Why *not* to use a SSD for Time Machine Backup
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I've been following this mailing list since the mid 1990s, and though it's gotten a lot less active than it used to be, every now and then there's a pearl of wisdom.
Some guy asked, "What's the best SSD to use as a Time Machine backup for my Mac?"
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An external SSD is a uniquely bad idea for use as a Time Machine backup. (Ironically, an SSD is almost a necessity for use as a clone backup.)The thing is that Time Machine is designed to fill up your external hard
drive with all of your old files and software until it becomes full, and
then it starts deleting the oldest backups to make room for newer
backups. In many cases, your TM backup drive becomes filled up rather
quickly.The problem is that SSD's start slowing down precipitously when
they become about 70% full."In practice, an SSD's performance begins to decline after it
reaches about 50% full."https://www.seagate.com/tech-insights/ssd-over-provisioning-benefits-maste
r-ti/"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."https://pureinfotech.com/why-solid-state-drive-ssd-performance-slows-down/
"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."
https://www.slrlounge.com/tips-to-get-the-most-from-your-ssds/This problem is made even worse by the fact that under all versions of
the Mac OS other than Monterey, an external hard drive can't use
the native TRIM routine built into the Mac OS. Some external SSD'
s do some TRIM-like things based in ROM, but it's not as good as
having TRIM enabled.This has changed under Monterey:
macOS Monterey (finally) supports the TRIM command over USB for external SSDs.
https://translate.yandex.com/translate?url 3Dhttps%3A%2F%2Fwww.journaldula
pin.com%2F2021%2F12%2F24%2Ftrim-usb-monterey%2F&lang 3Dfr-enI've heard from a number of users who have complained that their
external SSD being used for Time Machine filled up E2 80 A6and then
stopped working completely. To date there doesn't seem to be a
workable solution for this. You can't even erase your external
SSD and start over from scratch, because there are no Macintosh tools
available for implementing the secure erase function on SSD's.Of course, you can avoid the problem by purchasing an external SSD that is several times larger than your internal hard drive. That way,
assuming that you don't download or create huge files
constantly, you can avoid filling up your external SSD. But that is a
very expensive solution. It makes a lot more sense to just use an
inexpensive RDHD for your Time Machine backup."Now look here, you Baltic gas passer... " - Mik, 6/14/08
The saying, "Lite is just one damn thing after another," is a gross understatement. The damn things overlap.
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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."Back in the day it was common to have a "defragmentation" tool for the harddrive, which would move all free space to a continuous segment. It should be pretty simple to do the same for an SSD, no?
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@Klaus said in Why *not* to use a SSD for Time Machine Backup:
Back in the day it was common to have a "defragmentation" tool for the harddrive, which would move all free space to a continuous segment. It should be pretty simple to do the same for an SSD, no?
I would think so, but I also believe that SSDs are limited by the number of read/write cycles per block. Performance starts to suffer after a while.
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@Klaus said in Why *not* to use a SSD for Time Machine Backup:
Back in the day it was common to have a "defragmentation" tool for the harddrive, which would move all free space to a continuous segment. It should be pretty simple to do the same for an SSD, no?
It is already being done (to different degrees by different SSDs/systems), it’s called “garbage collection”:
https://datarecovery.com/rd/garbage-collection-ssd-simple-explanation/
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@Axtremus said in Why *not* to use a SSD for Time Machine Backup:
Re: the opening post … what I hope to see is performance comparison between filled SSD vs. HDD.
SSD can slow down after it is filled, but if after the slowdown it still outperforms the HDD, then it would still be worthwhile to use SSD.
How many have you built, Ax?
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@Axtremus said in [Why *not* to use a SSD for Time Machine
It is already being done (to different degrees by different SSDs/systems), it’s called “garbage collection”:
A terrible choice of name. Since the 1950s, the term "garbage collection" refers to a process of identifying blocks of memory that aren't needed anymore, either by tracing references start from a "root" set of references, or by a process called "reference counting". The "garbage collection" process of an SSD seems to not actually identify any garbage; rather, it seems to be only about compactifying and restructuring blocks.
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I recently replaced the HDD in a TimeCapsule with an SSD. It will be a while until that SSD fills up. I still have another TimeCapsule of the same model with an HDD. So if that HDD survives until the SSD fills up, I will have two systems that will let me compare their performances.
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@Axtremus said in Why *not* to use a SSD for Time Machine Backup:
Re: the opening post … what I hope to see is performance comparison between filled SSD vs. HDD.
SSD can slow down after it is filled, but if after the slowdown it still outperforms the HDD, then it would still be worthwhile to use SSD.
This is only about write speeds, no? So I assume it would still outperform HDDs in read speed.
I think it's a little sad that "hybrid" drives that have a smaller SSD and a big HDD aren't more common. A clever file system could combine their respective advantages.
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Yes, mostly affect write speed, not so much on read speed
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Hybrid drive with smaller SSD + bigger HDD … for a while that’s the standard for iMacs. Even Apple stopped doing that now. I guess people just don’t want to deal with the mechanical failures of HDD’s moving parts. For a system that never shuts down (servers), I am guessing an HDD with very big cache will likely work as well or better than a hybrid SSD+HDD.
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@Axtremus said in Why *not* to use a SSD for Time Machine Backup:
- Yes, mostly affect write speed, not so much on read speed
I would think that's not a major issue in backup, particularly if incremental, such as Time Machine.
- Hybrid drive with smaller SSD + bigger HDD … for a while that’s the standard for iMacs. Even Apple stopped doing that now. I guess people just don’t want to deal with the mechanical failures of HDD’s moving parts.
My 2014 iMac has such a drive. It failed after about 3 years. I had it replaced. But, I'm currently booting from an external SSD and it works great. My Time Machine backup is to a 4TB RDHD.
