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From: Ivan Voras <ivoras@fer.hr>
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Subject: Re: Slow RAID1 with gmirror
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Hr. Daniel Mikkelsen wrote:

> But reading from the RAID1 runs at half the normal rates, while I
> expected to get double rates.

Some possible things to check are:

- What did you use for bs= argument to dd? You should use sizes of 64KB 
or more (for example, a common size is 1MB)
- Did you use dd on a raw mirror device or on a file system built on the 
device? Try both :) The difference you're seeing might be because Linux 
buffers raw device I/O and FreeBSD doesn't.
- If you're using it on the filesystem you could try increasing 
vfs.read_max sysctl variable
- Try using stripe size of 64K or 128KB with gmirror ("-s" argument) and 
bs=X (of dd) with twice that size

In any case, please post results you get from experiments such as these 
(also from Linux) to this list.