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Re: Are the assigned capacities sufficient for my setup?

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On 8/2/2020 2:54 PM, David Wright wrote:
>> 	I never said it does.  It does take up a little space, but not a
>> significant amount.  What does take up space is /home, which can get
>> to be huge.  Even on a laptop, making /home a part of the largest
>> partition is a good idea.
> 
> What it sharing the partition with /home? And what is it's
> mountpoint?

	On my big servers, it doesn't have a mount point.  For consistency 
sake, the mount point for the data array on all the servers is /RAID, 
and the array is /dev/md0, which consists of varying numbers of 
individual drives (eight drives each plus one spare for the big servers) 
in a RAID 6 array.  On md0, I have a directory named home, which of 
course becomes /RAID/home when the md0 array is mounted.  On all the 
systems, I have two SSDs set up with three partitions each, the first 
two of which are assembled int RAID 1 arrays.  The second array, 
/dev/md2, is mounted on /, and a symlink points from /home to /RAID/home.

>> Giving it its own partition permanently
>> sequesters any space allocated to the purpose, to no particular
>> advantage.  Making it simply a directory on the largest drive target
>> frees up all the unused space for other use.  Even on most laptops,
>> the largest partition is usually not /,
> 
> Which one is, usually?

	The data target.  On my servers, this is /RAID, which is anywhere from 
9T to 48T.  On my workstations, this varies some depending upon whether 
the system is multi-boot or single boot, but it is usually named /DATA. 
It may be NTFS or EXT4.  It may r may not be a separate physical target, 
whether it is an array, a separate drive, or a separate partition on one 
drive.

>> and in any case, as mentioned
>> previously, it is generally better for / and /home to be on separate
>> targets.
> 
> In the previous post, we had "data array" aka "data repository",
> SSDs for booting, and a NAS or file server (whose data repository
> needs to be huge), plus a "single set of array spindles".
> 
> With that wide choice, I'm not clear what "targets" are, nor how

	A file system target is a logical storage location where a file system 
can be created.  It can be an unpartitioned drive, a partition on a 
drive, an array, a tmpfs system, a RAM drive, or a loop device.

> all these different items are arranged with respect to the PC
> itself (or which are entirely separate entities).

	In Linux, it can be done just about anyway one likes, although some 
topologies are rather senseless.


>> 	Journalling is only a safety mechanism if the file system is being
>> written.  Since /boot is to all intents and purposes never written,
>> journaling does nothing for it.  The /boot partition can be mounted
>> read-only, as a matter of fact.
> 
> What are the benefits of /boot on a RAID?

	The same as any other data repository.  RAID 0 would make no sense at 
all, because /boot is rather tiny, and RAID 0 offers no redundancy at 
all.  RAID 1 is perfect, however, allowing double or triple (or more) 
drive redundancy, virtually guaranteeing the system will boot even if a 
drive (or even two or three drives) fails.  Higher level RAID targets 
don't really provide any significant features for the boot system, and 
especially not /boot.  They really don't provide much for /, either, 
given its relatively small requirements.

> Does that mean that a RAID
> has to be assembled even before the PC can boot up?

	Not if it is a RAID 1.  A RAID 1 member is identical to any other 
target except for the addition of an MD superblock.  The I/O system only 
needs to be able to read the filesystem.  It can be completely ignorant 
of the RAID 1 superblock - at least for superblock versions 1.0 and 
above.  On a non-EFI, non-ZFS system, everything the system needs to 
boot is in the /boot directory.

	Indeed, the RAID 1 superblock can be added to or removed from almost 
any target to make it a part of an array or remove it from the array 
without touching the MBR, the partition table, file systems, or the data 
on the drive.  GRUB, or whatever bootloader one chooses, can read the 
contents of /boot from any one of the array members without 
understanding anything about arrays, or with only one - any one - of the 
RAID mirrors present.  It does not have to assemble the array, and in 
fact I don't think it does, or at least it did not in earlier versions 
of GRUB.  The same is actually true of any RAID 1 array, although it is 
not a good idea to mount a single array member, as opposed to mounting a 
single member RAID 1 array.  The bootloader will load the kernel, which 
*MUST* know how to assemble the arrays before mounting.  For RAID 1, it 
is not necessary for the bootloader to understand anything about arrays, 
and in fact to my knowledge GRUB does not.

	Note while GRUB does not need to know the details of a version 1.0 or 
above RAID 1 array containing the /boot directory, it *DOES* need to 
know enough about the filesystem to read it.

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Thread

Re: Are the assigned capacities sufficient for my setup? Charlie Gibbs <cgibbs@surfnaked.ca> - 2020-07-29 20:10 +0200
  Re: Are the assigned capacities sufficient for my setup? David Wright <deblis@lionunicorn.co.uk> - 2020-07-30 04:20 +0200
  Re: Are the assigned capacities sufficient for my setup? Stefan Monnier <monnier@iro.umontreal.ca> - 2020-07-30 04:40 +0200
  Re: Are the assigned capacities sufficient for my setup? Leslie Rhorer <lesrhorer@att.net> - 2020-08-02 02:30 +0200
    Re: Are the assigned capacities sufficient for my setup? Dan Ritter <dsr@randomstring.org> - 2020-08-02 05:50 +0200
      Re: Are the assigned capacities sufficient for my setup? David Wright <deblis@lionunicorn.co.uk> - 2020-08-02 06:20 +0200
      Re: Are the assigned capacities sufficient for my setup? Leslie Rhorer <lesrhorer@att.net> - 2020-08-02 21:20 +0200
        Re: Are the assigned capacities sufficient for my setup? David Wright <deblis@lionunicorn.co.uk> - 2020-08-02 22:00 +0200
          Re: Are the assigned capacities sufficient for my setup? Leslie Rhorer <lesrhorer@att.net> - 2020-08-02 23:40 +0200
            Re: Are the assigned capacities sufficient for my setup? David Wright <deblis@lionunicorn.co.uk> - 2020-08-11 06:20 +0200

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