Explanation

Choosing and preparing the type of underlying storage is one of the most important steps in production environment virtualization. One of the ways to speed up the storage for read/write operations and get better reliability is using RAID arrays. The RAID controller settings are very important and with different settings used the results may vary greatly. Even SSD disks in a RAID array can demonstrate results similar to the HDD arrays in case of using wrong RAID controller settings.

Recommended settings for hardware RAID arrays based on HDD:

RAID type: RAID10

Disk cache policy: Default (disabled by default)

Write policy: Write Back

Read policy: Read ahead

Stripe Size: 64K

Recommended settings for hardware RAID arrays based on SSD:

RAID type: RAID1 for 2x SSDs; RAID5 for 3x and more SSDs, or RAID10 for 4x and more pair SSD

Disk cache policy: Default (enabled by default)

Write policy: Write Through

Read policy: No read ahead.

Stripe Size: 64K

Disk Cache Policy: when enabled, allows writing to the cache of the disk prior to the medium

–       For virtual disks having SATA disks underneath, this policy is ENABLED by default;

–       For virtual disks having SAS disks underneath, this policy is DISABLED by default.

More information

https://www.dell.com/support/article/ca/en/cadhs1/sln163329/analyze-raid-controller-cache-policies-in-tty-log?lang=en

Recommended settings for Linux Software RAID with StarWind VSAN for vSphere.

Linux Software RAID (MDADM, MDRAID) can be used as an underlying storage device for StarWind Virtual SAN devices.

For 4k native HDD drives chunk size must be equal to 4KiB per one drive. For SSD drives chunk size must be equal to 8KiB (which equals the size of the SSD drive page cache).

Chunk size of the array should be calculated using following formulas:

RAID Level   Chunk size for HDD Arrays   Chunk size for SSD Arrays
0                    Disk quantity * 4Kb                Disk quantity * 8Kb
5                    (Disk quantity – 1) * 4Kb        (Disk quantity – 1) * 8Kb
6                    (Disk quantity – 2) * 4Kb        (Disk quantity – 2) * 8Kb
10                  (Disk quantity * 4Kb)/2          (Disk quantity * 8Kb)/2

The number of disks in RAID10 array should be equal to a power of 2 (e.g. 4,8,16…), while for RAID5 – power of 2 + 1 disk (e.g. 3,5,9…) and for RAID6 – power of 2 + 2 disks (e.g. 4,6,10…).

HDDs should be used in RAID10. SSD, NVMe disks can be used in parity arrays (RAID5, RAID6).

More information

https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm

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