October 04, 2006

MacPro disk access performance


I was curious about the hard disk access performance on MacPro. I tested this on a dual core 3GHz machine with 4GB RAM. The machine has the 160GB hard disk that shipped with the computer, and two additional 750GB Seagate perpendicular drives. The Seagate drives are configured as RAID 1 (mirroring) using MacOS X's built-in software RAID. I also used a CoolGear external RAID enclosure (hardware RAID), using two 400GB drives configured as RAID 1, and connected via USB2 to the MacPro. The external enclosure also uses eSATA, but the computer doesn't have an internal PCI SATA extension card to use it with.

I ran a very basic test, writing and reading a 1GB file. The file is written by reading from /dev/zero. The reading is by doing a cat to /dev/null. The commands look like this:

$ time dd if=/dev/zero of=<file> bs=1024 count=1048576

$ time cat <file> >/dev/null

Here are the numbers:

DriveRead time (seconds)Write time (seconds)
internal 160GB14.9014.64
internal, software RAID, 750GB18.0521.94
external, hardware RAID, USB2, 400GB49.4357.30

I guess there's no surprise how slow the USB2 drive is compared to the internal drive, although an almost 400% increase in read and write times are fairly extreme. The biggest surprise was the 50% performance drop of the software RAID compared to the raw disk access. Albeit the comparison is not quite fair as the drives are not identical. I wouldn't be surprised however to see the same number for a similarly configured software RAID.

Perhaps the situation would improve with either Firewire 800 or eSATA-connected drives, but I doubt it would change the number significantly.

My take on this is that external RAID systems should only be used for backups, not regular access. If possible, don't use software RAID either, instead do regular backups on an external storage device.

Posted by ovidiu at October 04, 2006 10:18 PM |

Do you have more storage on/under your desk there than whole NASA had thirty years ago?

Posted by: kit on October 4, 2006 11:14 PM

Something must be wrong, possibly the connection and possibly the software RAID driver. RAID 1 should be faster if you split the reads (http://en.wikipedia.org/wiki/RAID#RAID_1)... sometimes called RAID 0/1 or 0+1.

Posted by: Andy on October 5, 2006 04:27 AM

There might be some command line tools to allow tuning the performance of the internal software RAID. I'm not aware of them. The only thing that can be configured in DiskUtility is the sector size, which is set to 32KB for the software RAID.

I doubt there's something wrong with the connection, it's built in the MacPro.

Posted by: Ovidiu on October 5, 2006 08:11 AM
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