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

[David Lethe]

For faster random I/O:
 * Decrease chunk size
 * Migrate files that have higher random I/O to a RAID1 set, using disks
with the lowest access time/latency
 * If possible, use the /dev/shm file system 
 * Determine I/O size of apps that produce most of the random I/O, and
make sure that md+filesystem matches. If most random I/O is 32KB, then
don't waste bandwidth by making md read 256KB at a time, or making it
read 2x16KB I/Os. Also don't build md sets like 4-drive RAID5, (Do a
5-drive RAID5 set), because non-parity data isn't a multiple of 2. A
10-drive RAID5 set with heavy random I/O is also profoundly wrong
because you are just removing the opportunity to have all of those heads
processing random I/O. 
 * If you only have one partition on a md set, then partition it into a
few file systems. This may provide greater opportunity for caching I/Os.
 * Experiment with different file systems, and optimize accordingly.  
 * Turn of journaling, or at least move journals to RAID1 devices.
 * Add RAM and try to increase buffer cache in attempt to improve cache
hit percentage (this works up to a point)
 * Buy a small SSD and migrate files that get pounded with random I/O to
that device. (Make sure you don't get a flash SSD, but a DRAM based SSD
that satisfies random I/O in nanoseconds instead of millisecs). They are
expensive, but the appropriate device.  This is how companies such as
Google & Ebay manage to get things done. 
The biggest thing to remember about random I/O, is that they are
expensive, so just step back and think about ways to minimize the I/O
requests to disk in the first place, and/or to spread the I/O across
multiple raidsets that can work independently to satisfy your load.  All
suggestions above will not work for everybody. You must understand the
nature of the bottleneck. 

David

-----Original Message-----
From: linux-raid-owner@xxxxxxxxxxxxxxx
[mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of
thomas62186218@xxxxxxx
Sent: Monday, June 09, 2008 2:51 AM
To: dan.j.williams@xxxxxxxxx; jpiszcz@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-raid@xxxxxxxxxxxxxxx;
xfs@xxxxxxxxxxx; ap@xxxxxxxxxxxxx
Subject: Re: Linux MD RAID 5 Benchmarks Across (3 to 10) 300 Gigabyte
Veliciraptors

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-an
d-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-velicir
aptors/raptors.jpg
>
> Here are the bonnie++ results:
> 
http://home.comcast.net/~jpiszcz/20080607/raid5-benchmarks-3to10-velicir
aptors/veliciraptor-raid.html
>
> For those who want the results in text:
> 
http://home.comcast.net/~jpiszcz/20080607/raid5-benchmarks-3to10-velicir
aptors/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
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html





--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/