Greetings.

 I'm about to build a file server with at least a TB of disk space.  I'm
currently planning on hardware RAID-5, probably SATA. I've read a thread
or two about this in the NetBSD mailing list archives. Obviously, the file
systems are going to be big, in the hundreds of gigabytes.  A number of
things get more complicated with file systems this large, at least on a
limited budget like mine. Two items in particular are:

 - long fsck times for FFS
 - backups

 Two features of soft updates are particularly attractive to me in this
regard.

 First, if i understand things correctly, one of the helpful features of
soft updates (AKA soft dependencies) is supposedly the ability to mount a
dirty file system and fsck it while the file system is active.

 Second, soft updates includes a snapshot feature that can snapshot a file
system periodically. This would enable me to quickly and easily restore
overwritten, deleted, or changed files.  Obviously this isn't a complete
substitute for backups, but it could help reduce the required frequency of
backups to a seperate (or even remote) disk array.  (I can't really
imagine backing a 500 GB file system to a tape drive.  There are tape
drives that can hold hundreds of GBs, but i can't afford them. :)


 My questions are:

1) How well does NetBSD >=3 support fsck of a dirty FFS file
systems that were mounted with the "softdep" option before the event that
made them dirty?

2) How well does NetBSD >=3 support the snapshot functionality?  I see
that there's a kernel option to enable it.  Is it considered stable and
reliable? What opinion do people who have used it have of it?


Regarding question #1, i've done a very simple and preliminary test on
3.0_STABLE/sparc64.  I simply cut power to the sparc64 machine while it
was running this command:

dd if=/dev/zero of=/local/foo

(/local was mounted with the "softdep" option.)

 When the system came up, it began to fsck all the file systems.  When it
got to /local, i typed ^C. fsck exited, and the system did mount the file
system.  Good so far.

 However, when i fsck'd the active file system, i did need to give fsck
the -f arg. to force it to check a mounted file system.  Upon startup, it
informs me that it's running "NO WRITE".  When fsck finished, it would
make no changes to the file system, even when i gave it the "-y"
argument. Upon exit, fsck noted that "UNRESOLVED INCONSISTENCIES
REMAIN". I also tried the fsck_ffs command, with the same results.

 So it would appear that while i can mount a dirty file system, i cannot
clean it.  And to clean it, i would eventually need to fsck the file
system while it was unmounted. Is this correct?


 -johan

References:

http://www.mckusick.com/softdep/