FIXED: iMac Poor disk I/O

[Maverick]

I have a late 2013 27" iMac running MacOS High Sierra it has a 3TB Fusion Drive and 32GB RAM. The 3TB drive was partitioned into 2 x 1.5TB drives when I bought the machine in 2013 and has worked fine up to now.
A couple of weeks ago, before I upgraded to High Sierra, it slowed down dramatically. Disk intensive apps like Lightroom became almost unusable. Life has been a bit hectic so I'm only getting around to trying to sort it now. Performance monitor shows disk I/O performance to be dire - <5 Mb/sec read and write. I ran disk repair from the recovery boot and it tells me the disks are ok. On checking the disk configuration from the command line version of diskutil I get the output below. /dev/disk1 and /dev/disk4 can be ignored - they are external drives, I've booted the iMac with them disconnected and it makes no difference. disk0 is the SSD element of the fusion drive and disk2 the HDD part.
What does appear odd is the duplication of the Mackintosh HD partition - the first of the 1.5TB partitions on the fusion drive. It appears as physical drive disk2s2 and virtual drive disk3. The second partition appears as disk2s4 only and therefore decoupled from the fusion element.
I don't know enough to work out whether the data on the logical volumes/family is correct or not.

I think I'm looking at 2 scenarios (1) there is file corruption somewhere on the fusion drive that is slowing it all down (2) there is a hardware fault with either the SSD or the HDD. I had a couple of things to try - delete the second partition and resize the drive back to one 3TB drive which would hopefully mean I wouldn't have to do a clean install. Data on the second drive could be backed up and restored (mainly media files). Or I nuke the entire drive and do a clean install. All advice and explanations welcome.

[Maverick]

There is 1.1TB free on Mackintosh HD - it has programme files only on it. Other drive has >800Gb free. I'm guessing I need to try my suggestions    A while ago I repartioned the drive on my Air that had been split back into one with no problems. My worry is whith I/O being so dire it will take a huge amount of time to do the repartitioning.

[Maverick]

So, that didn't go well. Apple corestorage is the spawn of satan! Only after reading manuals on core storage, large manuals on the diskutil command line app and days trawling the internet was I able to find the one magic incantation* that defeated the big boss and allowed me to merge the two partitions and recreate a working fusion drive. Then it was a clean install of High Sierra and hours of fun with migration assistant. Now it won't let me re-install Adobe Creative Cloud  (add to to-do list)

However after all that, there are still problems with disk speed. Can the panel shed any light on these symptoms: small files copy ok on their own, directories with just a few files in them open ok, both locally and on the network. Large file transfers start off ok for the first couple of hundred MB and then progressively get slower. So it will copy around 1GB ok 20GB takes over an hour! This on a virtually empty 3TB disk with nothing else running.

I am prepared to upgrade the fusion drive to a 1TB SSD if this will solve the problem but as it will have to go to a repairer for this I am concerned the problem lies elsewhere so need convincing that the disk(s) are the problem. Any and all suggestions welcome.

* For those who may be interested this is the magic incantation

Code: [Select]

N=Macintosh\ HD; for d in /dev/disk?; do o=`diskutil info $d`; [[ ! "$SSD" ]] && grep -lqw 'APPLE SSD' <<< "$o" && SSD=$d; [[ ! "$HDD" ]] && grep -lqw 'APPLE HDD' <<< "$o" && HDD=$d; [[ "$SSD" && "$HDD" ]] && break; done; diskutil cs create "$N" $SSD $HDD && diskutil cs createLV "$N" jhfs+ "$N" 100%

[Maverick]

UPDATE
Eventually I got a program that reported all the S.M.A.R.T data from the disk. This showed a problem with 'end-to-end' errors. I understand this to mean that the buffer RAM has gone bad and this would result in the data being requested multiple times which fits perfectly with the symptoms - new disk time.

Do the panel know whether it is possible to build a new fusion drive with 'off the shelf items' i.e. non Apple branded products. I'm looking at several possibilities - a new fusion drive built with existing SSD + fast larger HDD (e.g. 6TB Seagate Barracuda Pro), replacing both elements of the fusion drive - HDD as above but with larger fast SSD (e.g. 256GB WD Blue) , ditching the fusion drive for either a single 2 TB SSD (WD Blue) or 500GB SSD + HDD run as seperate drives. Any advice on options gratefully received.

[ian]

I think Fusion is an Apple-ish thing so anything that doesn't fall out of a space grey Cupertino mothership is probably going to make someone cry.

Make life simple, my iMac has a 256GB SSD (I've never filled more 60GB, so you could do smaller) and an external USB for storage. I'd just have an straight SSD fitted and keep the spinning stuff external. No partitions or any of that crap.

[Maverick]

Thanks for the input ian.

I think Fusion is an Apple-ish thing so anything that doesn't fall out of a space grey Cupertino mothership is probably going to make someone cry.

The ssd part of the fusion drive has to be Apple proprietary one purchased from the 'mothership', the HDD is generic apparently.

The existing drives had 385GB used on one partition and 887GB used on the other. I also have 6TB of attached storage that has 2TB free (mostly backups of the internal drives). This is mainly films, photos and music - I have a large collection of music and films that are streamed around the house. Small internal SSD is not an option in this case. I visited my local (15 miles away) Mac repair place today to discuss what the options were.

• Replacing both elements requires the purchase of the SSD from Apple at Apple prices, not cost effective.
• I think that although probably the best option a large (>1TB) SSD is still pretty silly money.
• I'm going for replacing the HDD with fast 6TB drive and rebuilding a fusion drive. I'll split the HDD into 2 x 3TB - one as a fusion drive with the SSD and the other as data storage. The repair place will just do the hardware install and check it spins - I'll do the partitioning and build the fusion drive myself. I might as well use the hard won knowledge I've gained in the last week.

[Maverick]

Final post to round off this issue for anyone else who comes across it.
I decided, as stated in the last post above, to just go with a replacement HDD. I purchased a 6TB Seagate Barracuda Pro online for a reasonable price (circa £50/TB) and took it to my nearest Apple repairer who, after some discussion agreed to just do the disk replacement and nothing else. They were not convinced that the grey haired pensioner in their shop was capable of finishing the job off  Nothing like being stereotyped! Computer and disc left. I received a phone call 2 days later asking if I was sure I only wanted them to put the disk in <sigh>. Having confirmed this was the case, a couple of hours later I get a call to say it was ready. I get down to the shop to go through the quizzing once more - "you'll get the no entry symbol on the screen" (this being Scotland, spoken in the best 'Frazer from Dad's Army' accent). Having assured them that I was happy with that and paid the bill, they let me leave with it. Thanks Disk Depot, Dundee - the job was well done and speedy (if a little on the expensive side).
There is something quite gratifying when your backup works perfectly, the system was restored in a couple of hours and then an overnight restore of the 2.5TB of data and all was well.
The only issue was that the original factory installed drive (Seagate Barracuda) has a temp sensor, the off the shelf one doesn't. This meant that the fan ran flat out after the drive had been replaced. This was a known issue and the download of a fan control app sorted that out.
Now my 2nd Dell 27" monitor is refusing to turn on, bloody technology!

[Gattopardo]

The temp sensor is just stuck on the old drive, so wasn't transferred over.