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

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Date:	Mon, 28 Mar 2011 14:27:09 -0400
From:	Oren Elrad <elrad@...ndeis.edu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

On Mon, Mar 28, 2011 at 2:06 PM, Eric Sandeen <sandeen@...hat.com> wrote:
> On 3/28/11 1:02 PM, Oren Elrad wrote:
>> Undesired behavior; mke2fs defaults to reserving 5% of the volume for
>> the root user. 5% of a 2TB volume is 100GB. The rationale for root
>> reservation (syslogd, etc...) does not require 100GB. As volumes get
>> larger, this default makes less and less sense.
>>
>> Proposal; If the user does not specify their preferred reserve_ratio
>> on the command-line (-m), use the less of 5% or MAX_RSRV_SIZE. I
>> propose 10GiB as a sensible maximum default reservation for root.
>>
>> Patch: Follows and http://capsid.brandeis.edu/~elrad/e2fsprog.gitdiff
>>
>> Tested on the latest git+patch, RHEL5 (2.6.18-194.17.1.el5) with a
>> 12TB volume (which would reserve 600GB under the default!):
>
> There's been a bit of debate about this; is the space really saved
> for root, or is it to stop the allocator from going off the rails
> when the fs nears capacity?  Both, really.
>
> I don't really have a horse in the race, but the complaint has certainly
> come up before... it's just important to realize that the space isn't
> only there for root's eventual use.
>
> No other fs that I know of enforces this "don't fill the fs to capacity"
> common sense programatically, though.
>
> -Eric
>

[SNIP]

Well, in my version you still get some reservation to prevent whatever
woes (fragmentation, allocator slow-down) that accompany a nearly-full
disk. If you think 25 or 50GiB is a more appropriate maximum default,
I have no objections.

Whatever the reason for reservation, more than 100GB is totally
nonsensical IMHO.

Oren Elrad
Dept. of Physics
Brandeis University
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