Date:	Fri, 28 Jan 2011 22:08:42 -0800 (PST)
From:	david@...g.hm
To:	Dave Chinner <david@...morbit.com>
cc:	Stan Hoeppner <stan@...dwarefreak.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	Christoph Hellwig <hch@...radead.org>,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	Alex Elder <aelder@....com>, Mark Lord <kernel@...savvy.com>
Subject: Re: xfs: very slow after mount, very slow at umount

On Sat, 29 Jan 2011, Dave Chinner wrote:

> On Fri, Jan 28, 2011 at 11:26:00AM -0800, david@...g.hm wrote:
>> On Sat, 29 Jan 2011, Dave Chinner wrote:
>>
>>> On Thu, Jan 27, 2011 at 06:09:58PM -0800, david@...g.hm wrote:
>>>> On Thu, 27 Jan 2011, Stan Hoeppner wrote:
>>>>> david@...g.hm put forth on 1/27/2011 2:11 PM:
>>>>>
>>>>> Picking the perfect mkfs.xfs parameters for a hardware RAID array can be
>>>>> somewhat of a black art, mainly because no two vendor arrays act or perform
>>>>> identically.
>>>>
>>>> if mkfs.xfs can figure out how to do the 'right thing' for md raid
>>>> arrays, can there be a mode where it asks the users for the same
>>>> information that it gets from the kernel?
>>>
>>> mkfs.xfs can get the information it needs directly from dm and md
>>> devices. However, when hardware RAID luns present themselves to the
>>> OS in an identical manner to single drives, how does mkfs tell the
>>> difference between a 2TB hardware RAID lun made up of 30x73GB drives
>>> and a single 2TB SATA drive? The person running mkfs should already
>>> know this little detail....
>>
>> that's my point, the person running mkfs knows this information, and
>> can easily answer questions that mkfs asks (or provide this
>> information on the command line). but mkfs doesn't ask for this
>> infomation, instead it asks the user to define a whole bunch of
>> parameters that are not well understood.
>
> I'm going to be blunt - XFS is not a filesystem suited to use by
> clueless noobs. XFS is a highly complex filesystem designed for high
> end, high performance storage and therefore has the configurability
> and flexibility required by such environments. Hence I expect that
> anyone configuring an XFS filesystem for a production environments
> is a professional and has, at minimum, done their homework before
> they go fiddling with knobs. And we have a FAQ for a reason. ;)
>
>> An XFS guru can tell you
>> how to configure these parameters based on different hardware
>> layouts, but as long as it remains a 'back art' getting new people
>> up to speed is really hard. If this can be reduced down to
>>
>> is this a hardware raid device
>>   if yes
>>     how many drives are there
>>     what raid type is used (linear, raid 0, 1, 5, 6, 10)
>>
>> and whatever questions are needed, it would _greatly_ improve the
>> quality of the settings that non-guru people end up using.
>
> As opposed to just making mkfs DTRT without needing to ask
> questions?

but you just said that mkfs couldn't do this with hardware raid because it 
can't "tell the difference between a 2TB hardware RAID lun made up of 
30x73GB drives and a single 2TB SATA drive" if it could tell the 
difference, it should just do the right thing, but if it can't tell the 
difference, it should ask the user who can give it the answer.

also, keep in mind that what it learns about the 'disks' from md and dm 
may not be the complete picture. I have one system that thinks it's doing 
a raid0 across 10 drives, but it's really 160 drives, grouped into 10 
raid6 sets by hardware raid, than then gets combined by md.

I am all for the defaults and auto-config being as good as possible (one 
of my biggest gripes about postgres is how bad it's defaults are), but whe 
you can't tell what reality is, ask the admin who knows (or at least have 
the option of asking the admin)

> If you really think an interactive mkfs-for-dummies script is
> necessary, then go ahead and write one - you don't need to modify
> mkfs at all to do it.....

it doesn't have to be interactive, the answers to the questions could be 
comand-line options.

as for the reason that I don't do this, that's simple. I don't know enough 
of the black arts to know what the logic is to convert from knowing the 
disk layout to setting the existing parameters.

David Lang
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