Obtaining real-world aged file systems
Posted Jul 7, 2006 2:46 UTC (Fri) by dlang
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In reply to: Obtaining real-world aged file systems
Parent article: The 2006 Linux Filesystems Workshop (Part III)
the problem with aged file systems is that you may have a file system (say ext2) that has been in use for years, with lots of stuff added at different times.
now you have a new filesystem (newfs) that you want to test.
if you just copy everything from the old filesystem to the new one you end up with a optimaly layed out newfs filesystem since all the files were created at one time, useually with each file being created in one operation (no fragmentation). The resulting performance is vastly different then if the same contents had been put there the same way they were put on the origional ext2fs filesystem.
so the next step is to not record the filesystem, but record the operations on the filesystem (each write, delete, create, etc) and replay those against the new filesystems.
but since modern filsystems delay allocations for several seconds when something is done to them the replays end up with many of the operations canceling out in memory (never hitting disk) and so the result still doesn't match.
the work-around for this is to do lots of sync's to force the filesystem to actually perform the writes to disk instead of short-circuiting them.
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