Re: Linux MD RAID 5 Benchmarks Across (3 to 10) 300 Gigabyte Veliciraptors

[thomas62186218]

Thank you for sharing these results. One issue that I consistently see 
with these results is miserable random IO performance. Looking at these 
numbers, even a low-end RAID controller with 128MB of cache will outrun 
md-based RAIDs in random IO benchmarks. In today's world of virtual 
machines, etc, random IO is far more common than sequential IO. What 
can be done with md (or something else) to alleviate this problem?

-Thomas


-----Original Message-----
From: Dan Williams <dan.j.williams@xxxxxxxxx>
To: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-raid@xxxxxxxxxxxxxxx; 
xfs@xxxxxxxxxxx; Alan Piszcz <ap@xxxxxxxxxxxxx>
Sent: Sat, 7 Jun 2008 6:46 pm
Subject: Re: Linux MD RAID 5 Benchmarks Across (3 to 10) 300 Gigabyte 
Veliciraptors










On Sat, Jun 7, 2008 at 7:22 AM, Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx> 
wrote:
> First, the original benchmarks with 6-SATA drives with fixed 
formatting,
> using
> right justification and the same decimal point precision throughout:
http://home.comcast.net/~jpiszcz/20080607/raid-benchmarks-decimal-fix-and-right-justified/disks.html
> Now for for veliciraptors!  Ever wonder what kind of speed is 
possible with
> 3 disk, 4,5,6,7,8,9,10-disk RAID5s?  I ran a loop to find out, each 
run is
> executed three times and the average is taken of all three runs per 
each
> RAID5 disk set.
>
> In short? The 965 no longer does justice with faster drives, a new 
chipset
> and motherboard are needed.  After reading or writing to 4-5 
veliciraptors
> it saturates the bus/965 chipset.
>
> Here is a picture of the 12 veliciraptors I tested with:
http://home.comcast.net/~jpiszcz/20080607/raid5-benchmarks-3to10-veliciraptors/raptors.jpg
> Here are the bonnie++ results:
http://home.comcast.net/~jpiszcz/20080607/raid5-benchmarks-3to10-veliciraptors/veliciraptor-raid.html
> For those who want the results in text:
http://home.comcast.net/~jpiszcz/20080607/raid5-benchmarks-3to10-veliciraptors/veliciraptor-raid.txt
> System used, same/similar as before:
> Motherboard: Intel DG965WH
> Memory: 8GiB
> Kernel: 2.6.25.4
> Distribution: Debian Testing x86_64
> Filesystem: XFS with default mkfs.xfs parameters [auto-optimized for 
SW
> RAID]
> Mount options: defaults,noatime,nodiratime,logbufs=8,logbsize=262144 
0 1
> Chunk size: 1024KiB
> RAID5 Layout: Default (left-symmetric)
> Mdadm Superblock used: 0.90
>
> Optimizations used (last one is for the CFQ scheduler), it improves
> performance by a modest 5-10MiB/s:
> http://home.comcast.net/~jpiszcz/raid/20080601/raid5.html
>
> # Tell user what's going on.
> echo "Optimizing RAID Arrays..."
>
> # Define DISKS.
> cd /sys/block
> DISKS=$(/bin/ls -1d sd[a-z])
>
> # Set read-ahead.
> # > That's actually 65k x 512byte blocks so 32MiB
> echo "Setting read-ahead to 32 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"

Sorry to sound like a broken record,  16MiB is not correct.

size=$((num_disks * 4 * 16384 / 1024))
echo "Setting stripe_cache_size to $size MiB for /dev/md3"

...and commit 8b3e6cdc should improve the performance / 
stripe_cache_size ratio.

> echo 16384 > /sys/block/md3/md/stripe_cache_size
>
> # Disable NCQ on all disks.
> echo "Disabling NCQ on all disks..."
> for i in $DISKS
> do
>  echo "Disabling NCQ on $i"
>  echo 1 > /sys/block/"$i"/device/queue_depth
> done
>
> # Fix slice_idle.
> # See http://www.nextre.it/oracledocs/ioscheduler_03.html
> echo "Fixing slice_idle to 0..."
> for i in $DISKS
> do
>  echo "Changing slice_idle to 0 on $i"
>  echo 0 > /sys/block/"$i"/queue/iosched/slice_idle
> done

Thanks for putting this data together.

Regards,
Dan
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