USENIX Technical Program - Abstract - USENIX Annual
Conference, General Session - June 2000
Journaling Versus Soft Updates: Asynchronous Meta-data Protection in
Margo I. Seltzer, Harvard University; Gregory R. Granger, Carnegie
Mellon University; M. Kirk McKusick, Author & Consultant; Keith A.
Smith, Harvard University; Craig A. N. Soules, Carnegie Mellon
University; Christopher A. Stein, Harvard University
The UNIX Fast File System (FFS) is probably the most widely-used file
system for performance comparisons. However, such comparisons frequently
overlook many of the performance enhancements that have been added over
the past decade. In this paper, we explore the two most commonly used
approaches for improving the performance of meta-data operations and
recovery: journaling and Soft Updates. Journaling systems use an
auxiliary log to record meta-data operations and Soft Updates uses
ordered writes to ensure meta-data consistency.
The commercial sector has moved en masse to journaling file systems, as
evidenced by their presence on nearly every server platform available
today: Solaris, AIX, Digital UNIX, HP-UX, Irix, and Windows NT. On all
but Solaris, the default file system uses journaling. In the meantime,
Soft Updates holds the promise of providing stronger reliability
guarantees than journaling, with faster recovery and superior
performance in certain boundary cases.
In this paper, we explore the benefits of Soft Updates and journaling,
comparing their behavior on both micro-benchmarks and workload-based
macrobenchmarks. We find that journaling alone is not sufficient to
"solve" the meta-data update problem. If synchronous semantics are
required (i.e., meta-data operations are durable once the system call
returns), then the journaling systems cannot realize their full
potential. Only when this synchronicity requirement is relaxed can
journaling systems approach the performance of systems like Soft Updates
(which also relaxes this requirement). Our asynchronous journaling and
Soft Updates systems perform comparably in most cases. While Soft
Updates excels in some meta-data intensive microbenchmarks, the
macrobenchmark results are more ambiguous. In three cases Soft Updates
and journaling are comparable. In a file intensive news workload,
journaling prevails, and in a small ISP workload, Soft Updates prevails.
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