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Old Nov 23, 2012, 09:17 AM   #1
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Fusion Drive - Over Provisioning, Garbage Collection, TRIM

I am going to be adding a SSD to my base model 2011 Mac Mini using the OWC kit. I am trying to decide whether to set this up as a DIY Fusion drive or whether to just keep the SSD and HDD managed separately.

One of the factors that will come into play is how the Fusion system works in managing how full it will allow the SSD to become. An empty SSD performs at much higher levels than a half full SSD... and a full SSD can slow to a crawl. For reference look at the OCZ Vertex 4 where it will run in a "Performance Mode" as long as the drive is 50% full or less. Over 50% Vertex 4 performance is dropped. Also, when benchmarking my many SSD's I can never seem to achieve the same levels as the reviews which is due to the fact that I am benching a drive that is already in use and therefore not 100% empty like most reviews.

So, if the Fusion system manages the SSD such that it can fill it to capacity then you would expect much poorer performance than if the SSD was, say, half full. Of course the amount of degradation will depend on the aggressiveness of the garbage collection on the particular SSD model as well as whether TRIM has been enabled (for non Apple SSD's).

One of the things hampering a clear understanding of this is the rarity of freely available benchmarking tools for SSD's in OS X. Black Magic is nothing more than raw sequential performance which is only one piece of the puzzle. It is really the small 4K performance that will be most noticeable in day to day use. The only other tool freely available that I know of is Xbench but it is an old tool and the 4K numbers don't make much sense. In Windows I have access to all sorts of free benchmarking tools that give pretty consistent, comparable results like ATTO, Crystal Diskmark, AS SSD Bench, Anand Storage Bench, etc...

So the questions that this raises are:

1) What sort of algorithm does Fusion use to determine how full to allow the SSD to become?
2) How well does Garbage Collection alone maintain the performance of the Fusion Drive?
3) Is TRIM absolutely required for Fusion to maintain performance over time?

Any other thoughts or discussion are very welcome. Just trying to get my head around this.
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Old Nov 23, 2012, 09:33 AM   #2
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Keep things separate if you want to decide what resides where instead of leting OSX dictate what goes where. Also keep in mind with fusion what will happen if your HDD crashes, how will that affect fusion drive ? If you've data on HDD and programs on SSD it makes it easier to recover as well as to manage.
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Old Nov 23, 2012, 09:43 AM   #3
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Keep things separate if you want to decide what resides where instead of leting OSX dictate what goes where. Also keep in mind with fusion what will happen if your HDD crashes, how will that affect fusion drive ? If you've data on HDD and programs on SSD it makes it easier to recover as well as to manage.
Good point. I am pretty much decided that I will keep them separate. This mini will be used as my home server with the primary task of serving files and videos to the rest of the machines in the house. It has 8TB of external drives attached to it. So I basically only need the basic OS X on there and all other storage is external or on the 500GB internal HDD. The SSD is a 120GB drive so more than enough for basic OS install with lot's of space left over.

But I am still interested in understanding more about the Fusion technology so I might play with it a bit.
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Old Nov 23, 2012, 10:44 AM   #4
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Keep things separate if you want to decide what resides where instead of leting OSX dictate what goes where. Also keep in mind with fusion what will happen if your HDD crashes, how will that affect fusion drive ? If you've data on HDD and programs on SSD it makes it easier to recover as well as to manage.
I thought everyone had a backup system in place. No? That's kind of foolish. All drives fail - it's just a matter of time. Quite frankly, if one has an SSD and a spinning drive, it's tough to justify not having them as a fusion volume. I suspect there are a few reasons, but I can't think of any.
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Old Nov 23, 2012, 11:13 AM   #5
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I thought everyone had a backup system in place. No? That's kind of foolish. All drives fail - it's just a matter of time. Quite frankly, if one has an SSD and a spinning drive, it's tough to justify not having them as a fusion volume. I suspect there are a few reasons, but I can't think of any.
Well that's kinda what I'm trying to determine. There "might" be a reason not to use Fusion if it fills the SSD to capacity and results in slowing the performance of the SSD.

SSD's have built in over provisioning (to handle cell errors and manage write amplification) but that varies from model to model and controller to controller. For example the Sandforce based drives came in 120GB and 128GB versions. From what I understand they had the same amount of NAND but implemented different amount of spare cells.
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Old Nov 23, 2012, 12:12 PM   #6
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I thought everyone had a backup system in place. No? That's kind of foolish. All drives fail - it's just a matter of time. Quite frankly, if one has an SSD and a spinning drive, it's tough to justify not having them as a fusion volume. I suspect there are a few reasons, but I can't think of any.
Why would you go outside and consume one of your USB 3.0 port when you've space to put HDD right inside your system on SATA III ? Having backup is different from creating fusion drive.
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Old Nov 23, 2012, 12:28 PM   #7
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Why would you go outside and consume one of your USB 3.0 port when you've space to put HDD right inside your system on SATA III ? Having backup is different from creating fusion drive.
I'm at no loss for USB ports - even my display has three of them. But my Time Machine volume is a FW800 drive with 1.7x greater capacity than my 1.24 gb fusion drive. I suppose if you want to sacrifice valuable internal drive space, that's a personal decision.
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Old Nov 23, 2012, 06:42 PM   #8
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1) What sort of algorithm does Fusion use to determine how full to allow the SSD to become?
2) How well does Garbage Collection alone maintain the performance of the Fusion Drive?
3) Is TRIM absolutely required for Fusion to maintain performance over time?

Any other thoughts or discussion are very welcome. Just trying to get my head around this.
My understanding is that Apple's goal is to use the SSD for as much data as it can handle. The way Ars describes, you're using the HDD to augment the capacity of the SSD, as opposed to standard caching, which uses the SSD to augment the speed of the HDD.

Jury is still out on some of the technical details, but generally speaking, writes always go to the SSD first. It will keep writing to the SSD until it is "full". I know that at least 4GB is reserved for a write cache on a 128GB drive. This may be a fluid amount based on the size of the SSD, I'm not sure. I also don't know if any other space is held back for any reason.

Once the write cache is filled, if you're still writing data, it will seamlessly spill over onto the HDD. Over time, the OS will determine what data would benefit from being "promoted" to the SSD, and what data can be demoted, and uses idle time to rearrange the data.

This should speak quite a bit to the kind of SSD (and controller) you should get. If the OCZ drives suffer from degraded performance when they are only 50% full, I would highly suggest not using that kind of drive in a Fusion setup. (I also wouldn't recommend that kind of drive at all, because that's a ludicrous limitation to have on any storage device.) Again, philosophy matters here. Your primary storage is always the SSD, and all writes will go to the SSD, so write speed is going to be a very important stat. Likewise, most data will be read from the SSD first.

That said, I'm using a relatively low performing drive (Crucial M4) compared to some on the market, but it was cheap, and still a massive performance boost compared to the stock 5400RPM drive, so even a mid-quality drive should yield positive results.

As to your other questions, TRIM and garbage collection are two sides of the same coin. In most cases, the firmware of the SSD will take care of those issues. In my case, the Crucial M4 really needs to have TRIM enabled, so I ran the TRIM Enabler at Groths, and that did the trick. Garbage collection is handled by OS X, as far as I'm aware. Generally speaking, though, the lifespan on these drives will likely outstrip the useful life of the computer itself, so I wouldn't stress this too much. Find out if your drive needs the TRIM Enabler (a Google search should do it), and after that, you'll just need to trust things are working as they're supposed to.

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Old Nov 23, 2012, 07:22 PM   #9
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I thought everyone had a backup system in place. No? That's kind of foolish. All drives fail - it's just a matter of time. Quite frankly, if one has an SSD and a spinning drive, it's tough to justify not having them as a fusion volume. I suspect there are a few reasons, but I can't think of any.

One reason is that I am using the SSD for a commercial Windows 7 application which is crucial. The 7200 rpm HDD is running OS X and has Winclone for backing up the SSD bootcamp. Weekends OS X and during the week Windows 7 (24/5). If Windows / SSD goes down for one reason or another I can reboot in OS X and use Safari to do certain actions before investigating the Windows issues.
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Old Nov 23, 2012, 09:40 PM   #10
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My understanding is that Apple's goal is to use the SSD for as much data as it can handle. The way Ars describes, you're using the HDD to augment the capacity of the SSD, as opposed to standard caching, which uses the SSD to augment the speed of the HDD.

Jury is still out on some of the technical details, but generally speaking, writes always go to the SSD first. It will keep writing to the SSD until it is "full". I know that at least 4GB is reserved for a write cache on a 128GB drive. This may be a fluid amount based on the size of the SSD, I'm not sure. I also don't know if any other space is held back for any reason.

Once the write cache is filled, if you're still writing data, it will seamlessly spill over onto the HDD. Over time, the OS will determine what data would benefit from being "promoted" to the SSD, and what data can be demoted, and uses idle time to rearrange the data.

This should speak quite a bit to the kind of SSD (and controller) you should get. If the OCZ drives suffer from degraded performance when they are only 50% full, I would highly suggest not using that kind of drive in a Fusion setup. (I also wouldn't recommend that kind of drive at all, because that's a ludicrous limitation to have on any storage device.) Again, philosophy matters here. Your primary storage is always the SSD, and all writes will go to the SSD, so write speed is going to be a very important stat. Likewise, most data will be read from the SSD first.

That said, I'm using a relatively low performing drive (Crucial M4) compared to some on the market, but it was cheap, and still a massive performance boost compared to the stock 5400RPM drive, so even a mid-quality drive should yield positive results.

As to your other questions, TRIM and garbage collection are two sides of the same coin. In most cases, the firmware of the SSD will take care of those issues. In my case, the Crucial M4 really needs to have TRIM enabled, so I ran the TRIM Enabler at Groths, and that did the trick. Garbage collection is handled by OS X, as far as I'm aware. Generally speaking, though, the lifespan on these drives will likely outstrip the useful life of the computer itself, so I wouldn't stress this too much. Find out if your drive needs the TRIM Enabler (a Google search should do it), and after that, you'll just need to trust things are working as they're supposed to.
Excellent summary.

I have 5 Macs with SSD's in them so far (Intel 330 180GB, OCZ Vertex 4 128GB, OCZ Agility 4 128GB, OCZ Vertex Plus 60GB, OCZ Colossus 1TB). The Mac Mini will be the 6th with a Kingston V+200 120GB. I also use a Patriot Inferno 60GB on my workbench when working on Macbooks. I have enabled TRIM on 3 of them so far and have not seen any detrimental effects. My wife's iMac with a 180GB Intel 330 drive does not have TRIM enabled yet because she only uses about 50GB out of the 180GB so I figured it was OK to just let GC do its thing. Garbage collection is an internal function of the SSD, not OS X, so it depends on the SSD for how aggressive/effective it is.

My new Mini will probably only have about 25-30GB used out of 120GB so I might just let it go with GC for now and think about TRIM later if it starts to get a bit fuller.

So for now I think that I will follow the Fusion story from the fence. I don't need it for my application so it is more of a technical interest for now.

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One reason is that I am using the SSD for a commercial Windows 7 application which is crucial. The 7200 rpm HDD is running OS X and has Winclone for backing up the SSD bootcamp. Weekends OS X and during the week Windows 7 (24/5). If Windows / SSD goes down for one reason or another I can reboot in OS X and use Safari to do certain actions before investigating the Windows issues.
Good plan. I also use Winclone to backup my Windows partitions. Great little utility.
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Old Nov 24, 2012, 12:23 PM   #11
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This should speak quite a bit to the kind of SSD (and controller) you should get. If the OCZ drives suffer from degraded performance when they are only 50% full, I would highly suggest not using that kind of drive in a Fusion setup. (I also wouldn't recommend that kind of drive at all, because that's a ludicrous limitation to have on any storage device.) Again, philosophy matters here. Your primary storage is always the SSD, and all writes will go to the SSD, so write speed is going to be a very important stat. Likewise, most data will be read from the SSD first.
This. I've read in various reviews that the read/write rates in SSD drives can start to suffer once a certain percentage of their capacity has been reached (seemingly a byproduct of consumer-grade drives though over-provisioning is supposed to alleviate this). I've been going back and forth between creating a Fusion Drive or keeping a dual drive setup. After reading some of the great points made in the following article and comparison table, I've opted to keep my SSD and HDD separated.

http://macperformanceguide.com/macmi...vs-fusion.html

One of my considerations: I have just over 300GB of music; didn't want that clogging up valuable SSD space. As the article notes, there's no benefit to keeping video and music on a fast drive; media streaming rates are slow—most likely, a Fusion SSD would be wasting capacity with little benefit. Easier for me to leave the entire iTunes folder on the HDD rather than wondering how Fusion is spreading those files out across two drives unnecessarily. Ultimately, it all depends on each individual's intended computer use. Myself, I prefer the level of control that manual drive management allows.
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Old Nov 24, 2012, 05:42 PM   #12
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This. I've read in various reviews that the read/write rates in SSD drives can start to suffer once a certain percentage of their capacity has been reached (seemingly a byproduct of consumer-grade drives though over-provisioning is supposed to alleviate this). I've been going back and forth between creating a Fusion Drive or keeping a dual drive setup. After reading some of the great points made in the following article and comparison table, I've opted to keep my SSD and HDD separated.

http://macperformanceguide.com/macmi...vs-fusion.html

One of my considerations: I have just over 300GB of music; didn't want that clogging up valuable SSD space. As the article notes, there's no benefit to keeping video and music on a fast drive; media streaming rates are slow—most likely, a Fusion SSD would be wasting capacity with little benefit. Easier for me to leave the entire iTunes folder on the HDD rather than wondering how Fusion is spreading those files out across two drives unnecessarily. Ultimately, it all depends on each individual's intended computer use. Myself, I prefer the level of control that manual drive management allows.
I would caution you to take anything you read on this subject from MPG with a healthy dose of NaCL. He sells OWC products, and they have a vested interest in selling you faster drives, rather than allowing you to use a slower drive in a Fusion setup. He also doesn't completely understand the technology, so he's claiming problems that no one else has been able to reproduce. Lastly, he has a very specific workflow that is tied to have two drives managed separately, one that most people don't need to replicate.
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Old Nov 24, 2012, 07:28 PM   #13
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I think the OP is thinking too much. The ultimate speed tests are what counts, and they are speccing out very well. If Apple is "overstuffing" the SSD, who cares.
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Old Nov 25, 2012, 09:32 AM   #14
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I think the OP is thinking too much. The ultimate speed tests are what counts, and they are speccing out very well. If Apple is "overstuffing" the SSD, who cares.
You may be right but "over thinking" is not always a bad thing. My technical nature wants to understand the technology rather than just blindly following because Apple "said so".

The problem with "speed tests" is that there are so few proper benchmarking tools in OS X. Or at least not free ones. I would not just look at a black magic test result and assume all is just fine. The 4K performance could be at a quarter of where it should be and you would never know it from the benchmark. But you would notice it in the real world. 4K scores are the more important factor most of the time unless you are regularly copying large files back and forth between 2 SSD's. HDD to SSD transfers would not show the issue either.

In the end it is about real performance and Apple may very well have implemented something clever to keep overall performance over time. I just want to know how.
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Old Nov 25, 2012, 05:07 PM   #15
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Still, you're over thinking. Just create a fusion drive. Work with it, and if it ever slows down, separate it. Why the fuss?

(I've been running a DIY-fusion drive and am totally happy. Sure,mi can manage files myself, but the good thing is: I don't have to anymore)
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Old Nov 25, 2012, 05:17 PM   #16
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Still, you're over thinking. Just create a fusion drive. Work with it, and if it ever slows down, separate it. Why the fuss?

(I've been running a DIY-fusion drive and am totally happy. Sure,mi can manage files myself, but the good thing is: I don't have to anymore)
How easy is it to separate the drives? I know to join them you have to reformat / restore, would you have to do the same thing again? That would be a pain in the anus.
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Old Nov 26, 2012, 02:06 AM   #17
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How easy is it to separate the drives? I know to join them you have to reformat / restore, would you have to do the same thing again? That would be a pain in the anus.
There are instructions to seperate them online.
And of course it'd need a reformat/restore, as the files are shattered at the block level.

Not really a problem I'd think, because you'd have backups and you'd probably need to move files around yourself anyway as not everything would fit the ssd.

It took me like half an hour to restore a time machine backup from a USB3-hdd.
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Old Nov 26, 2012, 03:32 AM   #18
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...
One of the factors that will come into play is how the Fusion system works in managing how full it will allow the SSD to become. An empty SSD performs at much higher levels than a half full SSD... and a full SSD can slow to a crawl. For reference look at the OCZ Vertex 4 where it will run in a "Performance Mode" as long as the drive is 50% full or less. Over 50% Vertex 4 performance is dropped. ...
I am skeptical that full drives are slower... or maybe some models are, but I doubt most (or even many) are. Can you provide some links?

Of course read speed will not depend on how full a drive is since reading just involves finding the cells you want and checking their values.

Write speed should be constant as long as the drive does background wear leveling and garbage collection... unless you will be writing many gigabytes of data in one burst.

But ultimately, how much does it really matter, unless you have some very particular and unusual use cases? Even if an SSD is not performing optimally it will still be many times faster than a spinning disk.
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Old Nov 26, 2012, 08:32 AM   #19
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I am skeptical that full drives are slower... or maybe some models are, but I doubt most (or even many) are. Can you provide some links?

Of course read speed will not depend on how full a drive is since reading just involves finding the cells you want and checking their values.

Write speed should be constant as long as the drive does background wear leveling and garbage collection... unless you will be writing many gigabytes of data in one burst.

But ultimately, how much does it really matter, unless you have some very particular and unusual use cases? Even if an SSD is not performing optimally it will still be many times faster than a spinning disk.
This is an older article but it explains the principle.

http://www.anandtech.com/print/2738

Also, any decent review site reports on the efficiency of a drive in recovering speed after having a lot of data written to it. It is a measure of the effectiveness of the drive's garbage collection. It is a regular feature of the Anandtech reviews usually under "performance over time" section.

I personally have seen the slowdown happen on the Macbook Pro 13" 2009 that I use as the media center in my living room. After about a year of operating a Patriot Inferno Sandforce based SSD I noticed things "seemed" a little sluggish. Fortunately I had saved an original Xbench run from when I first installed it and, when I ran the test again, there was a significant slowdown in write speeds. I decided to use the Trim enable app at that point and when I restested about a week later the benchmark was back to normal.

This slowdown exists on all SSD's but to greater or lesser extent depending on the controller used. Check the reviews and they usually document this under garbage collection efficiency.

Do we know if Apple will be using Samsung SSD's in their Fusion setups? It may be more important in a Fusion setup for Apple to pay close attention to the SSD characteristics.

http://www.anandtech.com/show/4863/t...d-830-review/6
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Old Nov 26, 2012, 01:50 PM   #20
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This is an older article but it explains the principle.

http://www.anandtech.com/print/2738

...
Oh yeah, this is an old problem that only affects drives that aren't overprovisioned and don't support TRIM. I can't imagine Apple is selling drives that suffer from the same problem as these 2009 drives.
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Old Nov 26, 2012, 02:01 PM   #21
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Oh yeah, this is an old problem that only affects drives that aren't overprovisioned and don't support TRIM. I can't imagine Apple is selling drives that suffer from the same problem as these 2009 drives.
But check the second link. That is a review for the Samsung 830 which a current drive and is the same drive Apple has been using lately. This issue still exists.
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Old Nov 26, 2012, 02:09 PM   #22
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But check the second link. That is a review for the Samsung 830 which a current drive and is the same drive Apple has been using lately. This issue still exists.
I guess I don't know what you'll be doing with your drive but the 2nd link says performance decreases during (or immediately after) stress test runs. It makes sense and really any drive will be affected by this... if there aren't any free blocks to write data, the drive will have to flash blocks as you're writing, which is slow.

You would have to have a pretty unusual work pattern to be affected by such a slowdown though. Namely, writing gigabytes of data to the drive at a time with only a few minutes of "rest" in between for garbage collection. Personally, based on my own usage habits of using my computer for 8+ hours/day, I usually only write 2-3 gigabytes to my drive so I would never be affected by this...
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Old Nov 26, 2012, 02:37 PM   #23
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I guess I don't know what you'll be doing with your drive but the 2nd link says performance decreases during (or immediately after) stress test runs. It makes sense and really any drive will be affected by this... if there aren't any free blocks to write data, the drive will have to flash blocks as you're writing, which is slow.

You would have to have a pretty unusual work pattern to be affected by such a slowdown though. Namely, writing gigabytes of data to the drive at a time with only a few minutes of "rest" in between for garbage collection. Personally, based on my own usage habits of using my computer for 8+ hours/day, I usually only write 2-3 gigabytes to my drive so I would never be affected by this...
Yeah. That's probably the case. I am not trying to argue that there "is" a problem. The thread was intended to understand what techniques are implemented by Apple to address any potential problems. I am just trying to understand the technology. It seems to me that you would want to enable TRIM if you are brewing your own Fusion drive since it will not be enabled by default the way it is if you purchase the Apple Fusion Drive option from Apple.

As it turns out I think that I am going to keep my drives separate in my Mac Mini because, for that particular application, there is no need for additional internal storage (basic OS X installation, no significant apps). I will probably use it as a safe backup disk (bootable image of the SSD) so that I can recover quickly if the SSD were ever to have issues. This is my house server so high availability is the priority.

Thanks
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Old Nov 26, 2012, 02:43 PM   #24
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Yeah. That's probably the case. I am not trying to argue that there "is" a problem. The thread was intended to understand what techniques are implemented by Apple to address any potential problems. I am just trying to understand the technology. It seems to me that you would want to enable TRIM if you are brewing your own Fusion drive since it will not be enabled by default the way it is if you purchase the Apple Fusion Drive option from Apple.

As it turns out I think that I am going to keep my drives separate in my Mac Mini because, for that particular application, there is no need for additional internal storage (basic OS X installation, no significant apps). I will probably use it as a safe backup disk (bootable image of the SSD) so that I can recover quickly if the SSD were ever to have issues. This is my house server so high availability is the priority.

Thanks
What I've been doing for the past few years is to have an SSD and HD and I have two partitions on the HD--one as big as the SSD, basically to have a copy/backup of what's on the SSD, and another bigger partition for mass data storage. I think it works well. It offers a certain amount of flexibility, to be able to boot from the SSD or HD, which you may or may not find valuable. (I use SuperDuper! occasionally to copy the contents of the SSD to the appropriate hard drive partition.)

Personally I have no trouble managing my data on the two drives. Large files and/or media files like photos and songs all go on the hard drive. Otherwise everything goes on the SSD. Very easy once you get into the habit and works great. I understand the benefit of a Fusion drive and it seems nice but I like my setup as-is.
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Old Nov 26, 2012, 03:09 PM   #25
Dadioh
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Originally Posted by motrek View Post
What I've been doing for the past few years is to have an SSD and HD and I have two partitions on the HD--one as big as the SSD, basically to have a copy/backup of what's on the SSD, and another bigger partition for mass data storage. I think it works well. It offers a certain amount of flexibility, to be able to boot from the SSD or HD, which you may or may not find valuable. (I use SuperDuper! occasionally to copy the contents of the SSD to the appropriate hard drive partition.)

Personally I have no trouble managing my data on the two drives. Large files and/or media files like photos and songs all go on the hard drive. Otherwise everything goes on the SSD. Very easy once you get into the habit and works great. I understand the benefit of a Fusion drive and it seems nice but I like my setup as-is.
I have been an avid user of SuperDuper for a while. You have basically described my plan for this Mac Mini so we are on the same page. I am still not decided on whether to enable TRIM on this machine since the SSD will be Sandforce based and there seemed to be some beachball problems reported. Although it is not clear to me if those Sandforce issues were early in the development of Trim Enabler when it was copying an older kext rather than modifying the existing (which is how it does it now). Given that my SSD will probably only be about 25-30GB used out of 120GB TRIM is probably not required anyway.
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