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I bought a laptop with one 128GB M.2 SSD and one empty slot, then I got a 1TB SSD and installed it, then installed ubuntu on the 128GB and made the 1TB the home directory. Now I'm thinking, if I had installed both the Ubuntu system and the home directory on the 1TB, would that make the performance worse? Or make it better?

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    If both SSDs have the same read/write speeds, it would not matter whether you use one or the other, however if you have separate disks, you can access both at maximum speed independently at the same time (which normally would not happen that often probably, I think). There can be significant speed differences though. To benchmark the read speed of a disk, you can run sudo hdparm -t /dev/sdX, replacing the sdX with the actual name of your disk, as shown e.g. in the outpu of lsblk.
    – Byte Commander
    Apr 16 '17 at 18:50
  • I executed the command and found out that the 1TB disk is a bit faster. Do you recommend using it for both the system and the home dir?
    – Lawand
    Apr 16 '17 at 19:24
  • You should move your comment to an answer actually..
    – Lawand
    Apr 16 '17 at 19:33
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    No put / and /home on the ssd. That helps with performance. Move the dirs in /home/ to the 1Tb and set the system up to use those dirs (./config/user-dirs.dir). Keep your personal data from the ssd (that does not benefit from the speed of your ssd anyways)
    – Rinzwind
    Apr 16 '17 at 19:33
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    Oh sorry :D I forgot that when I made the comment :X Then it does not matter... maybe set the personal data on the 1tb so you can make an easy backup? That's the only benifit I can think of
    – Rinzwind
    Apr 16 '17 at 19:35
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If both SSDs have the same read/write speeds, it would not matter for what you use one or the other. There can be significant speed differences though. To benchmark the read speed of a disk, you can run

sudo hdparm -t /dev/sdX

replacing the sdX with the actual name of your disk, as shown e.g. in the output of lsblk.

However, if you have separate disks, you can access both at maximum speed independently at the same time, which gives you maybe up to twice the normal data rate, but only in the rare case where you have a large enough chunk of data to read from or write to both disks in parallel, which normally doesn't happen very often. It's hard to tell whether you would notice any difference during normal usage.

Another advantage of having /home on the separate, larger SSD would be that you can easily back up only your complete data disk, or reinstall the system without messing with it (carefully though!). Also if the smaller system SSD dies sooner, your data would not be affected. It can happen the other way round as well, in that case you could restore your data backup and keep using it with the old system installation.

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  • The most common situation where having two disks would speed things up is during startup, where it can load software from the system disk and read configuration and data files from the home disk.
    – Mark
    Apr 17 '17 at 1:33

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