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Fusion Volume Speed (OWC PCIe SSD + HDD)

Last updated 2012-10-31 - Send Feedback
Related: Storage, SSD

For all these tests, Apple’s Spotlight was disabled on the test volume.

After setting up the 480GB PCIe SSD + 2TB hard drive fusion volume, I tested the speed using DiskTester by creating 1000 files of 256MB each (256GB total), using the disktester create-files command.

Since the PCIe SSD is 480GB, these files should all fit onto the PCIe SSD, and should all see full SSD speed when written. Indeed, the performance is at 672 MB/sec = FAST.

It turns out that a special procedure is needed to actually make a Fusion volume, not just something that looks like a Fusion volume.

So what follows LOOKED exactly like a Fusion drive, but was not actually doing more than running in JBOD mode.

Tested on a Mac Pro 6-core 3.33 GHz, internal bays.

Write speed with a PCIe SSD + hard drive fusion volume

Clearly the Fusion technology is smart enough to send all the writes to the PCIe SSD when SSD space allows, and to keep the files there.

But is the “Fusion” file migration/optimization working? Or just JBOD hitting the SSD first? Keep reading.

Write and read speed of 256GB on 480GB SSD + 2TB Fusion volume
llcMule:~ lloyd$ disktester create-files -n 1000 -f 256M fused
DiskTester 2.2b1 64-bit, diglloydTools 2.2b1, 2012-02-10 14:40
Copyright 2006-2012 DIGLLOYD INC. All Rights Reserved
Use of this software requires a license. See
Mac OS X 10.8.2, 12 CPU cores, 24576MB memory
Wednesday, October 31, 2012 1:56:21 PM Pacific Daylight Time
disktester create-files -n 1000 -f 256M fused
Volume:               fused
Num files:            1000
File size:            256MB
Transfer size:        8192KB
Fill with:            "0x0000000000000000"
Verify:               true
Creating 1000 files of size 256MB on volume "fused"
Speed shown includes file system create/open/allocate/write-- real world time.
TARGET FOLDER: "/Volumes/fused/disktester-blobs"
Elapsed    File#        Qty   2sec   5sec  15sec   1min   2min   5min  15min  30min  60min 2s:15s  1m:5m ClockAvg
5s       13     3.30GB    674    675    675    675    675    675    675    675    675   1.00   1.00    676 
10s       26     6.57GB    674    671    674    674    674    674    674    674    674   1.00   1.00    672 
15s       39     9.85GB    672    671    672    672    672    672    672    672    672   1.00   1.00    671 
20s       52     13.1GB    683    676    671    673    673    673    673    673    673   1.02   1.00    672 
25s       65     16.4GB    654    670    672    672    672    672    672    672    672   0.97   1.00    672 
30s       79     19.8GB    687    686    678    674    674    674    674    674    674   1.01   1.00    674 
35s       92     23.1GB    678    676    677    674    674    674    674    674    674   1.00   1.00    674 
40s      105     26.4GB    674    670    677    674    674    674    674    674    674   1.00   1.00    674 
45s      118     29.6GB    661    671    673    674    674    674    674    674    674   0.98   1.00    674 
50s      131     32.9GB    679    676    673    674    674    674    674    674    674   1.01   1.00    673 
55s      144     36.2GB    677    674    672    674    674    674    674    674    674   1.01   1.00    674

Going beyond the 480GB capacity of the SSD

What happens when more than 480GB is written to the Fusion volume (480GB SSD + 2TB hard drive)?

When more stuff is stored than fits onto the SSD, the speed drops because the SSD has reached its capacity; Apple’s Fusion technology is not a caching solution.

At idle time, the Apple Fusion technology actively moves frequently accessed files to the SSD, and migrates infrequently accessed files back to the hard drive. Even portions of files (another consideration for recovery from a drive failure = a mess, be sure to have backups!).

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Adding more files

After writing the first 256GB of files, DiskTester was used again to write an additional 256GB of files (total of 512GB). Since a 480GB SSD is being used, the 512GB overflows its capacity, and one can then hear the hard drive kick into use as DiskTester creates those files.

Once the SSD fills up under the unrelenting disk I/O of disktester create-files, the system is forced to write to the hard drive. Without idle time to migrate files, the reads are impacted the same way (create-files writes all the files, then reads them back). The graph below captures this behavior.

Write and read speed of 256GB 2nd batch (512GB total) on 480GB SSD + 2TB Fusion volume

Adaptation to activity

After 256GB + 256GB were written to the Fusion drive, another 25.6GB were written in addition. Since the system had no time to adapt as yet, these additional 25.6GB of files ran at hard drive speed both when written and when read back.

Following this, determining whether the system adapts to frequently used files was simulated as follows:

  1. disktester read-files was used to read the 25.6GB of files, three times.
  2. The system was given some idle time to migrate files as it saw fit.
  3. I went for a nice bike ride, leaving the machine idle for ~2 hours (Energy Saver was set to not sleep for 3 hours).
  4. disktester read-files was used to read the 25.6GB of files.

And... no migration. Access to the 25.6GB of files is slow as mud, meaning hard drive speed. No migration was apparently done for that three hours. It’s just running at hard drives speeds.

disktester read-files --iterations 10 /Volumes/fused/test-folder

Reading that 25.6GB of data ten (10) times and giving the system idle time to migrate— no deal— no I/O (according to iostat) and no speedup reading the files— hard drive speeds only. So either I’m doing something wrong here, or the Fusion implementation code has some restrictions that make it inoperable in this scenario (e.g. sees that the Accelsior PCIe SSD is an “external drive”).

Also tested with a 200GB SATA SSD (internal to Mac Pro), the Fusion migration of files DOES NOT WORK (no files migrate to the SSD). Repeated (10 times) reads of 18GB of files shows that once the SSD overflows to the hard drive, the files stay there and performance stays at HDD speed (basic JBOD behavior).

The algorithm seems to be:

Keep 4GB SSD space free, push down items to the hard drive to maintain that 4GB.

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