Re: Provide a better free space estimate on RAID1

[Hugo Mills <hugo@carfax.org.uk>]

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On Sat, Feb 08, 2014 at 05:33:10PM +0600, Roman Mamedov wrote:
> On Fri, 07 Feb 2014 21:32:42 +0100
> Kai Krakow <hurikhan77+btrfs@gmail.com> wrote:
> 
> > It should show the raw space available. Btrfs also supports compression and 
> > doesn't try to be smart about how much compressed data would fit in the free 
> > space of the drive. If one is using RAID1, it's supposed to fill up with a 
> > rate of 2:1. If one is using compression, it's supposed to fill up with a 
> > rate of maybe 1:5 for mostly text files.
> 
> Imagine a small business with some 30-40 employees. There is a piece of paper
> near the door at the office so that everyone sees it when entering or leaving,
> which says:
> 
> "Dear employees,
> 
> Please keep in mind that on the fileserver '\\DepartmentC', in the directory
> '\PublicStorage7' the free space you see as being available needs to be divided
> by two; On the server '\\DepartmentD', in '\StorageArchive' and '\VideoFiles',
> multiplied by two-thirds. For more details please contact the IT operations
> team. Further assistance will be provided at the monthly training seminar.
> 
> Regards,
> John S, CTO.'

   In my experience, nobody who uses a shared filesystem *ever* looks
at the amount of free space on it, until it fills up, at which point
they may look at the free space and see "0". Or most likely, they'll
be alerted to the issue by an email from the systems people saying,
"please will everyone delete unnecessary files from the shared drive,
because it's full up."

   Having a more accurate estimate of the free space is a laudable
aim, and in principle I agree with attempts to do it, but I think the
argument above isn't exactly a strong one in practice.

   Even in the current code with only one RAID setting available for
data, if you have parity RAID, you've got to look at the number of
drives with available free space to make an estimate of available
space. I think your best bet, ultimately, is to write code to give
either a pessimistic (lower bound) or optimistic (upper bound)
estimate of available space based on the profiles in use and the
current distribution of free/unallocated space, and stick with that. I
think I'd prefer to see a pessimistic bound, although that could break
anything like an installer that attempts to see how much free space
there is before proceeding.

   Hugo.

-- 
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