James Bruce
15-712 Paper Review
Monday September 17th

"Virtual Log Based File Systems for a Programmable Disk"

Important Points:
 - Eager writing is fundamentally better than an update-in-place
 system given uniform distribution of free space over the disk.  As
 disk performance improves, this advantage increases.

 - The virtual log can enhance write performance, even for existing
 file systems, when implemented behind a standard disk interface.
 Current trends point in the direction of putting more powerful
 processing on the disk itself, and performance gains can be seen
 even with the same interface as current disks.

 - VLFS's chained log and eager writing avoid the problems of LFS's
 dependence on writing continuous logs.  In VLFS reads and writes need
 not interfere since eager writes can occur near reads, and eager
 writes handles syncing efficiently while plain LFS suffers in that
 case.  No semantic compromises or NVRAM is required in avoiding these
 issues.

Deficiencies:
  Does VLFS make reading even slower than LFS?  This detail is glossed
  over.  LFS's Achilles heel, even with enhancements, is read
  performance.  It may not actually get worse for VLFS, but no
  theoretical proof or experimental validation is performed.  The
  reader is only told that tools that help LFS will fix VLFS too,
  which is less than convincing.  Such tools would remove many of the
  listed advantages of VLFS over LFS, such as ability to use only
  short idle periods.

Conclusions:
  This paper shows how a more practical log based file system can be
  implemented using a technique called virtual logs.  The chained
  indirection map allow fast recovery given only a stable log tail
  pointer, and along with eager writes achieve high synchronous write
  performance.  This demonstrates that many of the performance
  benefits of LFS can be achieved without using modified sector
  formats, non-volatile storage, or sacrificing synchronous operation.