Gmirror performanc

[Oliver Fromme]

Guido van Rooij wrote:
 > Anyway, I created a gm device and a partition. Now the read performance
 > is not what I'd expect.
 > I have the partition on two SATA devices on different controlers.
 > I get around 60MB/s for each disk. I can get that speed from both disks
 > simultaneously.
 > Now when I dd from the gm device, I don't get any speed higher than that.

That's expected.

 > I tried with -b split -s <various sizes>, -b round-robin, -b load.
 > (dd-ing as done with a bs of 1m; I see the transaction size is 128Kb,
 > unless the split method is used, in which case the transaction size
 > gies down. When round-robin is used, the transaction size is 128Kb/s,
 > but the number of transaction per second goes down.).
 > 
 > I cannot explain why I should not get a higher read speed. Anyone?

dd is a sequential, single-threaded operation, so it will
only use one disk at a time.  It's not really suitable as
a benchmark for real-world things.

In the real world [TM] you have multiple processes that
access the file system at random.  Here you will benefit
from the mirror, because accesses will be distributed
among the disks.

Best regards
   Oliver

-- 
Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
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