Dell PowerVault MD3000i Посібник з налаштування - Сторінка 11

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Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices

first 128KiB of an I/O is written to the first drive, the next 128KiB to the next drive,

and so on with a total stripe size of 512KiB. For a RAID 1, 2 + 2 virtual disk

group, 128KiB would be written to each of the two drives (and same for the

mirrored drives). If the I/O size is larger than this (the number of physical disks

multiplied by a 128KiB segment), this pattern repeats until the entire I/O is

complete.

For very large I/O requests, the optimal segment size for a RAID volume group is

one that distributes a single host I/O across all data drives within a single stripe.

The formula for maximal stripe size is as follows:

LUN segment size = Maximal I/O Size ÷ number of data drives

However, the LUN segment size should be rounded up to the nearest supported

power of two value.

For RAID5 and 6, the number of data drives is equal to the number of drives in

the volume group minus 1 and 2 respectively. For example:

RAID5, 4+1 with a 64KiB segment size => (5-1) x 64KiB = 256KiB stripe

size

Optimally, this RAID group is sufficient for handling I/O requests less than or

equal to 256KiB.

For RAID1, the number of data drives is equal to the number of drives divided by

2. For example:

RAID1/10, 2+2 with a 64KiB segment size => (4-2) x 64KiB = 128KiB

stripe size

It is important to remember that depending on the application's I/O parameters,

segment and strip size will vary.

For application profiles with small I/O requests, set the segment size large

enough to minimize the number of segments (drives in the LUN) that are

accessed to satisfy the I/O request, that is, to minimize segment boundary

crossings. Unless specified otherwise by the application, starting with the default

segment size of 128KiB is recommended.

It is imperative that the stripe size be correctly selected so that the host operating

system is always making requests properly aligned with full stripes or full

segments when possible.

December 2008 – Revision A01

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