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Re: Raid 1

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Andy Smith writes:

> Hi Pankaj,
>
> Not wishing to put words in Linux-Fan's mouth, but my own views
> are…
>
> On Mon, Jan 25, 2021 at 11:04:09AM +0530, Pankaj Jangid wrote:
> > Linux-Fan <Ma_Sys.ma@web.de> writes:
> >
> > > * OS data bitrot is not covered, but OS single HDD failure is.
> > >   I achieve this by having OS and Swap on MDADM RAID 1
> > >   i.e. mirrored but without ZFS.
> >
> > I am still learning.
> >
> > 1. By "by having OS and Swap on MDADM", did you mean the /boot partition
> >    and swap.
>
> When people say, "I put OS and Swap on MDADM" they typically mean
> the entire installed system before user/service data is put on it.
> So that's / and all its usual sub-directories, and swap, possibly
> with things later split off after install.

Yes, that is exactly how I meant it :)

My current setup has two disks each partitioned as follows:

 * first   partition ESP          for /boot/efi (does not support RAID)
 * sencond partition MDADM RAID 1 for / (including /boot and /home)
 * third   partition MDADM RAID 1 for swap
 * fourth  partition ZFS mirror   for virtual machines and containers

Some may like to have /home separately. I personally prefer to store all my  
user-created data outside of the /home tree because many programs are using  
/home structures for cache and configuration files that are automatically  
generated and should (IMHO) not be mixed with what I consider important data.

> > 2. Why did you put Swap on RAID? What is the advantage?
>
> If you have swap used, and the device behind it goes away, your
> system will likely crash.
>
> The point of RAID is to increase availability. If you have the OS
> itself in RAID and you have swap, the swap should be in RAID too.

That was exactly my reasoning, too. I can add that I did not use a ZFS  
volume for the swap mostly because of
https://github.com/openzfs/zfs/issues/7734
and I did not use it for the OS (/, /boot, /home) mainly because I wanted to  
avoid getting a non-booting system in case anything fails with the ZFS  
module DKMS build. The added benefit was a less complex installation  
procedure i.e. using Debian installer was possible and all ZFS stuff could  
be done from the installed and running system.

I would advise against replicating my setup for first-time RAID users  
because restore after a failed disk will require invoking the respective  
restoration procedures of both technologies.

> There are use cases where the software itself provides the
> availability. For example, there is Ceph, which typically uses
> simple block devices from multiple hosts and distributes the data
> around.

Yes.

[...]

> > How do you decide which partition to cover and which not?
>
> For each of the storage devices in your system, ask yourself:
>
> - Would your system still run if that device suddenly went away?
>
> - Would your application(s) still run if that device suddenly went
>   away?
>
> - Could finding a replacement device and restoring your data from
>   backups be done in a time span that you consider reasonable?
>
> If the answer to those questions are not what you could tolerate,
> add some redundancy in order to reduce unavailability. If you decide
> you can tolerate the possible unavailability then so be it.

[...]

My rule of thumb: RAID 1 whenever possible i.e. on all actively relied-upon  
computers that are not laptops or other special form factors with tightly  
limited HDD/SSD options.

The replacement drive considerations are important for RAID setups, too. I  
used to have a "cold spare" HDD but given the rate at which the  
capacity/price ratio rises I thought it to be overly cautious/expensive to  
keep that scheme.

HTH
Linux-Fan

öö

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Thread

Raid 1 mick crane <mick.crane@gmail.com> - 2021-01-22 23:30 +0100
  Re: Raid 1 Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-23 00:10 +0100
  Re: Raid 1 David Christensen <dpchrist@holgerdanske.com> - 2021-01-23 00:20 +0100
  Re: Raid 1 David Christensen <dpchrist@holgerdanske.com> - 2021-01-23 00:20 +0100
  Re: Raid 1 Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-23 13:30 +0100
    Re: Raid 1 Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-23 18:20 +0100
      Re: Raid 1 Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-23 19:00 +0100
        Re: Raid 1 Pankaj Jangid <pankaj@codeisgreat.org> - 2021-01-25 07:00 +0100
          Re: Raid 1 Andy Smith <andy@strugglers.net> - 2021-01-25 08:20 +0100
            Re: Raid 1 Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-25 12:10 +0100
              Re: Raid 1 Pankaj Jangid <pankaj@codeisgreat.org> - 2021-01-25 13:50 +0100
    Re: Raid 1 David Christensen <dpchrist@holgerdanske.com> - 2021-01-23 23:10 +0100
      Re: Raid 1 Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-24 18:40 +0100
        Re: Raid 1 Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-24 23:30 +0100
          Re: Raid 1 Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-25 07:40 +0100
      Re: Raid 1 Andy Smith <andy@strugglers.net> - 2021-01-24 19:00 +0100
        Re: Raid 1 Andrei POPESCU <andreimpopescu@gmail.com> - 2021-01-24 21:20 +0100
      Re: Raid 1 David Christensen <dpchrist@holgerdanske.com> - 2021-01-24 21:20 +0100
        Re: Raid 1 deloptes <deloptes@gmail.com> - 2021-01-25 09:50 +0100
        Re: Raid 1 David Christensen <dpchrist@holgerdanske.com> - 2021-01-25 19:00 +0100

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