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I think I have a pretty good Idea how my TB drives will be laid out in a zpool using ZFS. I also have a 120GB SSD. My intent was to install the program on the SSD.

Is that outside the pool?

Is that a different format (eg EXT4)?

Should I consider using the SSD elsewhere (e.g. some sort of cache)?

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You have three solutions depending on the use case.

  1. If your disks are going to store a lot of data not accessed often and which do not need special read or write speed. And if you have a need for programs which will fit on the ssd, you can use your ssd separately to your zpool and format it simply as ext4.
  2. If you need high read performance on your ZFS pool, you should use at least part of the ssd as l2arc cache. The ZFS filesystem caches data in ram first (arc cache), and can use a ssd to store a level 2 (l2 arc) cache. It will contain the files uses most frequently.
  3. If instead you need fast write performance on your zfs pool, you can use your ssd as SLOG, which will cache the write requests before they are sequentially written to the spinning disks. However, it is a good practice to use a mirror of ssd to store the slog, as losing the ssd may corrupt recently written data.

In the first and third cases, beware of the reliability issues linked to the potential failure of the ssd.

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  • Could you add more details (e.g add a small quote from where you got these information) about links of second and third case? It will help people to understand better the possible solution. – Danibix Apr 20 '16 at 17:31
  • Due to my rank, I can only post 2 links, a link with more data is here: open-zfs.org/wiki/Hardware#NAND_Flash_SSDs – ptonelli Apr 24 '16 at 19:41

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