disk. The block may be cached, but if it isn’t then a disk read is necessary. It is not uncommon for such a read to take of the order of 10ms to complete, and the request is likely to be queued behind other requests. This is then returned to the processor.

This process is basically the same whether the disk is a storage device or a local disk. For a network device, however, there is a cache at the client and another at the server and network communication between them. The storage on a network device, however, is also likely to be spread over more disks and so the number of requests being queued to each disk will be lower. The data may also be stored as copies in multiple places, which again allows optimisation of requests and reduced queuing. The server side cache is also likely to be much larger than the cache on a standard production server. The network communication overheads are therefore traded for improved disk storage mechanisms in a dedicated device. To look at a particular instance, however, would need consideration of:

a)     Network interface efficiency;

b)    Network loading and communication efficiency;

c)     Relative disk performance;

d)    Disk duplication mechanisms, allocation strategy and efficiency;

In most cases a high quality storage device with a well designed network link will outperform a standard set of local server disks.

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