(Kernel: 2.6.18, x86_64)
Is it normal for an MD RAID1 partition with 1 active disk to perform
differently from a non-RAID partition?
md0 : active raid1 sda2[0]
8193024 blocks [2/1] [U_]
I'm building a search engine database onto this partition. All of the
source data is cached into memory already (i.e., only writes should be
hitting the disk).
If I mount the partition as /dev/md0, building the database consistently
takes 18 minutes.
If I stop /dev/md0 and mount the partition as /dev/sda2, building the
database consistently takes 31 minutes.
Why the difference?
The "fast" time seen when the partition is mounted as /dev/md0 actually
creates a serious problem: the kernel apparently flushes dirty pages so
aggressively that other processes attempting to write to the same
partition during the database build become blocked for several
minutes(!) at a time.
When mounted as /dev/sda2, that doesn't happen: other processes writing
to the same partition are blocked for no more than a few seconds at a time.
I don't know if it's relevant, but the results from iostat when writing
large chunks of data to RAID1 partitions seem somewhat curious, as if MD
is telling the I/O layer "all done!" before it's actually finished
writing the data out to the member disks. Note the unrealistically high
kB_wrtn/s numbers on md0 in the following test. (And why does it show
50000 tps?)
# iostat -dk 1 md0 sda
# fgrep MemTotal /proc/meminfo
MemTotal: 2059784 kB
# cat /proc/sys/vm/dirty_ratio
40
# cat /proc/sys/vm/dirty_background_ratio
10
# dd if=/dev/zero of=/testpart/bigfile bs=1M count=400
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 91.00 0.00 46592.00 0 46592
md0 48692.00 0.00 194768.00 0 194768
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 99.00 0.00 50176.00 0 50176
md0 0.00 0.00 0.00 0 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 96.00 0.00 49152.00 0 49152
md0 0.00 0.00 0.00 0 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 86.00 0.00 44032.00 0 44032
md0 0.00 0.00 0.00 0 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 96.00 0.00 48160.00 0 48160
md0 51636.00 0.00 206544.00 0 206544
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 95.05 0.00 48665.35 0 49152
md0 0.00 0.00 0.00 0 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 92.00 0.00 46596.00 0 46596
md0 0.00 0.00 0.00 0 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 95.96 0.00 48614.14 0 48128
md0 0.00 0.00 0.00 0 0
...
--
Jordan Russell
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