Time Machine + External SSD May Be Very Bad

[Randy B. Singer]

Getting an external SSD to use for a versioned backup (e.g. Time Machine) can end up being A HORRIBLE IDEA! I'm already hearing from folks who have told me that their TM SSD fills up, and then stops backing up.

While using an SSD for a clone backup (e.g. Carbon Copy Cloner or SuperDuper!) is a very good idea…

Six Lessons Learned from Dealing with an iMac's Dead SSD - TidBITS

Six Lessons Learned from Dealing with an iMac's Dead SSD - TidBITS

After working through the failure of his iMac’s SSD, Adam Engst shares the lessons he learned with regard to backing up, restoring, and eking out the most life from old hardware. Hint: waiting to upgrade hardware and software can make troubleshooting and recovery more difficult.

tidbits.com

(see Lesson #1)

because of the nature of Time Machine, it plays right to the weaknesses of external SSD’s.

External hard drive manufacturers for some reason think of all external SSD’s as portable drives. I’ve yet to find an external SSD that is robustly designed. Cooling tends to be so bad that even though external SSD’s often are blazingly fast, they tend to quickly overheat and then throttle back precipitously.

How fast SSDs slow to a crawl: thermal throttling 3

How fast SSDs slow to a crawl: thermal throttling

Fast compact external SSDs are prone to thermal throttling. This is demonstrated during tests writing 96 files, when transfer rates fell from 2 GB/s to around 700 MB/s.

eclecticlight.co

Safe SSD Operating Temperature: Is Your SSD Running Too Hot?

Safe SSD Operating Temperature: Is Your SSD Running Too Hot?

Find the safe operating temperature range for your SSD. SSDs that run too hot will slow down and can cause data loss or complete failure of the drive.

harddrivegeek.com

"SSDs that run too hot will slow down and can cause data loss or complete failure of the drive."


This is especially a problem when backing up using TM, because TM backups can really exercise a hard drive.

But that isn’t the only problem SSD’s have when used for Time Machine backups. TM works by keeping versions of all of your past files, quickly filling up your hard drive. Once your hard drive is filled up, TM is designed to started deleting the oldest files to make room for new files. The problem is that SSD’s start drastically slowing down when they get about 70% full. So, after that, your TM backup will become molasses slow, and even possibly fail.

"The rule of thumb to keep SSDs at top speeds is to never completely fill them up. To avoid performance issues, you should never use more than 70% of its total capacity.
“When you’re getting close to the 70% threshold, you should consider upgrading your computer’s SSD with a larger drive.”

Why solid-state drive (SSD) performance slows down as it becomes full

Why solid-state drive (SSD) performance slows down as it becomes full - Pureinfotech

Ever wondered why your solid-state drive degrades performance over time? Here we look into the problem and what to do to keep drive speedy.

pureinfotech.com

“SSDs may suffer performance issues, especially in writing speed, when the drive reaches full capacity. It is easier for the drive to write to an empty cell when there is free space available. When the drive is full, the SSD needs to find out which blocks are partially filled, move that information into a cache and then write it back to the drive. It is best to have 10-15% of your drive set aside for free space, to keep a good balance between performance and space utilization."

Top 7 Tips to Get the Most from your SSDs 2

Top 7 Tips to Get the Most from your SSDs

SSDs are are not without their problems. Here are my top 7 tips for getting the most from your SSDs.

www.slrlounge.com

I just wanted to warn folks. For now, using a large RDHD is probably the best idea for TM backups.

 

[mfpv]

Many thanks for the warning I was made aware of this potential issue a while ago so I'm keeping a close eye on the amount of available space on the external SSD drive I'm using for TM backups.

 

[Randy B. Singer]

Some random thoughts regarding the above, in response to things that I've heard others say.

You can get a huge SSD to work around the problem of your Time Machine drive filling up, but the price of huge SSD’s is silly compared to similarly large RDHD’s.

I don’t know of any external SSD’s that use Apple’s implementation of TRIM. They may do some TRIM-like things based in their own ROM, but it’s not like having a full implementation of TRIM enabled.

Note that Samsung’s SSD’s are very popular because of their price and initial performance. But they tend to slow down early in their lifespan and have no over-provisioning whatsoever. Tradeoffs for the low buy-in price.

Lots of folks think of SSD’s as being magical devices that are always fast and which are ultra reliable. Unless you purchase an extremely well made one (which will noticeably be priced to reflect this), like many Intel models, you can bet that your SSD has had some corners cut to be reasonably priced. That will likely come to haunt you at some point.

There are external case kits, and I’ve considered building my own high quality external SSD by using only the best components. Note that what you are looking for is a case kit that includes a very robust power supply, and a fan. To my knowledge, this doesn’t describe any case kits from OWC, a favorite supplier among Mac users. In fact, when you find these rare case kits, they will be surprisingly expensive. Somewhere around $100 just for the case with no drive in it. Example:

Avastor HDX Pro USB-C External Hard Drive

Check out the deal on Avastor HDX Pro USB-C External Hard Drive at Oyen Digital

oyendigital.com

Note that external case kits with aluminum cases (supposedly used as heat sinks) aren’t doing much unless the hot parts of the internal mechanism are in physical contact with the external case. It is extremely rare to find an aluminum case kit where this is actually how things are mounted.

 

[pine man]

An awful lot to digest, and understand, in all of this information!

When I got my new M1 iMac I also bought a pair of Seagate SSDs. One for CCC and the other for TM. My old Intel iMac still has a pair of RDHDs attached for the same purposes.

The RDHDs are always fairly warm/hot to touch but the SSDs are actually cold! Both of the SSDs, being small, are fixed side by side onto the back of the iMac metal stand with a cable tie, which hopefully will act as a heat sink in the event that they do ever get hot?

Before I got the SSDs I looked at several makes and most appeared to recommend them for TM use without any concerns or warnings about capacity problems. The advice, 'You should never use more than 70% of the total capacity to avoid performance issues.' is all very well but if taken literally means starting a new backup as I'm not aware of any way of reducing the size of a TM backup.

Thanks for pointing out the potential pitfalls of SSDs - now I've got something else to worry about in addition to Covid!!

 

[IWT]

Peter,

I share your feelings. But if you follow the recommendation that the EHD for a Time Machine backup should ideally be 1.5x the capacity of your Mac's Drive, then it will take a long time to reach 70% of its capacity.

By that time, your TM backups are likely to go back many months, even a year or so. Question: who needs a TM backup dating back 9-12 months, or longer?**

TM's great strength is that if you inadvertently erase a document, file, app even; it can be reclaimed by returning to a moment in time prior to that loss.

TM's other great asset is that it can be used to transfer all your settings, passwords, photos, data of any kind to a new Mac - provided there isn't a huge gap between Operating Systems.

**But TM is not intended as a long term BU of critical data. Music, Photos, business paraphernalia, Wills & all sorts of crucial/personal data should be backed up on separate EHDs, the Cloud etc.

So, it's not a big deal wiping your TM Hard Drive at 70% and starting afresh. In fact, doing so ensures that the EHD is still viable and all the new BU data on it in working order.

Ian

 

[pine man]

Hi Ian

Thanks for your views and excellent advice it really helps.

My SSDs are both 2x the capacity of the Mac's drive, so as you say, it will be a long time before the TM drive reaches 70% and anything that old probably has lost it's use.

Anyway, as I have adopted a 'belt and braces' approach to backing up using CCC as well, it is unlikely that both drives will reach capacity at the same time so I will always have access to some older stuff if really necessary.

Phew!

Peter

 

[Randy B. Singer]

So, it's not a big deal wiping your TM Hard Drive at 70% and starting afresh. In fact, doing so ensures that the EHD is still viable and all the new BU data on it in working order.

No. It is a big deal. An SSD is not a RDHD. The way that you deal with it is significantly different.

You should never be "wiping" a SSD if you can avoid it, and you certainly shouldn't be doing it routinely (e.g. every time it reaches 70% full). Each block in an SSD has a limited number of reads and writes. Erasing your entire hard drive damages and decreases the lifespan of your SSD.

I guess that, if you already have an SSD for use as a TM backup, that you have kind of put yourself into a corner in that you will probably have to erase it when it becomes too full. That's a shame, because it's really not what you want to do. (Note that Apple has removed the ability to erase your hard drive from Disk Utility for just this reason. We've discussed this elsewhere. I don't have my hands on a link just now, but since erasing an SSD damages it, and most Macs now come with SSD's, Apple has removed this ability from Disk Utility and they recommend that you use File Vault to keep your data secure, rather than erasing it.)

Of course, if you erase your entire SSD, you wipe out your ENTIRE TM backup, not just a few iterations of that backup. You have to start from scratch.

 

[Randy B. Singer]

The RDHDs are always fairly warm/hot to touch but the SSDs are actually cold! Both of the SSDs, being small, are fixed side by side onto the back of the iMac metal stand with a cable tie, which hopefully will act as a heat sink in the event that they do ever get hot?

I doubt that it will work that way. For it to work that way:

- The hot part of the internal SSD mechanism has to be in physical contact with the case internally

and

- The case itself has to be metal, preferable aluminum, and preferably with adequate cooling fins to provide area for convection of the heat

If your SSD's are cold to the touch I can guaranty you that one or both of the above are not the case. Because SSD mechanisms don't run cold. If fact, the few external SSD's on the market that do have both of the above tend to get so hot that they can be painful to touch. Read the reviews on Amazon, if you don't believe me. e.g.

https://www.amazon.com/gp/product/B07YCR1S3K/?tag=macforums0e4-20

Most (but not all) external SSD's that I've seen have cheap plastic cases that provide no cooling whatsoever.

Before I got the SSDs I looked at several makes and most appeared to recommend them for TM use without any concerns or warnings about capacity problems.

Hmmm...it's almost like they really, really want you to purchase their product, without regard for how suitable it might be for certain purposes...

The advice, 'You should never use more than 70% of the total capacity to avoid performance issues.' is all very well but if taken literally means starting a new backup as I'm not aware of any way of reducing the size of a TM backup.

I'm not saying that you should erase your SSD routinely if you are using it with TM. I'm saying that it's a bad idea to use an SSD with TM, period. Sorry to be the bearer of bad news if you have already purchased and started to use an SSD for that purpose. The only minor grace if you already have a TM SSD is now you know why things have gone to heck when your TM SSD fills up.

 

[Slydude]

Thanks for this discussion, guys. I've had a couple of RHDs fail in the last few months. I was starting to think about using SSDs for TM backups, so this discussion comes at a good time. I hadn't given much thought to the possibility that the way an SSD behaves might be problematic for TM backups.

 

[pine man]

Well I've been doing some more research into this SSD issue - and I'm really none the wiser!

My views may be a bit simplistic but here goes.

There is no doubt that wiping an SSD shortens its life, however it's not clear by how much it shortens its life, or the amount of times you need to do it to significantly affect its performance, but years are involved as far as I can make out.

My old Intel iMac has a Fusion drive, which, by its very nature, involves the moving of files back and forward from HDD to SSD continually. That iMac is about 9 years old now and hasn't missed a beat.

My old PC ran from an SSD and was at least ten years old. During that time it had numerous reformats and reinstallations of various operating systems. The SSD never gave up.

Excess heat in an SSD can definitely throttle its speed and there isn't an awful lot, it appears, that I can do about that, even if I knew that my SSDs were overheating.

I am going to adopt a 'head in the sand' attitiude and assume that my SSDs probably aren't overheating and that just maybe wiping them once every couple of years will not seriously affect their performance too much.

I have two independant methods of backup, CCC and TM, and hopefully if I ever need some sort of restoration one of them will be able to help me out.

The bottom line - if my iMac goes 'belly up' and I have no means of restoration the world will not come to an end. Life's too short - and getting shorter!

 

[Randy B. Singer]

I have two independant methods of backup, CCC and TM, and hopefully if I ever need some sort of restoration one of them will be able to help me out.

The rules that I often suggest to people are:

The more that you like your data, the more backups you should have. Having two backups is probably plenty for just about anyone.

The other rule is to test your backup(s) regularly. I can't tell you how often I hear from folks whose internal hard drive has died, and they thought that they were covered because they had a backup, and then they tried their backup and it wasn't as functional/intact as they expected it to be.

For folks in my attorney's user group, for whom their data is wildly important, I often recommend a three tier backup plan. I'll tell anyone who needs such a plan all about it if they ask.

 

[AMACL]

Thanks to all for this thread. I had no idea about this potential issue. At present I'm using an SSD to back up my new M1 iMac, mostly because it was there and easy to plug in. But it's only 500 GB so maybe not a great idea for TM in the long run.

Question to @Randy B. Singer : How does one "test" a backup? just plug in the drive from time to time and see if it's still alive?

 

[Slydude]

FWIW I think you need to not only plug in the drive but also try restoring some of the files. If the drive spins up but something prevents restoring files (directory error etc), the fact that the drive spins up becomes irrelevant.

 

[Randy B. Singer]

Question to @Randy B. Singer : How does one "test" a backup? just plug in the drive from time to time and see if it's still alive?

Mike Bombich of Carbon Copy Cloner has an article on this for clone backups:

How to verify or test a bootable backup - Bombich Software

bombich.com

Here is an article for Time Machine backups:

How to Verify That Your Mac's Time Machine Backups Are Working Properly

You regularly back up your Mac with Time Machine, but how do you know that it's working?Related: What's the Best Way to Back Up My Computer?There are a few ways to check.

www.howtogeek.com

 

[AMACL]

Thank you both.

 

[jpdemers]

I guess that, if you already have an SSD for use as a TM backup, that you have kind of put yourself into a corner in that you will probably have to erase it when it becomes too full.

There's no need to "erase" it. Just re-format the SSD, and carry on, no thrashing of the drive required. It's quicker even than trashing + emptying the trash (the latter step takes a long time b/c the file directory is huge.)

 

[Randy B. Singer]

There's no need to "erase" it. Just re-format the SSD, and carry on, no thrashing of the drive required. It's quicker even than trashing + emptying the trash (the latter step takes a long time b/c the file directory is huge.)

You can do that, and it will certainly be a fast and easy thing to do, and it will extend the life of the SSD unlike erasing it. However, it won't fix the underlying problem that has caused the SSD to slow to a crawl and be unsuitable for use with Time Machine. All of the drive's blocks will still be filled, and the drive will continue to be slow. See the links that I gave that explain the problem in my original post.

And of course, if you re-format your external SSD, all of your TM backup will be gone. The fact remains that using an SSD for your TM backup, at this time, isn't a good idea.

 

[Rod]

Thank you Randy for another thought provoking topic discussion.
I am still using a RDHD fot Time machine because I erase it at each new major iteration of macOS. It is twice the storage of my internal SSD so never fills.
I use my T5 Samsung SSD for CCC data backups only it's also twice the size of my internal drive.
I too only reformat the T5 if I need to erase such as when I experimented with CCC's Legacy method of creating a bootable backup of Monterey.
With a new M1 coming up soon I have given up on that process.

 

[chscag]

The price of SSDs are going down. For example, the Samsung T5 500 GB can be had for less than $100 at most on line stores that sell them.

A good SSD of quality can take many reads and writes. Chances are before your Time Machine or CCC backup SSD wears out or slows down, the technology will have moved on to something better.

Another consideration is just how long do you want to keep your TM backups? I know some folks get all frustrated having to wipe their TM external drive because it filled up. How useful is a TM backup that you made one year ago?

it seems some folks use their Time Machine backups to archive data. That's not what it's for.

 

[jpdemers]

The "Garbage Collection" feature of all decent SSDs today (it may be lacking from cheap no-name imports) addresses the slow-down problem. It requires the drive to be idle, but powered up, which you can ensure by unchecking "sleep drives when possible" in the Energy Saver control panel.