Does your application depend on, or report, free disk space? Re: F20 Self Contained Change: OS Installer Support for LVM Thin Provisioning

[Dennis Jacobfeuerborn]

On 27.07.2013 05:07, Chris Murphy wrote:
>
> On Jul 26, 2013, at 4:53 PM, Pádraig Brady <P at draigBrady.com> wrote:
>
>> On 07/26/2013 09:13 PM, Miloslav Trmač wrote:
>>> Hello all,
>>> with thin provisioning available, the total and free space values
>>> reported by a filesystem do not necessarily mean that that much space
>>> is _actually_ available (the actual backing storage may be smaller, or
>>> shared with other filesystems).
>>>
>>> If your package reports disk space usage to users, and bases this on
>>> filesystem free space, please consider whether it might need to take
>>> LVM thin provisioning into account.
>>>
>>> The same applies if your package automatically allocates a certain
>>> proportion of the total or available space.
>>>
>>> A quick way to check whether your package is likely to be affected, is
>>> to look for statfs() or statvfs() calls in C, or the equivalent in
>>> your higher-level library / programming language.
>>
>> Anything df(1) should do here?
>
> Example: Creating a btrfs raid1 volume from two 2TB drives, df shows it as having 4TB available:
>
> # parted -l
>
> Error: /dev/sdb: unrecognised disk label
> Model: ATA VBOX HARDDISK (scsi)
> Disk /dev/sdb: 2199GB
> Sector size (logical/physical): 512B/512B
> Partition Table: unknown
> Disk Flags:
>
> Error: /dev/sdc: unrecognised disk label
> Model: ATA VBOX HARDDISK (scsi)
> Disk /dev/sdc: 2199GB
> Sector size (logical/physical): 512B/512B
> Partition Table: unknown
> Disk Flags:
>
> # mkfs.btrfs -d raid1 -m raid1 /dev/sd[bc]
>
> WARNING! - Btrfs v0.20-rc1 IS EXPERIMENTAL
> WARNING! - see http://btrfs.wiki.kernel.org before using
>
> adding device /dev/sdc id 2
> fs created label (null) on /dev/sdb
> 	nodesize 4096 leafsize 4096 sectorsize 4096 size 4.00TB
> Btrfs v0.20-rc1
>
> # mount /dev/sdb /mnt
> #  df -h
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sda1        79G  4.2G   71G   6% /
> devtmpfs        1.5G     0  1.5G   0% /dev
> tmpfs           1.5G     0  1.5G   0% /dev/shm
> tmpfs           1.5G  680K  1.5G   1% /run
> tmpfs           1.5G     0  1.5G   0% /sys/fs/cgroup
> tmpfs           1.5G  4.0K  1.5G   1% /tmp
> none            224G   87G  138G  39% /media/sf_chris
> /dev/sdb        4.0T   56K  4.0T   1% /mnt
>
>
> The explanation is that the file system isn't raid1, but rather the allocated chunks have this attribute. Presently a volume only allocates with one profile, but the future plan is per subvolume and even per file raid profiles. So establishing how much free space there is on a btrfs volume is absolutely less than clear.
>
> Anyway, I think it will cause some confusion if by "available" an application thinks it can write out more than 2TB of data to this example volume.

I thought one of the features of combining the block layer and 
filesystem layer like btrfs does is that the filesystem can actually 
know the state/topology of the block layer and work more efficiently. 
Combined with the already existing problem of getting out of diskspace 
errors long before use hits 100% (has this been fixed since?) this makes 
any sort of capacity planning difficult if not impossible.

Regards,
   Dennis