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Re: xfs: very slow after mount, very slow at umount

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: xfs: very slow after mount, very slow at umount
From: david@xxxxxxx
Date: Fri, 28 Jan 2011 22:08:42 -0800 (PST)
Cc: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>, Alex Elder <aelder@xxxxxxx>, Mark Lord <kernel@xxxxxxxxxxxx>
In-reply-to: <20110129054021.GB21311@dastard>
References: <4D40EB2F.2050809@xxxxxxxxxxxx> <4D418B57.1000501@xxxxxxxxxxxx> <alpine.DEB.2.00.1101271040000.31246@xxxxxxxxxxxxxxxx> <4D419765.4070805@xxxxxxxxxxxx> <4D41CA16.8070001@xxxxxxxxxxxxxxxxx> <alpine.DEB.2.00.1101271209370.17579@xxxxxxxxxxxxxx> <4D42056E.2050505@xxxxxxxxxxxxxxxxx> <alpine.DEB.2.00.1101271730030.17579@xxxxxxxxxxxxxx> <20110128135629.GX21311@dastard> <alpine.DEB.2.00.1101281119250.17579@xxxxxxxxxxxxxx> <20110129054021.GB21311@dastard>
User-agent: Alpine 2.00 (DEB 1167 2008-08-23)
On Sat, 29 Jan 2011, Dave Chinner wrote:

On Fri, Jan 28, 2011 at 11:26:00AM -0800, david@xxxxxxx wrote:
On Sat, 29 Jan 2011, Dave Chinner wrote:

On Thu, Jan 27, 2011 at 06:09:58PM -0800, david@xxxxxxx wrote:
On Thu, 27 Jan 2011, Stan Hoeppner wrote:
david@xxxxxxx 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

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

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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