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Dell PowerVault ME4 Series Storage System Administrator’s Guide

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Disk count per RAID level

The controller breaks virtual volumes into 4-MB pages, which are referenced paged tables in memory. The 4-MB page is a fixed unit of allocation. Therefore, 4-MB units of data are pushed to a disk group. A write performance penalty is introduced in RAID-5 or RAID-6 disk groups when the stripe size of the disk group isn't a multiple of the 4-MB page.
  • Example 1: Consider a RAID-5 disk group with five disks. The equivalent of four disks provide usable capacity, and the equivalent of one disk is used for parity. Parity is distributed among disks. The four disks providing usable capacity are the data disks and the one disk providing parity is the parity disk. In reality, the parity is distributed among all the disks, but conceiving of it in this way helps with the example.

    Note that the number of data disks is a power of two (2, 4, and 8). The controller will use a 512-KB stripe unit size when the data disks are a power of two. This results in a 4-MB page being evenly distributed across two stripes. This is ideal for performance.

  • Example 2: Consider a RAID-5 disk group with six disks. The equivalent of five disks now provide usable capacity. Assume the controller again uses a stripe unit of 512-KB. When a 4-MB page is pushed to the disk group, one stripe will contain a full page, but the controller must read old data and old parity from two of the disks in combination with the new data in order to calculate new parity. This is known as a read-modify-write, and it's a performance killer with sequential workloads. In essence, every page push to a disk group would result in a read-modify-write.

    To mitigate this issue, the controllers use a stripe unit of 64-KB when a RAID-5 or RAID-6 disk group isn't created with a power-of-two data disks. This results in many more full-stripe writes, but at the cost of many more I/O transactions per disk to push the same 4-MB page.

The following table shows recommended disk counts for RAID-6 and RAID-5 disk groups. Each entry specifies the total number of disks and the equivalent numbers of data and parity disks in the disk group. Note that parity is actually distributed among all the disks.

Table 1. Recommended disk group sizes
RAID levelTotal disksData disks (equivalent)Parity disks (equivalent)
RAID 6422
642
1082
RAID 5321
541
981

To ensure best performance with sequential workloads and RAID-5 and RAID-6 disk groups, use a power-of-two data disks.

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