[CentOS] Filesystem writes unexpectedly slow (CentOS 6.4)

I have a rather large box (2x8-core Xeon, 96GB RAM) where I have a couple of 
disk arrays connected on an Areca controller. I just added a new external array, 
8 3TB drives in RAID5, and the testing I'm doing right now is on this array, but 
this seems to be a problem on this machine in general, on all file systems 
(even, possibly, NFS, but I'm not sure about that one yet).

So, if I use iozone -a to test write speeds on the raw device, I get results in 
the 500-800MB/sec range, depending on write sizes, which is about what I'd expect.

However, when I have an ext4 filesystem on this device, mounted with noatime and 
data=writeback, (the filesystem is completely empty) and I test with dd, the 
results are less encouraging:

dd bs=1M if=/dev/zero of=/Volumes/data_10-2/test.bin count=40000
40000+0 records in
40000+0 records out
41943040000 bytes (42 GB) copied, 292.288 s, 143 MB/s

Now, I'm not expecting to get the raw device speeds, but this seems at least to 
be 2-3 times slower than what I'd expect.

Using conv=fsync oflag=direct makes it utterly pathetic:

dd bs=1M if=/dev/zero of=/Volumes/data_10-2/test.bin oflag=direct conv=fsync 
count=5000
5000+0 records in
5000+0 records out
5242880000 bytes (5.2 GB) copied, 178.791 s, 29.3 MB/s

Now, I'm sure there can be many reasons for this, but I wonder where I should 
start looking to debug this.

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Joakim Ziegler  -  Supervisor de postproducción  -  Terminal
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