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Operating Systems Solaris Understanding ZFS Snapshots - why will it utilize space ? Post 303013375 by javanoob on Tuesday 20th of February 2018 12:44:45 PM
Old 02-20-2018
Understanding ZFS Snapshots - why will it utilize space ?

Hi all,

I am moving to Solaris11 and is trying to understand how ZFS snapshot works.

Chances upon this Oracle Blog and can't wrap my head around it.
https://blogs.oracle.com/solaris/understanding-the-space-used-by-zfs-v2

Hope gurus here can shed some light .

=======

Here it goes,

I understand snapshot works on copy on write mechanism, so if there isn't any changes, there wouldn't be extra space taken.

q1) Why is it that when file1 is deleted, both snap1 and snap2 isn't utilizing any space ?

Code:
bleonard@os200906:~# rm /tank/file1
bleonard@os200906:~# zpool scrub tank
bleonard@os200906:~# zfs list -t all -r tank
NAME         USED  AVAIL  REFER  MOUNTPOINT
tank         200M   784M   100M  /tank
tank@snap1    17K      -   100M  -
tank@snap2    17K      -   200M  -

Yet when using zfs list as below, it is showing that 100M is use by snapshot
(USEDSNAP - 100M)

Code:
 
 bleonard@os200906:~# zfs list -t all -o space -r tank
NAME        AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
tank         784M   200M      100M    100M              0       154K

So when file1 is deleted, snap1 and snap2 is not utilizing any space but 100M is used by snapshots ?...

==============

q2) However, when snap1 is deleted, snap2 will show that it is using 100M of space.. why so ?

Code:
 
 bleonard@os200906:~# zfs destroy tank@snap1
bleonard@os200906:~# zfs list -t all -r tank
NAME         USED  AVAIL  REFER  MOUNTPOINT
tank         200M   784M   100M  /tank
tank@snap2   100M      -   200M  -

SmilieSmilieSmilie
I am sure I am looking at this the wrong way...

Regards,
Noob
 

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AMZFS-SNAPSHOT(8)					  System Administration Commands					 AMZFS-SNAPSHOT(8)

NAME
amzfs-snapshot - Amanda script to create zfs snapshot DESCRIPTION
amzfs-snapshot is an Amanda script implementing the Script API. It should not be run by users directly. It create a zfs snapshot of the filesystem where the path specified is mounted. PRE-DLE-* create a snapshot and the POST-DLE-* destroy the snapshot, *-DLE-AMCHECK, *-DLE-ESTIMATE and *-DLE-BACKUP must be set to be executed on the client: execute-on pre-dle-amcheck, post-dle-amcheck, pre-dle-estimate, post-dle-estimate, pre-dle-backup, post-dle-backup execute-where client The PRE_DLE_* script output a DIRECTORY property telling where the directory is located in the snapshot. The application must be able to use the DIRECTORY property, amgtar can do it. The script is run as the amanda user, it must have the priviledge to create and destroy snapshot: zfs allow -ldu AMANDA_USER mount,snapshot,destroy FILESYSTEM Some system doesn't have "zfs allow", but you can give the Amanda backup user the rights to manipulate ZFS filesystems by using the following command: usermod -P "ZFS File System Management,ZFS Storage Management" AMANDA_USER This will require that your run zfs under pfexec, set the PFEXEC property to YES. The format of the DLE must be one of: Desciption Example ---------- ------- Mountpoint /data Arbitrary mounted dir /data/interesting_dir ZFS pool name datapool ZFS filesystem datapool/database ZFS logical volume datapool/dbvol The filesystem must be mounted. PROPERTIES
This section lists the properties that control amzfs-snapshot's functionality. See amanda-scripts(7) for information on the Script API, script configuration. DF-PATH Path to the 'df' binary, search in $PATH by default. ZFS-PATH Path to the 'zfs' binary, search in $PATH by default. PFEXEC-PATH Path to the 'pfexec' binary, search in $PATH by default. PFEXEC If "NO" (the default), pfexec is not used, if set to "YES" then pfexec is used. EXAMPLE
In this example, a dumptype is defined to use amzfs-snapshot script to create a snapshot and use amgtar to backup the snapshot. define script-tool amzfs_snapshot { comment "backup of zfs snapshot" plugin "amzfs-snapshot" execute-on pre-dle-amcheck, post-dle-amcheck, pre-dle-estimate, post-dle-estimate, pre-dle-backup, post-dle-backup execute-where client #property "DF-PATH" "/usr/sbin/df" #property "ZFS-PATH" "/usr/sbin/zfs" #property "PFEXEC-PATH" "/usr/sbin/pfexec" #property "PFEXEC" "NO" } define dumptype user-zfs-amgtar { dt_amgtar script "amzfs_snapshot" } SEE ALSO
amanda(8), amanda.conf(5), amanda-client.conf(5), amanda-scripts(7) The Amanda Wiki: : http://wiki.zmanda.com/ AUTHORS
Jean-Louis Martineau <martineau@zmanda.com> Zmanda, Inc. (http://www.zmanda.com) Dustin J. Mitchell <dustin@zmanda.com> Zmanda, Inc. (http://www.zmanda.com) Amanda 3.3.1 02/21/2012 AMZFS-SNAPSHOT(8)
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