HP DL360 G5 RAID5 3 x 500GB SATA Hitachi under load: (stock controller P400i 512MB Cache + battery)

hdparm -tT /dev/cciss/c0d0
Timing cached reads: 12700 MB in 2.00 seconds = 6357.24 MB/sec
Timing buffered disk reads: 288 MB in 3.03 seconds = 94.98 MB/sec

#dd of=/tmpfile if=/dev/zero bs=1024 count=100000
102400000 bytes (102 MB) copied, 0.44822 seconds, 228 MB/s

#dd of=/tmpfile if=/dev/zero bs=1024 count=1000000
1024000000 bytes (1.0 GB) copied, 6.70012 seconds, 153 MB/s

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host ESX 4.1 / guest: centos5.5 over NFSv3 from SUN 7310 10 x 1TB SATA RAID5
(SUN 7310 1x2Ghz 16GB Mem, J4400)
under heavy load

hdparm -tT /dev/sda5 (system disk)

/dev/sda5:
Timing cached reads: 27616 MB in 2.00 seconds = 13819.79 MB/sec
Timing buffered disk reads: 554 MB in 3.02 seconds = 183.47 MB/sec

#dd of=/tmpfile if=/dev/zero bs=1024 count=100000
102400000 bytes (102 MB) copied, 0.285821 seconds, 358 MB/s

#dd of=/tmpfile if=/dev/zero bs=1024 count=1000000
1024000000 bytes (1.0 GB) copied, 28.8918 seconds, 35.4 MB/s

We are talking heavily used NAS! Expensive one

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Same ESX guest centos 5.5 local datastore 2 x sas raid1 (SUN FIRE X4170 stock controller)

hdparm -tT
/dev/sda5:
Timing cached reads: 28268 MB in 2.00 seconds = 14145.97 MB/sec
Timing buffered disk reads: 564 MB in 3.00 seconds = 187.90 MB/sec

#dd of=/tmpfile if=/dev/zero bs=1024 count=100000
102400000 bytes (102 MB) copied, 0.234233 seconds, 437 MB/s

#dd of=/tmpfile if=/dev/zero bs=1024 count=1000000
1024000000 bytes (1.0 GB) copied, 4.92702 seconds, 208 MB/s

Just proves you want to use local storage
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For comparison slightly busy HP Laptop 2.5in drive:

hdparm -tT /dev/sda

Timing cached reads: 1306 MB in 2.00 seconds = 653.21 MB/sec
Timing buffered disk reads: 88 MB in 3.03 seconds = 29.06 MB/sec

#dd of=/tmpfile if=/dev/zero bs=1024 count=100000
102400000 bytes (102 MB) copied, 2.6486 s, 38.7 MB/s

#dd of=/tmpfile if=/dev/zero bs=1024 count=1000000
1024000000 bytes (1.0 GB) copied, 44.0648 s, 23.2 MB/s
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SSD Test: Corsair F120GB3 (dell vostro 410)

#dd of=/sdc/tmpfile if=/dev/zero bs=1024 count=100000
102400000 bytes (102 MB) copied, 0.371003 s, 276 MB/s

# dd of=/sdc/tmpfile if=/dev/zero bs=1024 count=1000000
1024000000 bytes (1.0 GB) copied, 4.21027 s, 243 MB/s

# hdparm -tT /dev/sdc

/dev/sdc:
Timing cached reads: 12212 MB in 2.00 seconds = 6113.14 MB/sec
Timing buffered disk reads: 502 MB in 3.00 seconds = 167.11 MB/sec
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New HP DL380 G7 4 x 7k6 SAS RAID50 on ESX Guest Centos 6

/dev/sda:
Timing cached reads: 13052 MB in 2.09 seconds = 6234.07 MB/sec
Timing buffered disk reads: 214 MB in 3.00 seconds = 71.33 MB/sec

1024000000 bytes (1.0 GB) copied, 19.3192 seconds, 53.0 MB/s
102400000 bytes (102 MB) copied, 0.335534 seconds, 305 MB/s
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HP DL380 G7 ESX4.1 + SUN/Oracle 7310 24x HDD SATA, Guest Centos 6

1024000000 bytes (1.0 GB) copied, 67.3101 s, 15.2 MB/s
102400000 bytes (102 MB) copied, 0.994895 s, 103 MB/s

Timing cached reads: 18232 MB in 2.00 seconds = 9129.09 MB/sec
Timing buffered disk reads: 276 MB in 3.00 seconds = 91.98 MB/sec