Question regarding memory disks

[Shrivats]

Thanks a lot for the response. 

--- On Wed, 1/6/10, Julian Elischer <julian at elischer.org> wrote:

From: Julian Elischer <julian at elischer.org>
Subject: Re: Question regarding memory disks
To: "Pieter de Goeje" <pieter at degoeje.nl>
Cc: freebsd-hackers at freebsd.org, "shrivatsan" <shrivatsan at gmail.com>
Date: Wednesday, January 6, 2010, 7:27 PM

Pieter de Goeje wrote:
> On Wednesday 06 January 2010 22:49:44 shrivatsan wrote:
>> Hi,
>> 
>> I have configured a malloc-backed memory disk, and I mount the device on to
>> the file system. I write some data onto the file system. I see that the
>> free memory indicated by kmem_map_free goes down, and this is proportional
>> to the size of the data written. However, even after removing all the
>> data, kmem_map_free doesn't seem to go up. Its only after detaching the
>> memory disk does the free memory go up. May I know the reason for this
>> behavior?
>> 
>> 
>> Thanks,
>> -shrivatsan
> 
> Because when you "erase" something, all it does is unlink (delete the reference to) the data. So there is currently no way the memory disk can free the memory associated with the data. That is also why you should normally use swap backed memory disks instead, or use tmpfs. These can return memory to the system.
> 
> The ability of the filesystem to mark certain blocks as "erased" is important not only for memory disks but also for solid state drives. It is a feature UFS2 is currently lacking unfortunately.

but is being worked on

> 
> - Pieter
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