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Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)

To: Chris Snook <csnook@xxxxxxxxxx>
Subject: Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)
From: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Date: Wed, 28 May 2008 13:32:44 -0400 (EDT)
Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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On Wed, 28 May 2008, Chris Snook wrote:

Justin Piszcz wrote:

1. Utilized (6) 400 gigabyte sata hard drives.
2. Everything is on PCI-e (965 chipset & a 2port sata card)

Used the following 'optimizations' for all tests.

# Set read-ahead.
echo "Setting read-ahead to 64 MiB for /dev/md3"
blockdev --setra 65536 /dev/md3

# Set stripe-cache_size for RAID5.
echo "Setting stripe_cache_size to 16 MiB for /dev/md3"
echo 16384 > /sys/block/md3/md/stripe_cache_size

# Disable NCQ on all disks.
echo "Disabling NCQ on all disks..."
for i in $DISKS
  echo "Disabling NCQ on $i"
  echo 1 > /sys/block/"$i"/device/queue_depth

Given that one of the greatest benefits of NCQ/TCQ is with parity RAID, I'd be fascinated to see how enabling NCQ changes your results. Of course, you'd want to use a single SATA controller with a known good NCQ implementation, and hard drives known to not do stupid things like disable readahead when NCQ is enabled.
Only/usually on multi-threaded jobs/tasks, yes?

Also, I turn off NCQ on all of my hosts that has it enabled by default because
there are many bugs that occur when NCQ is on, they are working on it in the
libata layer but IMO it is not safe at all for running SATA disks w/NCQ as
with it on I have seen drives drop out of the array (with it off, no problems).

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