Question about some of the features in Hammer / DragonflyBSD

[Justin Sherrill]

It does not have its own volume manager; Hammer (1) isn't designed to be as
DWIMmy as ZFS was, though the ability to adapt more tools to Hammer2 is
being worked on.  It may do you some good to look at the design document
for Hammer:

http://www.dragonflybsd.org/hammer/hammer.pdf

For an anecdote: I was doing the bulk builds of pkgsrc on DragonFly for
quite a while.  I'd do the work on a building machine, and then rsync them
over to the host machine for distribution.  When I first set it up, I
reversed something in the rsync and managed to copy over the files and then
promptly delete all of them.  That's several gigabytes of files - somewhere
around 9 or 10,000 files.

I copied in the files, saw they were there, and put a statement in cron to
keep updating, and figurative walked away thinking I was done.  The next
day, someone said on IRC "Where did those files go?"  It wasn't a "oops I
deleted too much scenario" where some sort of undelete can fix things - it
was some 24 hours gone at that point.

However, since Hammer takes fine-grained snapshots automatically, I was
able to locate the time before it was all deleted, and copy the files back
into place from the snapshot with a single cp command.

I managed to do something similar at least one other time - deleted a
complex mess of files, and was able to put them all back immediately:

http://www.shiningsilence.com/dbsdlog/2009/12/08/5138.html



On Sun, May 4, 2014 at 5:38 AM, Olav Gjerde <olav at backupbay.com> wrote:

> Hello everyone,
>
> I'm going to have a presentation at work later this month for my
> superiors about next generation file systems. Hammer is of particular
> interest as it has some really nice features and strengths compared
> with other next generation file systems.
>
> However, as I understand, Hammer does not come with its own volume
> manager? You have to rely on another separate implementation to get
> raid? Does this mean that if a bad block is being read, that Hammer
> can't automatically repair it (as ZFS self healing)?
>
> Another question I have is if some of you have a small story to share
> about how you use Hammer and take good advantage of its features, as
> compared to a traditional file system.
>
> Kind regards,
> --
> Olav Grønås Gjerde
>
>
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