Mastering Windows Server 2016 Hyper-V

Figure 4.8 Storage Spaces Direct workings under the hood

The manual configuration of tiers is not used with Storage Spaces Direct, and instead
the new Software Bus Cache (SBC) that is part of the new SSB (which is the
foundation of Storage Spaces Direct) performs automatic storage caching and tiering
configuration. For example, if you have a storage solution with NVMe, SSDs, and
HDDs, the NVMe disks will be used for caching the hottest data, the SSDs for hot data,
and the HDDs for the cold data (everything else). The NVMe disks provide a read
cache for the hottest data as well as providing the initial target for writes, enabling
high performance; then data is destaged to SSD/HDD capacity devices as needed. By
default, the cache works as follows:

All writes  up  to  256KB   are cached.

Reads   of  64KB    or  less    are cached  on  first   miss    (that   is, they’re not in  the cache

already).

Reads   larger  than    64KB    are cached  on  second  miss    within  a   10-minute   interval.

Sequential  reads   larger  than    32KB    are not cached.

A   write   cache   is  provided    only    in  all flash   systems (since  read    caching with    NVMe

for SSDs    would   not provide much    benefit).

Table 4.1 provides a full list of caching and capacity utilization with Storage Spaces
Direct. As you can see, the highest-performance storage is used for caching, while the
lower-performing storage is used for capacity. In a system with only one type of
storage, the SBC is disabled. Microsoft has a great article that walks through the
various combinations in detail:

https://blogs.technet.microsoft.com/filecab/2016/04/28/s2dtp5nvmessdhdd/

Table 4.1: Storage Usage with Storage Spaces Direct

STORAGE

CONFIGURATION

CACHING

DEVICES

CAPACITY

DEVICES

CACHING BEHAVIOR

SATA SSD + SATA

HDD

All SATA

SSD

All SATA    HDD Read    +   Write

NVMe    SSD +   SATA

HDD

All NVMe

SSD

All SATA    HDD Read    +   Write

NVMe    SSD +   SATA

SSD

All NVMe

SSD

All SATA    SSD Write   only

NVMe    SSD +   SATA

SSD +   SATA    HDD

All NVMe

SSD

All SATA    SSD

and SATA    HDD

Write   only    for SATA    SSD and

Read    +   Write   for SATA    HDD

https://technet.microsoft.com/en-us/library/mt651665.aspx

Behind the scenes, the disks used for the SBC have a special partition created on them
that consumes all of the space except for 32GB, which is saved for storage pool and