Managing metadata under Veritas File System
Rick Cook
The Veritas (www.veritas.com) File System (VxFS) is a journaling file system for handling backup, recovery and restoration. Its performance benefits from careful planning to avoid significant performance problems when it is managing many files.
This is sometimes called the "small files problem," although it isn't actually a problem with file size at all. Rather, the number of files and the way that VxFS handles information on files causes the difficulty.
VxFS uses metadata to store directory information and intent logs, usually putting this information at the beginning of the file system. Having such vital information collected in one place enhances performance, but as the number of files grows, so does the number of metadata entries. Keeping the metadata constantly updated on millions of files steals back a significant fraction of the performance gains the VxFS system provides.
According to Karl Larson and Todd Stansell, in their presentation "Improving availability in Veritas environments" (www.vxideas.org/lisa2000_veritas.ppt), on the Vxideas web site, the solution depends on which kind of information is causing the performance problem. If the problem comes from a lot of writes to the intent log you should consider moving them to dedicated storage. The Quicklog facility lets you move just the intent logs to other disks.
If directory updates are what is slowing you down, Larson and Stansell recommend striping across more disks to improve read-write performance.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
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