Re: [Xen-users] Physical Disk VBD vs File VBD Performance

[Goswin von Brederlow <brederlo@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

"Michael T. Babcock" <mike@xxxxxxxxxxxxxx> writes:

> Matias wrote:
>> Correct me if I'm wrong, when writing
>> to RAID5 the data gets striped up and your write performance increases.
>> Now if I write to the dom2 it has to write the whole data to one spindle.
>
> RAID5 frequently causes performance losses, not gains, although with
> high performance controllers and disks you can see performance
> improvements over a raw disk.  The problem is that the data isn't just
> striped like in a RAID-0 set, the data has to have parity information
> calculated and written out as well, and all disks get hit with write
> requests at the same time when writing out data.  On reads (in a
> non-degraded set), you may see performance increases because of the
> striped nature of the data, but no guarantees.

Note that you only gain anything when writing a full stripe. Updating
a stripe entails reading the parity block, updating it, writing
it. That costs you more than the speedup of striped data.

Databases are know to cause small random writes killing your raid5
performance while filesystem try to keep data continious and don't
suffer (much).

MfG
        Goswin

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