On Apr 08, 2008 13:08 -0400, Josef Bacik wrote: > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
