On Apr 08, 2008  13:08 -0400, Josef Bacik wrote:
quoted text
> On Tue, Apr 08, 2008 at 09:33:32AM -0700, Bock, Tony wrote:
> > Is there any ext3 feature that would cause disk layout to vary if
> > one were to delete all the files on disk and then rewrite them in the
> > same order?  We've been doing some simple read/write file system tests
> > in our lab that sometimes result in short delays being distributed
> > throughout several files.  As long as we don't re-wipe the drive,
> > these delays occur at repeatable offsets within the affected files,
> > suggesting disk seeks at the affected locations.
> > 
> > To my understanding, deleting a file should mark all the data blocks
> > as free.  Thus, any previous disk state should no longer affect files
> > that are written thereafter.  Is this correct?
> 
> As a general rule you cannot re-use the blocks (in ordered mode anyway)
> that are involved in a truncate until the transaction for the truncate
> is completed in order to make sure that everything is consistent after
> a recovery in the case of a crash.  So if you are rm -rf *'ing and then
> immediately re-writing stuff you are going to end up with weirdness, as
> there will be blocks that cannot be reused until the entire truncate is
> completed.  Best bet is to do  you rm -rf * and then run sync, and then
> do your writing, that should garuntee that the blocks that you have freed
> up from the truncate are actually able to be used by new stuff.

In addition, every 125MB or so there has to be a break in the file data
because of metadata.  In ext4 with the "flex_bg" patch the metadata will
be more localized and spread around 16GB apart (configurable).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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