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RE: XFS Preallocation

To: "Dave Chinner" <david@xxxxxxxxxxxxx>
Subject: RE: XFS Preallocation
From: "Jef Fox" <jef.fox@xxxxxxxxxx>
Date: Fri, 28 Jan 2011 10:33:03 -0700
Cc: <xfs@xxxxxxxxxxx>
References: <155CAEA5D902E7429569DD197567724A01534D42@xxxxxxxxxxxxxxxxxxx> <20110128045205.GR21311@dastard>
Thread-index: Acu+pysludNFV+ZTSrKZi8mbuuVqeAAadf3Q
Thread-topic: XFS Preallocation
I guess, disregard my previous message.  After some further testing of
examining the hard disk blocks, we see what you are saying - the file is
presented at 0s to the user even if the blocks are changed on the hard
disk.  So, we will always see 0s until we write to the extent.

So, I think our only question now is if there is a way to force the
extents to be marked as allocated without writing all of the data?  That
is, is there a fast way to lay down a file(s) of 1G size without
actually writing 1G of info.

> -----Original Message-----
> From: Dave Chinner [mailto:david@xxxxxxxxxxxxx]
> Sent: Thursday, January 27, 2011 9:52 PM
> To: Jef Fox
> Cc: xfs@xxxxxxxxxxx
> Subject: Re: XFS Preallocation
> 
> On Thu, Jan 27, 2011 at 07:05:33PM -0700, Jef Fox wrote:
> > We are having some problems with preallocation of large files.  We
have
> > found that we can preallocate about 500 1GB files on a volume using
the
> > resvsp and truncate commands, but the extents are still showing up
as
> > preallocated.  Is this a problem?  The OS appears to think the files
are
> > allocated and correctly sized.
> 
> That's the way it's supposed to work. Preallocated space stays
> preallocated (i.e reads as zeros) until it is written to, regardless
> of whether you change the file size via truncate commands.
> 
> > For reference, we are trying to create files for an external piece
of
> > equipment to write to a SSD with.  The SSD would then be mounted in
RHEL
> > and the data pulled off in the 1G chunks.  Because of the nature of
the
> > data, we need to constantly erase and recreate the files and
> > preallocation seems to be the fastest option.
> 
> What do you mean by "erase and recreate"? Do you mean you rm the
> files, then preallocate them again?
> 
> If you were running 2.6.37+ and a TOT xfsprogs, there's also the
> "zero" command that converts allocated space back to the
> preallocated (zeroed) state without doing any IO. It's the
> equivalent unresvsp + resvsp in a single operation.
> 
> > We don't really care if
> > the data gets 0'ed out.  Is there another method - allocsp takes too
> > long for this application?
> 
> allocsp is historical interface, pretty much useless and should
> probably be removed. I can't think of any situation where allocsp
> would be better than resvsp or zero....
> 
> > Or, does it matter if XFS thinks the extents
> > are preallocated but unwritten if no other files are written to the
> > disk?
> 
> I'm not sure what you are asking there...
> 
> Cheers,
> 
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx

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