On Thu, May 5, 2011 at 9:06 AM, Nikolay Kichukov [off-list ref] wrote:

-----BE
It seems like he is reading directly from the raid device and not through the filesystem. So there are no filesystem
caches in this way.

phew!  ... i think (see below)


I installed ubuntu 11.04 on the new system last night.
this morning i went to reconnect the old system drive (id disconnected
it for safety)  and 'pop' a small piece of metal must have touched the
back of the loose drive and fried the board! ... oh joy.

--luckily its an old drive and i have an identical spare so im hoping
i can swap the board and save all my work since last git commit.

anyway.. this look different in 11.04. mdadm 3.1.4

/dev/md0:
 Timing buffered disk reads: 594 MB in  3.00 seconds = 197.79 MB/sec
zoizoi@shankara:~$ sudo dd if=/dev/md0 of=/dev/null bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 5.84755 s, 179 MB/s
zoizoi@shankara:~$ sudo dd if=/dev/md0 of=/dev/null bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 0.152353 s, 6.9 GB/s

even though im accessing the block device directly it does look like
im getting buffering in natty! I don't think i was in 10.10 and
certainly wasnt getting 7GB/s

raid10 f2 performance is right down vs what i got last night (i got
470MB/s first try after creating the array so dont think there was
buffering.

my md1 raid5 was also slow again. readahead on both defaulted down to
256 on 11.04

I applied Neil's x8 fix to both md0 and md1 and now the dd test look
much better.

sudo dd if=/dev/md0 of=/dev/null bs=1M count=4000
4000+0 records in
4000+0 records out
4194304000 bytes (4.2 GB) copied, 7.62018 s, 550 MB/s

a little too good!
i could see in the system monitor that i didnt have the large 4G
buffer (i do post this test). Something i did must have reset the
buffer. I could see a small amount of buffer in system monitor so
maybe it was 1GB. I appreciate these are not the best test but not
that hdparm is much worse. Once i set the 2048 sector readahead i gets
totally unrealistic.

zoizoi@shankara:~$ sudo blockdev --setra 2048 /dev/md0
zoizoi@shankara:~$ sudo hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads: 5294 MB in  3.00 seconds = 1762.34 MB/sec
zoizoi@shankara:~$ sudo blockdev --setra 256 /dev/md0
zoizoi@shankara:~$ sudo hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads: 582 MB in  3.00 seconds = 193.78 MB/sec

anyway, it looks like i'm getting good read speed now with 2048
lookahead, ill do another dd test on reboot.


thanks to you all for the helpful responses,

Liam