Re: best base / worst case RAID 5,6 write speeds

[Dallas Clement <dallas.a.clement@gmail.com>]

On Thu, Dec 10, 2015 at 2:06 PM, Phil Turmel <philip@turmel.org> wrote:
> On 12/09/2015 08:34 PM, Dallas Clement wrote:
>> Hi all.  I'm trying to determine best and worst case expected
>> sequential write speeds for Linux software RAID with spinning disks.
>>
>> I have been assuming on the following:
>>
>> Best case RAID 6 sequential write speed is (N-2) * X, where is is
>> number of drives and X is write speed of a single drive.
>>
>> Worst case RAID 6 sequential write speed is (N-2) * X / 2.
>>
>> Best case RAID 5 sequential write speed is (N-1) * X.
>>
>> Worst case RAID 5 sequential write speed is (N-1) * X / 2.
>>
>> Could someone please confirm whether these formulas are accurate or not?
>
> Confirm these?  No.  In fact, I see no theoretical basis for stating a
> worst case speed as half the best case speed.  Or any other fraction.
> It's dependent on numerous variables -- block size, processor load, I/O
> bandwidth at various choke points (Northbridge, southbridge, PCI/PCIe,
> SATA/SAS channels, port mux...), I/O latency vs. queue depth vs. drive
> buffers, sector positioning at block boundaries, drive firmware
> housekeeping, etc.
>
> Where'd you get the worst case formulas?
>

> Where'd you get the worst case formulas?

Google search I'm afraid.  I think the assumption for RAID 5,6 worst
case is having to read and write the parity + data every cycle.