linux-ext4 - Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

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Date:	Tue, 29 Mar 2011 10:26:29 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Theodore Tso <tytso@....edu>
CC:	Oren Elrad <elrad@...ndeis.edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

On 3/29/11 9:05 AM, Theodore Tso wrote:
> 
> On Mar 28, 2011, at 2:06 PM, Eric Sandeen wrote:
>> 
>> No other fs that I know of enforces this "don't fill the fs to
>> capacity" common sense programatically, though.
> 
> Actually, we (ext2) copied this from the BSD Fast File System (FFS)
> which used a default MINFREE of 10%.   For ext2 we decided to bring
> it down to 5%.   FreeBSD currently uses 8% as their default free
> ratio.

Clearly I don't know enough filesystems, I guess ;)

Should have said "linux filesystem" perhaps.

> The decrease does seem to be relative to the percentage of free
> space, from empirical experience, although no one I know of has done
> a formal analysis of the slowdown.   A lot depends on your workload,
> how much memory pressure you place on your system, etc.  I've
> actually started seeing slowdowns starting as early as 80% full when
> you're trying to allocate large chunks (1M to 8M) at a time, although
> this isn't something where I've gathered hard data; just what I've
> noticed from looking at different systems and their performance
> characteristics.
> 
> Fortunately disks are cheap, and lots of people end up buying far
> more disk space than they need, and so they naturally keep their file
> systems well under 75-80% full.
> 
> If someone wants to add some tuning parameters to mke2fs.conf, so
> they can set their own personal default free ratios, or even
> min_reserved_blocks and max_reserved_blocks settings, that's probably
> a reasonable patch to e2fsprogs that I'd be willing to accept. 

Hm I thought I had sent that, but it was only for the other two
semi-controversial behaviors.  :)

I agree, it seems like at least a decent first step to make it
more site/admin-configurable. 

-Eric

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