Memorex Telex NV reckons that its 3937 Disk Array Subsystem for the AS/400 is about six months ahead of equivalent IBM Corp technology (CI No 2,213). According to marketing manager for mid-range systems Phil Townsend, several major performance enhancing features have been made to the new product, which is currently in beta testing and is not due to be released until October or November this year. First, the 3937 has fully cached read and write facilities. This means that user data is stored in cache memory, which reads and writes it to and from disks in the SCSI disk array in between one and two milliseconds. With IBM’s 9337, conversely, user data is processed by the AS/400 CPU. But because electronic signals from the CPU travel faster than the disk drive can deal with them, there is some mechanical delay in reading and writing to disk – it takes on average between 10mS and 18mS. While Townsend said that IBM would be inclined to sell customers a larger processor to speed up the disk drive, he claims there is no need with the 3937 as it maximises use of their existing CPU. Second, the cache has fault tolerance built in in three ways – with the aim of covering every eventuality. Data from one cache card is automatically mirrored on another sitting beside it – they are essentially like two bits of toast in a toaster.
Four can fail
So if one fails, information can be retrieved from the other, even when the system is operational. Faulty cards can also be replaced while the system is live. Following a power failure, the cache is immediately given battery back-up, which lasts for eight days. And just before the battery runs out, it powers up a small internal Winchester drive, and transfers all the data held in the cache to it. At the disk level, Memorex Telex provides RAID Level 5 protection. This means the same data is striped across each of the 10 disks in the disk array, and regular checks are made as to its integrity. Up to four of the 10 disks can then fail – compared with one for IBM’s product, says Townsend – but the information can still be reconstructed. A 1.7Gb version of the 3937 Disk Array Subsystem will cost UKP12,000, the top end 13.8Gb model UKP72,000.
