Raid 5 Write Speed Slowness

cyberblitz · 06-16-2013, 06:56 PM

I have problems with severe write speed slowness after setting up a Raid 5 using mdadm.

First, list of hardware:

I have:
- 4 x Barracuda 3TB drives (all connected to below)
- Syba SATA II PCI-X 4 Port Host Raid Controller (not using it as a raid controller)
- this card is suppose to provide 3GB/s transfer rate.
- only required this card to support an additional 4 drives
- 8GB DDR Ram
- i5 Intel Processor

Analysis i have done:

Using hdparm showed on each drive:

- Before Raid set up:

Code:

 Timing cached reads:   12142 MB in  2.00 seconds = 6074.54 MB/sec
 Timing buffered disk reads: 320 MB in  3.01 seconds = 106.23 MB/sec

- After Raid Set up:

(now here's the problem, immediately after Raid had synced or the Server had restarted the readings are similar to above, but after 20mins or so being idle, hdparm shows

Code:

 Timing cached reads:   12142 MB in  2.00 seconds = 6074.54 MB/sec
 Timing buffered disk reads: 4 MB in  4 seconds = 1.2 MB/sec

dd has similar results:

Code:

sudo dd if=/dev/zero of=/mnt/Media/test.out bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 10.1847 s, 103.2 MB/s

then after a few minutes of idle,

Code:

/mnt/Media$ sudo dd if=/dev/zero of=/mnt/Media/test.out bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 185.886 s, 5.8 MB/s

What have i done to rectify this thus far:

- I have changed the block size of the array several times.
- I have changed the stripe cache size to different sizes (with marginal improvements)
- I have followed pretty good instructions from this link: http://dennisfleurbaaij.blogspot.com...rray-with.html
- using parted to format the drive
- specifying explicit`s of the array (using meta 1.0)
- specifying the File System parameters (no super user space/stripe sizes etc)
again with marginal improvements..

I am really pulling my hair out in this one. I have a 2nd Raid 5 array with no problems at all. But this is directly attached to the board. I wondering if is the card causing buffering issues. Can anyone assist????? Please!!!

lleb · 06-17-2013, 07:32 PM

FRAID is always slow no matter how strong your system is. get rid of the FRAID (your software RAID) and put in on a proper hardware solution with true RAID capabilities and your performance will improve.

cyberblitz · 06-23-2013, 06:34 PM

Unfortunately, I receive the same speed without RAID. I experimented with the drives in isolation and i get the same results. I attached two of the drives directly to the board and two to the 4xPort card. The ones I attached to the board are where you'd expect the speeds to be, but the ones on the card are around 1.5mb/s (rubbish).. I conclude then it is the bloody card. I don't know why. I will investigate further. I will post on here if i find anything.