When a business operates with volumetric data, the unit of storage is not a single disk, but a set of disks, their totality, an aggregate of the required volume. And it must be managed as a whole entity. The storage scaling logic for large-block aggregates is well described using the example of JBOD - as a disk pooling format and as a physical device.
You can scale the disk infrastructure not only "up", cascading JBOD, but also "inside", using various filling scenarios. How it works, let's look at the Western Digital Ultastar Data60 as an example.
About filling
JBOD is a separate class of server hardware for dense disk placement, with multi-channel access to them by management hosts via SAS. JBOD manufacturers sell them empty, partially or completely clogged discs - who knows. Gradually filling storage with disks as needs grow allows you to spread capital costs over time. It is profitable to buy JBOD with all 60 disks from Western Digital - it's much cheaper. But you can also take a partially filled one: the minimum configuration of the Ultastar Data60 is 24 drives.
Why 24? The answer is simple: aerodynamics. The "gold standard" JBOD 4U / 60 x 3.5” has taken root in the industry for practical reasons - reasonable device sizes, accessibility, good cooling. 60 disks are typed as 5 rows of 12 HDDs each. Partially filled rows or shortages of disks in a JBOD (for example, just one row) lead to poor heat dissipation or even reverse airflow in the central channel - a design feature of the Ultastar Data60, its distinguishing feature.
In its JBODs, WD uses ArcticFlow drive technology, carefully modeled and verified. Everything for HDD - for their performance, survivability, data safety.
The essence of ArcticFlow comes down to the formation of two independent air flows with the help of fans: the front one cools the front rows of drives, and the air that enters through the internal air corridor deep into the case is used to blow the drives in the back zone of the JBOD.
It is clear why for the efficient operation of ArcticFlow it is necessary to monitor the filling of empty compartments. In a minimum configuration of 24 disks, the Ultastar Data60 must be spaced from the rear.
In a 12-drive configuration, without encountering the resistance that a two-row layout must create, the airflow at the exit from the JBOD again enters through the frontal zone into the cooling system.
There is a way to improve the situation - about it later.
About hybridity
It is worth immediately accepting as an axiom that the purpose of JBOD is in scaled data storage. From this conclusion: we use it for a population of homogeneous devices. With an eye - eventually reach the design storage volume, filling all the compartments.
But what about SSD? The best (and correct) solution is to build a separate high-performance storage on JBOF. Solid state is more comfortable there. However, Ultastar Data60 allows the installation of flash drives. Before proceeding with JBOD hybridization, you should first weigh all the pros - choose an SSD from the list of compatible ones (unlike HDD, the situation with SSD support is full of nuances). You will also have to spend money on mounting 2,5-inch drives in 3,5-inch bays.
Single SSD devices should be located in the back zone of the JBOD, closing unused bays with special plugs - Drive Blanks. This blocks the free flow of cooling air in order, as mentioned above, to prevent its recirculation.
A total of 24 SSDs are allowed per Ultastar Data60 chassis. In any case, these should be the last rows of the back zone.
Why 24? The heat dissipation of SSDs is higher than that of HDDs, for this reason, a multi-row arrangement of drives with different media natures will not be effectively blown by ArcticFlow. And heat dissipation will be a risk factor for JBOD operation.
It is pertinent to note here that with the help of Drive Blanks it is possible to reduce the effect of hot air recirculation. A JBOD layout with 12 HDDs will cool better if the empty bays are covered with blanks. The manufacturer did not say a word about such a trick, but the right to experiment is always with us. By the way, WD does not prohibit 12-disk padding, although it does not recommend it.
Practical Conclusions
Even a superficial acquaintance with JBOD aerodynamics gives an idea that it is better to rely on the developer's experience and recommendations for reliable storage operation. The processes that take place inside the disk basket require fundamental research. Neglecting the acquired knowledge is fraught with problems that are sensitive in every sense on storage volumes of hundreds of terabytes.
It is known how military regulations are written. Something similar happens with the JBOD architecture. If the solutions of the recent past sinned with the layout, in which the interface part was in the "exhaust" zone, blown by hot air, then today Ultastar Data60 is free from this drawback. All other design finds are just a technological miracle. This is how it should be treated.
Source: habr.com