First commit.

.gitignore

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+# Emacs
+*~

README.md

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+This script produces csv output giving *useful* details of the usage
+of ZFS snapshots.
+
+When you ask ZFS about the disk space used by snapshots, it really
+only wants to tell you about how much space is being *uniquely* used
+by individual snapshots. If a two adjacent snapshots use a Gigabyte of
+space between them, but have only 1 Megabyte of space unique to each
+of them, "zfs list" will tell you that each snapshot is consuming only
+a *one* Megabyte of space. This is helpful if you're trying to work
+out which snapshots to delete to free up diskspace.
+
+If you use "zfs destroy -nv filesystem@snap1%snap2" ZFS *will*
+actually tell you how much space the sequence of snapshots between
+snap1 and snap2 is using.  This is much more useful for working out
+which snapshots are using all the space.  In the example from the
+previous paragraph, this would tell you that deleting both snapshots
+together would 1.002 Gigabytes - obviously much more useful
+information.
+
+This script runs "zfs destroy -nv" for all pairs of snapshots in a
+filesystem, and shows how much space would be freed by deleting the
+corresponding sequence of snapshots.
+
+Output from this script:
+(a) converts sizes shown into bytes
+(b) strips any common prefix from snapshot names (e.g. "zfs-auto-snap")
+
+Options could be added to enable/disable this, but I can't be bothered.
+
+Example:
+  zfs-snapshot-disk-usage-matrix.py local-fast-tank-machine0/Virtual-Machines/VirtualBox/vpn-linux-u14 | tee vpn-snapshot-usage.csv

zfs-snapshot-disk-usage-matrix.py

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+#!/usr/bin/env python
+"""Usage: zfs-snapshot-disk-usage-matrix.py <filesystem>
+
+This script produces csv output giving useful details of the usage of
+ZFS snapshots.
+
+When you ask ZFS about the disk space used by snapshots, it really
+only wants to tell you about how much space is being *uniquely* used
+by individual snapshots. If a two adjacent snapshots use a Gigabyte of
+space between them, but have only 1 Megabyte of space unique to each
+of them, "zfs list" will tell you that each snapshot is consuming only
+a *one* Megabyte of space. This is helpful if you're trying to work
+out which snapshots to delete to free up diskspace.
+
+If you use "zfs destroy -nv filesystem@snap1%snap2" ZFS *will*
+actually tell you how much space the sequence of snapshots between
+snap1 and snap2 is using.  This is much more useful for working out
+which snapshots are using all the space.  In the example from the
+previous paragraph, this would tell you that deleting both snapshots
+together would 1.002 Gigabytes - obviously much more useful
+information.
+
+This script runs "zfs destroy -nv" for all pairs of snapshots in a
+filesystem, and shows how much space you can free up by deleting the
+corresponding sequence of snapshots.
+
+Output from this script:
+(a) converts sizes shown into bytes
+(b) strips any common prefix from snapshot names (e.g. "zfs-auto-snap")
+
+Options could be added to enable/disable this, but I can't be bothered.
+
+Example:
+  zfs-snapshot-disk-usage-matrix.py local-fast-tank-machine0/Virtual-Machines/VirtualBox/vpn-linux-u14 | tee vpn-snapshot-usage.csv
+
+"""
+
+from docopt import docopt
+
+import subprocess, sys, fcntl
+from os.path import commonprefix
+
+def strip_filesystem_name(snapshot_name):
+    """Given the name of a snapshot, strip the filesystem part.
+
+    We require (and check) that the snapshot name contains a single
+    '@' separating filesystem name from the 'snapshot' part of the name.
+    """
+    assert snapshot_name.count("@")==1
+    return snapshot_name.split("@")[1]
+
+def maybe_ssh(host):
+    if (host == 'localhost'):
+        ## no need to ssh host @ start of command - empty string
+        return ""
+    ##else
+    ## will need the ssh in there
+    return "ssh -C {}".format(host)
+
+def snapshots_in_creation_order(filesystem, host='localhost', strip_filesystem=False):
+    "Return list of snapshots on FILESYSTEM in order of creation."
+    result = []
+    cmd = "{} sudo zfs list -r -t snapshot -s creation {} -o name".format(maybe_ssh(host), filesystem)
+    lines = subprocess.check_output(cmd, stderr=subprocess.STDOUT, shell=True).split('\n')
+    snapshot_prefix = filesystem + "@"
+    for line in lines:
+        if line.startswith(snapshot_prefix):
+            result.append(line)
+    if strip_filesystem:
+        return map(strip_filesystem_name, result)
+    return result
+
+def space_between_snapshots(filesystem, first_snap, last_snap, host='localhost'):
+    "Space used by a sequence of snapshots."
+    cmd = "{} sudo  zfs destroy -nv {}@{}%{} | grep \"would reclaim\"".format(maybe_ssh(host), filesystem, first_snap, last_snap)
+    lines = subprocess.check_output(cmd, stderr=subprocess.STDOUT, shell=True).split('\n')
+    return lines[0].split()[-1]
+
+def print_csv(lines):
+    """Write out a list of lists as CSV.
+
+    Not robust against odd input."""
+    for line in lines:
+        for item in line:
+            if item <> None:
+                print item,
+            print ",",
+        print
+
+def size_in_bytes(size):
+    "Convert a ZFS size string into bytes"
+    if size.endswith("K"):
+        return float(size[:-1])*1024.0
+    if size.endswith("M"):
+        return float(size[:-1])*pow(1024.0,2)
+    if size.endswith("G"):
+        return float(size[:-1])*pow(1024.0,3)
+    if size.endswith("T"):
+        return float(size[:-1])*pow(1024.0,4)
+    return int(size)
+
+def write_snapshot_disk_usage_matrix(filesystem, suppress_common_prefix=True):
+    snapshot_names = snapshots_in_creation_order(filesystem, strip_filesystem=True)
+    if suppress_common_prefix:
+        suppressed_prefix_len = len(commonprefix(snapshot_names))
+    else:
+        suppressed_prefix_len = 0
+    print_csv([[None]+[name[suppressed_prefix_len:] for name in snapshot_names]]) # Start with Column headers
+    for end in range(len(snapshot_names)):
+        this_line = [snapshot_names[end][suppressed_prefix_len:]]
+        for start in range(len(snapshot_names)):
+            if start <= end:
+                start_snap = snapshot_names[start]
+                end_snap = snapshot_names[end]
+                space_used = space_between_snapshots(filesystem,
+                                                     start_snap,
+                                                     end_snap)
+                this_line.append(size_in_bytes(space_used))
+            else:
+                this_line.append(None)
+        ## Show line we've just done
+        print_csv([this_line])
+
+if __name__ == '__main__':
+    arguments=docopt(__doc__)
+    write_snapshot_disk_usage_matrix(arguments['<filesystem>'])
+
+# Useful for     
+# snapshots_in_creation_order('local-fast-tank-machine0/Virtual-Machines/VirtualBox/vpn-linux-u14')
+# space_between_snapshots('local-fast-tank-machine0/Virtual-Machines/VirtualBox/vpn-linux-u14',
+#                         'zfs-auto-snap_monthly-2015-03-18-2345',
+#                         'zfs-auto-snap_frequent-2015-09-28-0245')
+# write_snapshot_disk_usage_matrix('local-fast-tank-machine0/Virtual-Machines/VirtualBox/vpn-linux-u14')