Beginners questions

Cassandra von Ahrcanburg proudmouse at
Wed Dec 4 10:03:21 PST 2013

On Sun, 1 Dec 2013 22:11:29 +0100
Sven Gaerner <sgaerner at> wrote:

> ZFS has some features that are not available in HAMMER, but IMHO the
> biggest advantage of HAMMER is that it is running with less memory and
> it's running on 32bit systems, too. So there is no need for more than
> 4 GB just to use HAMMER. Also 1 to 2 GB should be fine.

Well, the machine I have chosen for this server will have 32GB of ECC
RAM, which should be enough either way. I'm guessing that the extra
memory won't get in Dragonfly's way. :-)
> Keeping the history is also a nice feature that is not available in
> ZFS. It's sort of built-in backup. This needs some extra space,
> especially if a lot of files are changed very often.

This is cold storage, so I would probably deactivate this feature
anyway - or at least reduce it.
> Why do you want something like a pool? Do you need to extend your
> storage that often? Or do you need one big file system?

Both. As written before, this is cold storage and things will usually
not get removed, but instead, the archive will tend to grow. The
problem is, that I can't really say where. So setting a mount point in
a good position within the archives directory tree is near to
impossible. What I have done until now (with UFS) is to give each drive
a mount point outside of the archive. Beside all of these there is also
a directory tree (without files) of for the archive. Then I connect the
last directory level with smylinks to the tree. This way, when I add a
drive, adding its contents to the tree is a matter of clevis, but in
terms of skill, rather trivial.

I was hoping, I could avoid the clevis part. Expanding a RAID would
take a while because the file system would have to copy stuff onto the
new drive. But waiting a day or so is not so much an issue.

> The master-slave copying (mirror-copy) helps a lot. If the allocated
> space grows, you can use a new disk, mirror-copy the whole FS while
> working and then just switch the mount points. This is very different
> from extending a pool, but in this case you get a backup volume for
> free.

You only get it for free if you get HDs for free. :-)
This is the main reason I am no big fan of mirrors: They are very
costly in terms of HD space. A mirror is cheap in terms of CPU time but
it offers no benefits compared to a RAID5 (in redundancy) while always
"eating" 50% of your disc space.

> If I remember correct creating a HAMMER FS on more than one volume is
> not recommended.

Just to be sure: In this context a volume could be a physical drive. So
creating a single fs over both ad1 and ad2 is a bad idea. I can
understand that, because this constellation would be something like a
RAID0. If one drive dies, all data is lost. If however ad1 and ad2 are
combined to a single volume either with vinum oder LVM, creating a
hammer on that would be ok.

Kind regards,

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