JBOD modular storage and degrees of freedom

    When a business operates with voluminous data, the storage unit is not a separate disk, but a set of disks, their combination, an aggregate of the required volume. And you need to manage it as a whole entity. The logic of scaling storage with large-block aggregates is well described using the example of JBOD - as a format for combining disks and as a physical device.

    You can scale the disk infrastructure not only “up” by cascading the JBOD, but also “inside” using various filling scripts. How it works, consider the example of Western Digital Ultastar Data60.

    About filling


    JBOD - a separate class of server equipment for the dense allocation of disks, with multi-channel access to them control hosts via SAS. JBOD manufacturers sell them with empty, partially or completely clogged disks - whoever. The gradual filling of storage with disks - as needs grow - allows you to spread capital costs over time. It’s profitable for Western Digital to buy a JBOD with all 60 discs — much cheaper. But you can take it partially filled: 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 - a reasonable size of the device, organization of access, good cooling. 60 disks are typed as 5 rows of 12 HDDs each. Partial filled rows or shortage of disks in a JBOD (for example, only one row) lead to a deterioration in heat dissipation or even to reverse air flow in the central channel - a design feature of the Ultastar Data60, its distinguishing feature.

    In its JBOD, Western Digital uses ArcticFlow disc blow technology, carefully crafted and calibrated. Everything for HDD - for their performance, survivability, data safety.

    The essence of ArcticFlow is reduced 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 coming through the internal air corridor deep into the case is used to blow discs in the back zone of the JBOD.

    It is clear why, for the ArcticFlow to work effectively, it is necessary to monitor the filling of free compartments. In a minimum configuration of 24 disks, the placement in Ultastar Data60 should start from the back zone.

    The essence of ArcticFlow is reduced 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 coming through the internal air corridor deep into the case is used to blow discs in the rear JBOD



    In a configuration of 12 drives, without encountering the resistance that a two-row arrangement should create, the air flow at the outlet of the JBOD again enters the cooling system through the front zone.
    In a configuration of 12 drives, without encountering the resistance that a two-row layout should create, the air flow at the outlet of the JBOD again enters the cooling system through the front zone
    There is a way to improve the situation - more on that later.

    About hybridity


    It should immediately be accepted, as an axiom, that the purpose of JBOD is in scalable data storage. From this conclusion: we use it for a population of homogeneous devices. With an eye on - eventually reach the design storage volume, filling all the compartments.

    But what about the SSD? The best (and correct) solution is to build a separate production storage on JBOF. Solid state is more comfortable there. At the same time, Ultastar Data60 allows the installation of flash drives. Before embarking on JBOD hybridization, you should first weigh all the pros - choose an SSD, choose one from the list of compatible ones (unlike an 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 to prevent its recirculation, as mentioned above.
    Single SSD-devices should be located in the back zone of the JBOD, closing unused bays with special plugs - Drive Blanks
    In total, no more than 24 SSDs can be installed in the Ultastar Data60 chassis. In any case, it should be the last rows of the back zone.
    You can install no more than 24 SSDs in the Ultastar Data60 chassis - these should be the last rows of the back zone
    Why 24? The heat dissipation of solid-state drives is higher than the similar characteristics of the HDD, for this reason the multi-row layout of disks with different nature of the media will not be effectively blown by ArcticFlow. And heat dissipation will become a risk factor for the operation of the JBOD.

    It is appropriate to note here that with Drive Blanks you can reduce the effect of recirculation of hot air. The layout of the JBOD with 12 HDDs will cool better if the empty bays are closed with blanks. The manufacturer did not say a word about such a trick, but the right to experiment is always ours. By the way, WD does not prohibit 12-disk padding, although it does not recommend it.

    Practical findings


    Even a superficial acquaintance with the JBOD aerodynamics gives an idea that it is better to rely on the experience and recommendations of the developer for reliable storage operation. The processes taking place inside the disk basket require basic research. Neglecting the knowledge gained is fraught with problems that are sensitive in every sense on storage volumes of hundreds of terabytes.

    It is known how military manuals are written. Something similar is happening with the JBOD architecture. If the decisions of the recent past sinned by the layout, in which the interface part was in the “exhaust” zone, blown by hot air, today Ultastar Data60 is devoid of this drawback. All other design discoveries are just a technological miracle. So it’s worth it to relate to.

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