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

[Dave Chinner]

On Fri, Jan 28, 2011 at 10:08:42PM -0800, david@xxxxxxx wrote:
> 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
> >>>>>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.

Just because we can't do it right now doesn't mean it is not
possible. Array/raid controller vendors need to implement the SCSI
block limit VPD page, and if they do then stripe unit/stripe width
may be exposed for the device in sysfs. However, I haven't seen any
devices except for md and dm that actually export values that
reflect sunit/swidth in the files:

/sys/block/<dev>/queue/minimum_io_size
/sys/block/<dev>/queue/optimal_io_size

There's information about it here:

http://www.kernel.org/doc/ols/2009/ols2009-pages-235-238.pdf

But what we really need here is for RAID vendors to implement the
part of the SCSI protocol that gives us the necessary information.

> 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.

MD doesn't care whether the block devices are single disks or RAID
LUNS.  In this case, it's up to you to configure the md chunk size
appropriately for those devices. i.e. the MD chunk size needs to be
the RAID6 lun stripe width. If you get the MD config right, then
mkfs will do exactly the right thing without needing to be tweaked.
The same goes for any sort of heirarchical aggregation of storage -
if you don't get the geometry right at each level, then performance
will suck.

FWIW, SGI has been using XFS in complex, multilayer, multipath,
heirarchical configurations like this for 15 years.  What you
describe is a typical, everyday configuration that XFS is used on
and it is this sort of configuration we tend to optimise the
default behaviour for....

> 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.

Which means you're assuming a competent admin is running the tool,
in which case they could just run mkfs directly.  Anyway, it still
doesn't need mkfs changes.

> 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.

Writing such a script would be a good way to learn the art and
document the information that people are complaining that is
lacking. I don't have the time (or need) to write such a script, but
I can answer questions when they arise should someone decide to do
it....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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