Mdadm

Software RAID

Create md device

Create a raid5 device with 256 chunk size on 4 devices with out any hot spares.

# mdadm --create /dev/md0 --level=raid5 --chunk=256 --raid-devices=4 --spare-devices=0 /dev/sd[b-e]

Create /etc/mdadm.conf

# mdadm  --examine  --scan --config=mdadm.conf >> /etc/mdadm.conf</pre>
Modify as appropriate, ex;
<pre>
DEVICE partitions
CREATE owner=root group=disk mode=0660 auto=yes
HOMEHOST <system>
MAILADDR root

Replace dead device

# mdadm /dev/md0 -a /dev/sdc

Force a degraded array to start

If a drive fails, reboot happens, and we need to restart an array with 3 out of 4 drives running

# mdadm -Af /dev/md0 -Af /dev/md0 /dev/sda /dev/sdb /dev/sdd

Create filesystem

Ext3

Create an ext3 file system with 0% space reserved for root, a 4096 block size, and a raid stride of 16 ( 16 * 256 = 4096 | stride*chunk=block)

# mkfs.ext3 -m 0 -b 4096 -E stride=16 /dev/md0

Performance Testing

Bonnie++

# bonnie++ -d /mnt/raid5/tmp -u drew -f

Iozone

• -a auto
• -b output_excel file
• -i 0 run read test
• -i 1 run write test

# iozone -a -b werd.xls -i 0 -i 1 -C -E

Additional Tunables

Most of this were pulled from http://www.3ware.com/KB/article.aspx?id=11050

max_sectors_kb

echo "Setting max_sectors_kb to chunk size of RAID5 arrays..."
for i in sdb sdc sdd sde
do
   echo "Setting /dev/$i to 128K..."
   echo 128 > /sys/block/"$i"/queue/max_sectors_kb
done

Read-ahead on md0

• I hear this eats a lot of RAM

echo "Setting read-ahead to 64MB for /dev/md3"
blockdev --setra 65536 /dev/md0

stripe_cache_size

• + stripe_cache_size (raid4, raid5 and raid6)
• number of entries in the stripe cache. This is writable, but there are upper and lower limits (32768, 16). Default is 128.
• + stripe_cache_active (raid4, raid5 and raid6)
• number of active entries in the stripe cache

• + The stripe cache memory is locked down and not available for other uses.
• + The total size of the stripe cache is determined by this formula:
• +
• + PAGE_SIZE * raid_disks * stripe_cache_size = memory used

echo "Setting stripe_cache_size to 16MB for /dev/md3"
echo 16384 > /sys/block/md0/md/stripe_cache_size

Array resync speed

• Dramatically improves resync performance...

# Increase the minimum / maximum resync speed of the array..
echo "Setting minimum and maximum resync speed to 100MB/s..."
echo 100000 > /sys/block/md0/md/sync_speed_min
echo 100000 > /sys/block/md0/md/sync_speed_max

Disable NCQ

• Disabling native command queuing ... Benefits?

# Disable NCQ.
echo "Disabling NCQ..."
for i in sdc sdd sde sdf sdg sdh sdi sdj sdk sdl
do
   echo "Disabling NCQ on $i"
   echo 1 > /sys/block/"$i"/device/queue_depth
done