From owner-freebsd-hackers@FreeBSD.ORG  Sat Jun 16 19:38:00 2012
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Date: Sat, 16 Jun 2012 12:37:58 -0700
Message-ID: <CAGMYy3sWPMCnttUtJhJCFTtPtgfWWUO1tgK+RYOE3QQpg=+w1A@mail.gmail.com>
From: Xin LI <delphij@gmail.com>
To: Chris Rees <utisoft@gmail.com>
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Cc: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>,
	freebsd-hackers@freebsd.org
Subject: Re: BIO_DELETE equivalent for file on FFS filesystem
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On Sat, Jun 16, 2012 at 12:01 PM, Chris Rees <utisoft@gmail.com> wrote:
> On Jun 14, 2012 5:49 AM, "Wojciech Puchar" <wojtek@wojtek.tensor.gdynia.pl>
> wrote:
>>>>
>>>> file to take 900MB or... can i call some system function to "punch"
>>>> holes?
>>>
>>>
>>> I think you can only truncate the file at this time, pretty much like
>>> brk() works for memory.
>>
>>
>>
>> BAD. suppose i keep windoze VM image on filesystem which takes 10GB but
> uses 5GB.
>>
>> i could write simple program to find out what blocks are unused and
> then...do nothing.
>>
>
> What if you cp it?

That would be a dd(1) unless we teach cp(1) how to do sparse.  I think
what he wanted is to tell the OS "I don't need block XX - YY anymore"
and the OS creates a sparse hole, which is not available at this time.

Cheers,
-- 
Xin LI <delphij@delphij.net> https://www.delphij.net/
FreeBSD - The Power to Serve! Live free or die