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[ale] ZFS on Linux

On Mon, Apr 1, 2013 at 7:14 AM, Wolf Halton <wolf.halton at gmail.com> wrote:

> A gotcha on ZFS:
> If it ever fills up a drive array, for instance the backup process starts
> writing recursively, you can't remove anything, since it uses disk space to
> write a cache of what you delete.  You have to copy the files to a larger
> array and reformat your stuffed ZFS array.  Ask me how I know?

from Oracle Solaris ZFS Administration Guide ? August 2010 page 58

" Out of Space Behavior
File system snapshots are inexpensive and easy to create in ZFS. Snapshots
are common in most
ZFS environments. For information about ZFS snapshots, see Chapter 7,
?Working With
Oracle Solaris ZFS Snapshots and Clones.?
The presence of snapshots can cause some unexpected behavior when you
attempt to free disk
space. Typically, given appropriate permissions, you can remove a file from
a full file system,
and this action results in more disk space becoming available in the file
system.However, if the
file to be removed exists in a snapshot of the file system, then no disk
space is gained from the
file deletion. The blocks used by the file continue to be referenced from
the snapshot.
As a result, the file deletion can consume more disk space because a new
version of the directory
needs to be created to reflect the new state of the namespace. This
behavior means that you can
receive an unexpected ENOSPC or EDQUOT error when attempting to remove a

yeah. that looks fun. So snapshots are a double-edged sword.

The data deduplication is very useful until it's backup time. It looks like
the backup will un-deduplicate and use full-size storage less backup
compression abilities.

James P. Kinney III
*Every time you stop a school, you will have to build a jail. What you gain
at one end you lose at the other. It's like feeding a dog on his own tail.
It won't fatten the dog.
- Speech 11/23/1900 Mark Twain
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