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I have a late 2013 27” iMac with the max specs possible at the time (i7, 32GB RAM, 3GB Fusion drive (with 128GB SSD blade)). I don’t really know when it happened but I feel it has become slower. I don’t think it’s because of the CPU nor the RAM (both are never at 100%). I ran some benchmarks on the fusion drive and here are the results: Blackmagic disk speed test shows between 120 to 200MB/s write and 450 MB/s read speedI checked with iostat and most of read / write operations use the SSD part of the fusion drive first, so nothing seems wrong here I would need help with the following questions: What speed should I normally get with a fusion drive with these specs (didn’t bench the machine when it was brand new so I don’t really know what I should expect from the fusion drive) ?Is there some other benchs / tests I can do to point out a drive problem ? What would make more sense if I were to upgrade the drive: Replace the 3TB HDD by a SSD and still use fusion drive with 128GB SSD blade and 2.5” SSDRemove completely the 3TB HDD and replace the SSD blade with a larger pcie nvme SSD ?Or is there anything else that would make more sense ? Just let me precise that I won’t have trouble disassembling everything to replace the SSD blade and that I am well aware of the speed limits (at least for sequential read/write) : 500MB/s for a 2.5” SSD, and most likely 700MB/s for the SSD blade (the pcie is only 2.0 X2 which will limit the speed even if I put a samsung 980 pro which is able of 2000 MB/s read / write). The thing which is out of my knowledge is actually the fusion drive part … Thanks in advance for your help. Nicolas.
Well your hitting a common problem with HDD’s as over time they do become sluggish as the you use them the files become fragmented so they take more time to read and write. Normally I would recommend you use a defragmentation tool to defrag the drive, but we can’t here! You have a double whammy which makes this much harder, You have a Fusion Drive which gets in the way and if you are using a newer OS Mojave or newer your HDD file system was upgraded to APFS from its original file system HFS+. APFS does have a defrag feature hidden but it really is useless! To add to it APFS copy on write makes things worse for HDD’s. So what to do?? But! do I really need to defrag my drives all I read tells me I don’t need to? Sadly, Apple has mesmerized people into thinking it doesn’t sadly the built-in function is not very good in either HFS+ or APFS. Heres a bit more Should you enable defragmentation on APFS ‘‘‘‘‘‘hard drives? So far none of the major drive maintenance apps have any way to defrag HDD’s or Fusion Drive sets. Apple still hasn’t released any documentation to the app makers, I think I understand why, if you look through the four versions of APFS you’ll note the structures are not the same! So Apple is still tweaking it! Time to buy a nice big HDD to setup as a TimeMachine backup. Make a fresh backup, You’ll want a to setup a USB thumb drive as a bootable OS installer with the OS you are using. If you are using the ver latests we can jump over this step. Now the nail bitting point reboot your system either with the USB drive or in recovery mode to get to disk utility and gulp! Reformat your drive and re-install your OS. Then at the end of the re-install you’ll be asked if you have data to recover connect your external drive and restore your stuff. So what to do moving forward: First I’m not a fan of Fusion Drives at one point I understood the need as SDD’s where very expensive in larger sizes. today SSD drive cost have dropped. But sadly Apple’s custom PCIe/NVMe blade SSD’s as still hard to get and as they are in such demand the price has not dropped as well. Apple has dropped the price for new systems with SSD’s (still not as cheap as other systems with the same sized SSD). So you’ll need to think if replacing your SATA drive to a SSD makes sense if you need so much space as your current 3 TB HDD as a 4 TB drive is still not cheap! As SATA drives are still limited to SATA III 6 Gb/s I/O speed the function of your system starts to come into play. A person who has lots of stuff like music and photos and what not will be fine with a large HDD (what not a SSD!) Just setup differently! Here we are going to break the Fusion Drive back to its discreet drives. Using the blade SSD as the boot drive and the HDD as your data drive. But we do have a problem still as the blade SSD you have is just too small for an effective boot drive. Here we really want either a 256 or 512 GB blade SSD. I personally think this is by far the better direction! We will also need to alter the HDD file system back to HFS+ which is the better one for SATA drives.
