Review and test drives WD Purple 2, 4 and 6 TB

Western Digital’s great devised by dividing the diversity of its HDD consumer focus on a few well-remembered “color” of sorts. This approach not only facilitates the selection of the hard drive to the destination, but also guarantees attention to the new colors are added to the basic trio Black-Green-Blue, existed back in the days when the WD drives called the word “caviar”. So, what kind of a beast – WD Purple?

A series of hard disk drives Western Digital CCTV did not appear out of anywhere. Previously, these functions were performed by the same drives WD-AV GP. Now the old line was abolished, and the successors appear under the new brand of catchy, signaling a shift device that appointment of a corporate consumer sector.

But why generally there was a need to allocate storage for such a specific application in a separate family? It would seem that the performance of any modern HDD is more than enough to simultaneously keep a record multiple video streams, even in the resolution of 1080p. For example, the bit rate 3000 kbit / s is acceptable even for films in high quality, and it is only 375 KB / s – nothing against the backdrop of sequential write speed of 150-200 MB / s, the available 3.5-inch HDD.

The task is complicated when flows really a lot, to the same hard disk periodically need to be distracted by reading the previously recorded data. In addition, we can not exclude sporadic failures of performance caused by error correction. As a consequence, in the long term, there is a non-zero risk of losing a frame, which may be unacceptable for observing objects serious.

ATA Streaming Feature Set

The best defense against data loss – flows in DRAM cache server surveillance, but the devastation of the cache can not be excluded completely. At least the problem is significant enough that, despite the low – for amateurish standards – the probability of failure, had to take action at the level of HDD, making the final link in the chain of transmission more aware of what is going on in the previous stages.

The instruction set ATA / ATAPI there is an optional section – Streaming Feature Set, which aims to provide isochronous (constant time) Execution of read / write, even if in some circumstances for that you have to sacrifice their integrity.

CONFIGURE STREAM – use this command to the host controller reports the disk, with how many threads it will have to deal with, which flows from the execution time limit read / write commands, and more. With this information, the firmware will optimize the strategy of using the cache. There can be eight threads created.

Reading and writing data from a group of commands executed flows READ / WRITE (DMA) STREAM EXT, which have the following additional options:

• READ / WRITE CONTINUOUS. Execution of the command continues, despite the error occurs. If in the process of recording the controller does not have time to correct an error in the time allotted to perform commands on the plate will be recorded corrupted data. In addition, the disc keeps a log of errors read / write streams.
• Data sent along with the write command must be reset to the magnetic plate of the DRAM-cache.
• NOT SEQUENTIAL. This parameter applies only to commands and read reports that the next read of the same flow will not happen in the near LBA (logical block). Thus, the controller can not worry about that, just in case beforehand to read the following in the track blocks (pre-fetching).

This is the interface that all hard drives with support for Streaming Feature Set provides the host controller. And here is how to dispose of the disk obtained parameters – a question to the firmware. A set of algorithms AllFrame, designed for WD drives Purple, replaced the “technology» SilkStream, which was used in the disks WD-RE GP. The difference is primarily expressed in the number of supported streams of HD-video: 12 SilkStream and 32 – for AllFrame. The latter number, apparently relates to installations with 8 disks.

It should be noted that drives WD Red and Red Pro also supports ATA Streaming Feature Set, but they do not have the firmware AllFrame.But server-class drives – WD Se and Re, oddly enough, with ATA Sreaming Feature Set, is not compatible.

Specifications, prices

Basis for WD Purple taken HDD Series WD Red, designed with the expectation to work in an array of multiple disks 24/7. It is not clear, whether in WD Purple balancing of the magnetic plates, as is done in Red (at least on the official website among the advantages of this technology Purple is not specified).

From its pre-image WD Purple inherited function TLER (Time-Limited Error Recovery), which sets a limit on the time of error correction in order to RAID-controller did not rule thoughtful disk from the array. But, like Red, WD Purple have RAFF (Rotary Accelerated Feed Forward) – a technology that helps to cope with the vibrations of the disks in the rackmount-buildings.

The line includes discs with the capacity from 1 to 6 TB. All have a spindle speed of about 5400 revs / min, and, judging by its kinship with the WD Red, must use the plate 1 TB (with a volume of 2 to 4 TB) or 1.2 TB (5-6 TB). Only model of 1 TB in question – it was too low in her speed sequential read / write. Judging by weight, WD Purple 2 and TB 3 and 5 and 6 have the same number of TB plates only at a lesser capacity disks trimmed address space (short-stroked).

The specifications Purple and Red can find the difference in the number of cycles Capping (300 000 in Purple and 600 000 in Red), but it is not critical for the drive is one continuous recording.

On the whole line is given three-year warranty. At prices similar to Purple and Red.

Outset about testing WD Purple: ideally should be tested in practice, how effective these drives work with streaming read / write, and we compare WD Re / Se in the same task, but without using the ATA Streaming Feature Set. Alas, it would need specific software and / or hardware, and public benchmarks that could replace it all, does not exist. For this reason, we restrict ourselves to the adopted methodology 3DNews testing hard drives, which focuses on isolated Performance Options HDD. In the end, nobody forbids to use WD Purple as an analogue of WD Red, only with a specific supplement (especially at the same price), and hence, a broad assessment of the performance of these drives are not meaningless.