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

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Message-ID: <4D90D3DD.9000709@redhat.com>
Date:	Mon, 28 Mar 2011 13:30:53 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Oren Elrad <elrad@...ndeis.edu>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mke2fs reserved_ratio default value is nonsensical

On 3/28/11 1:27 PM, Oren Elrad wrote:
> 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.

the question is, how much is enough?  (isn't that always the question?) :)
What constitutes "nearly full?"

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

That depends; 1% sounds small, until the total is a Petabyte.

For overall performance, it may well be the % that matters, not the 
absolute number.  It really could probably use more real investigation,
and less hand-waving (of which I am also guilty).  :)

-Eric

> Oren Elrad
> Dept. of Physics
> Brandeis University

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