Real-world speed differences between Fusion drive and SSD on latest iMac 27"

[marzer]

I wanted a discussion mainly, and have this thread be helpful to users who are potentially choosing between the two options. I genuinely wanted to focus on real world performance because too often the discussion of SSD vs FD quickly becomes "FD is outdated technology, transitional, unreliable, loud, spinning." All of these are valid points of course, but then no one talks about real-world performance which is what a lot of people (including myself) care about.

My thoughts on the subject from an earlier thread...

https://forums.macrumors.com/threads/ssd-vs-fusion-drive-imac.1846485/page-2#post-20740051

 

[AppleHater]

A good reference for this matter:
http://www.anandtech.com/show/6679/a-month-with-apples-fusion-drive

 

[makrom]

While the FD is pretty fast it has limited I/O bandwidth relative to SSD. In addition to the OS and apps if the media is on there plus the hidden work/render files from an app like FCP X, it can saturate the capability.

For lower end things you can put media on the FD and it is OK. There is no clear, fixed limit on this because all apps and workflows are different. In general using iMovie to edit short cell phone videos is OK, but using FCP X to edit 4k video is not OK. In between there is some threshold where it becomes increasingly difficult. What that is varies with different situations.

The problem with internal SSD is even though it has the bandwidth it's not big enough to put much media on it -- depending on what that media is. It can hold a lot of .jpg photos. It cannot hold a lot of 4k video and larger numbers of 40 megapixel raw stills from new cameras will exhaust it faster than you'd expect.

My wife has a 2012 iMac 27 with 3TB FD and all her media is on it -- but that is only .jpg cell phone photos and short videos, plus a few iTunes movies and things downloaded from our Tivo. It works fine and there are no performance problems and she'll have plenty of space for years.

But isn't the internal vs. external issue mostly about the speed of the involved drives? Or does FCPX really cause that many concurrent drive accesses (media and application data)?
My active raw photo library is like 800GB, so I'm pretty sure it exhausted the SSD part of the FD by a long shot. But the only time I truly notice a real speed disadvantage is when I copy a huge amount of data to my external SSD.
The fact that those files are on the FD when I work with them doesn't seem to have a significant impact. Programs like LR or PS hardly access the drive once they are started except for interacting with media files, but since these accesses aren't concurrent, it shouldn't matter much whether these files are on the ~200 MB/s FD HDD compared to an external ~200 MB/s HDD. Reading a 25MB RAW should take like 125ms + 10ms access time (as long as it lies consecutively on the HDD). Obviously there's more to open an image than just reading it from the drive, but beyond that, speed should only be affected by CPU and RAM.

 

[joema2]

But isn't the internal vs. external issue mostly about the speed of the involved drives? Or does FCPX really cause that many concurrent drive accesses (media and application data)?...Programs like LR or PS hardly access the drive once they are started except for interacting with media files, but since these accesses aren't concurrent, it shouldn't matter much whether these files are on the ~200 MB/s FD HDD compared to an external ~200 MB/s HDD....there's more to open an image than just reading it from the drive, but beyond that, speed should only be affected by CPU and RAM.

Yes you are correct in all these and I did not mean to imply otherwise. Video editing can be a little different since it's a rapid stream of stills. In fact 4k is 30 eight-megapixel stills per second. But even in this case, due to H264 compression the I/O rate is not as rapid as you'd think, but the CPU load is very high.

Even when Lightroom is doing bulk import and preview generation or bulk export, the I/O load is not that high -- it is mostly CPU. Anyone can examine this themselves by using Activity Monitor or iStat Menus.

As you said, since there is so much non-I/O work in image processing, even if the I/O was infinitely fast it would not make a huge difference. Usually I have video media on a Thunderbolt RAID but I've done many FCP X video tests with the media on my 2015 iMac 27 1TB SSD and compared that to media on my 2013 iMac 27 with 3TB SSD. Despite the SSD having 2,000 MB/sec reading rate, it not significantly faster due to the I/O. It is faster because the new iMac has a faster CPU and GPU. That is obvious because the I/O rate doesn't reach 10% of the SSD capability.

I/O can be very important in video -- when dealing with lower compression codecs like ProRes. When dealing with H264 material is it not unimportant, just not the dominating issue. Depending on the software there can be a lot of I/O happening in the background you don't see, such as FCP X writing render files.

You can get into an I/O-bound situation quickly when dealing with 4k because it's so CPU-intensive that transcoding to a lower-compression codec is often required. That increases I/O rate by 8x or 10x, but the size also goes up so it usually won't fit on an internal SSD.