freebsd stable - Jul 2006 - Slow RAID1 with gmirror

Hi.

I've been setting up some gmirror based software RAID1s lately, and I
keep running into some odd behaviour.

Writing to the RAID1 runs at near to normal rates, as expected.

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

I've tested this on 6.0, 6.1, with PATA and SATA drives, and I
consistently get the same results.

I've tested with the same hardware running Linux 2.6 with software RAID1
and there I get the expected double rate.

I've tried all the available balance algorithms, and none of them help.

I haven't tried with SCSI disks.

I'm thinking that it might be the test I'm running that's a corner
case,
I simply dd to or from the filesystem with various settings for bs=.

Is this a know issue? Is it an issue at all?

-- 
Daniel Mikkelsen
Copyleft Software AS


-- 
Daniel Mikkelsen
Copyleft Software AS

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.