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I'd like to build a cheap NAS with two drives in RAID1 configuration using Linux software RAID. What are the pros and cons of having a separate system drive? My main concern is reliability of the data, I don't require high availability (failure of system drive without compromising the data on RAID would not pose a big problem).

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up vote 1 down vote accepted

There is no need for a whole system drive, though I recommend separating your OS and your data on different partitions.

Since Linux can boot from a RAID 1 device you have at least these options:

  1. Make a single RAID/mda device of all disks. Create a separate partitions for /, /tmp, /var, /usr and /home (aka the normal way you set up a Linux installation). Put all data on /home.
  2. Create separate RAID volumes for each and every partition. Advantage: RAID can break down for one of those but leave the rest intact. Disadvantage: more complex than a single mda device.
  3. Create a plain simple non RAID device for the OS (easiest to recover in case of disasters). Optionally copy all of that to the second drive as a backup/rescue boot. Use the rest of the disk for a RAID 1'ed /home. (E.g like this:)
    Disk 1: /, /usr, /tmp, /var , RAIDed /home
    Disk 2: (backup (dd'ed?) /, /usr, /tmp, /var , RAIDed /home
    

    Downside of this is that the system is not part of the RAID array, which may give poorer performance.

[Edit]

Better performance compared to [part of] 1 drive OS, most of 2 drives for data.

Not compared to 2 drives RAID data and an extra drive for the OS.
(Then again, with 3 drives you could use RAID 1 over 3 drives).

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I recommend a separate system drive because of the following points

  • separation of system and data
  • easier to move to different hardware
  • easy and small backup of the system drive
  • better performance (not for the system but to access the data)
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