Only 80MB/s Disk Speed on 3 different hard drives (MacPro 8-core)

As a data point, writing from a 1TB SSD internal to an internal Seagate Barracuda XT ST33000651AS 3TB 7200 RPM's first partition, the write speed is roughly 140 MB/sec using Disk Utility's Disk Activity display. Same source, same file, same Seagate, but this time to the last partition, write speed is about 80 MB/s. The first write speed would be lower if the source was on a HD rather than an SSD. Takeaway is that where on the HD the data is being written to makes a big difference.

The thing with HD's is that since the rotational speed is usually consistent and there are more sectors passing the read/write head per rotation at the outside than the inside of the disk, where the data is both read from and written to can affect transfer speed. I'd assume that Blackmagic would write to empty sectors rather than overwrite your data so where it's going matters.

You'd mentioned partitions. In Disk Utility, the volume at the top of each Media group is the outside partition and the bottom is at the inside. Does the transfer rate change if you use the source and destination on the top partition of each list?

I mentioned the SSD because that's my boot and source disk and it's both simply faster reading and has no data position issues to worry about. And the file I was using was a Virtual Machine of some 6GB so the transfer lasted long enough to give a reliable reading.

philomat wrote:

...Copying from e.g. "Workstation" to "BACKUPS" I only have about 50MB/s read and write. Even slower than copying from one HD to another (e.g. "Workstation" to "Dontuse").

Copying between partitions on the same disk is bound to be slower because the reading and writing can't be done simultaneously. With the single partition drives, how full they are will impact where the data can be written to since if nearly all the space is used up, the space that's left will usually be at the center. Another thing to consider is what else is going on during the transfer. Do you have antivirus software that's scanning the file as it's moved? Is Time Machine backing up hourly automatically in the background? Is Spotlight indexing? Indeed, since you have a Mac Pro, are you getting ECC errors due to failing DIMMs? (You can spot that using the Memory section of System Information [About this Mac, more info]; if the status isn't OK, it'll say so.)

philomat wrote:

...Spotlight indexing: all boxes are checked. Might that be a reason?

I appreciate your time and help so far, thanks! 🙂

Message was edited by: philomat

Glad to help. I don't know much about Spotlight since I have it turned on only enough to support the App Store (I use EasyFInd to actually find things), but I think if there's a dot in the middle of the magnifying glass on the menu bar, it's indexing. You'll also see mds and mdworker processes in Activity Monitor.

how free and fragmented space,

whether the disk head does a lot of seeks or not

somet imes the ideal is to just stripe 4x2TB drives, sometimes not.

An empty drive partition is the only way to get a single drive to 140MB/sec

Want some SSDs on PCIe? or some 4TB Seagates? or WD 10K VRs now that those higher end drives can get 180MB... .on an empty drive using outer tracks.

you could have some remapped out bad sectors

The inner tracks tend to be about 70MB/sec on today's drives.

Seek latency and head arm movement and finding file segments or next file play a large role.

So I run 4 hard drives, only one performs at 120-130MB/s.

How can that be?

Starting with the specs for the first drive listed, the manufacturer claims a MAXIMUM transfer speed OD (Off the Disc platters -- that would be After the correct data are already under the read heads) of 125MBytes/sec.

Table 1: Drive specifications summary for 1000 and 750 Gbyte models

The problem appear to me to be:

Your expectations are too high.

The numbers you have attained are perfectly reasonable. The drive takes time to seek to the next track and for the platter to spin around. 10-20msec of dead time while seeking and spinning is common, and this will detract from Maximum performance.

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