Re: Provide a better free space estimate on RAID1

[Roman Mamedov <rm@romanrm.net>]

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On Sun, 9 Feb 2014 06:38:53 +0000 (UTC)
Duncan <1i5t5.duncan@cox.net> wrote:

> RAID or multi-device filesystems aren't 1970s features and break 1970s 
> behavior and the assumptions associated with it.  If you're not prepared 
> to deal with those broken assumptions, don't.  Use mdraid or dmraid or lvm 
> or whatever to combine your multiple devices into one logical devices as 
> presented, and put your filesystem (either traditional filesystem, or 
> even btrfs using traditional single-device functionality) on top of the 
> single device the layer beneath the filesystem presents.  Problem solved! 
> =:^)
> 
> Note that df only lists a single device as well, not the multiple 
> component devices of the filesystem.  That's broken functionality by your 
> definition, too, and again, using some other layer like lvm or mdraid to 
> present multiple devices as a single virtual device, with a traditional 
> single-device filesystem layout on top of that single device... solves 
> the problem!

No reason BTRFS can't work well in a similar simplistic usage scenario.

You seem to insist there is no way around it being "too flexible for its own
good", but all those advanced features absolutely don't *have* to get in the
way of everyday usage for users who don't require them.

> Meanwhile, what I've done here is use one of df's commandline options to 
> set its block size to 2 MiB, and further used bash's alias functionality 
> to setup an alias accordingly:
> 
> alias df='df -B2M'
> 
> $ df /h
> Filesystem     2M-blocks  Used Available Use% Mounted on
> /dev/sda6          20480 12186      7909  61% /h
> 
> $ sudo btrfs fi show /h
> Label: hm0238gcnx+35l0  uuid: ce23242a-b0a9-423f-a9c3-7db2729f48d6
>         Total devices 2 FS bytes used 11.90GiB
>         devid    1 size 20.00GiB used 14.78GiB path /dev/sda6
>         devid    2 size 20.00GiB used 14.78GiB path /dev/sdb6
> 
> $ sudo btrfs fi df /h
> Data, RAID1: total=14.00GiB, used=11.49GiB
> System, RAID1: total=32.00MiB, used=16.00KiB
> Metadata, RAID1: total=768.00MiB, used=414.94MiB
> 
> 
> On btrfs such as the above I can read the 2M blocks as 1M and be happy.
>
> On btrfs such as my /boot, which aren't raid1 (I have two separate 
> /boots, one on each device, with grub2 configured separately for each to 
> provide a backup), or if I df my media partitions still on reiserfs on 
> the old spinning rust, I can either double the figures DF gives me, or 
> add a second -B option at the CLI, overriding the aliased option.

Congratulations, you broke your df readings on all other filesystems to fix
them on btrfs.

> If I wanted something fully automated, it'd be easy enough to setup a 
> script that checked what filesystem I was df-ing, matched that against a 
> table of filesystems to preferred df block sizes, and supplied the 
> appropriate -BxX option accordingly.

I am not sure this would work well in the network share scenario described
earlier, with clients which in the real world are largely Windows-based.

-- 
With respect,
Roman

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