zfs discuss - Jul 2010 - zfs - filesystem versus directory

Folks,

While going through a quick tutorial on zfs, I came across a way to create zfs
filesystem within a filesystem. For example:

# zfs create mytest/peter

where mytest is a zpool filesystem.

When does this way, the new filesystem has the mount point as /mytest/peter.

When does it make sense to create such a filesystem versus just creating a
directory?

# mkdir mytest/peter

Thank you in advance for your help.

Regards,
Peter
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This message posted from opensolaris.org

I created a zpool called ''data'' from 7 disks.
I created zfs filesystems on the zpool for each Xen vm

I can choose to recursively snapshot all ''data''
I can choose to snapshot the individual ''directories''

If you use mkdir, I don''t believe you can snapshot/restore  at that
level


Malachi de ?lfweald
http://www.google.com/profiles/malachid


On Thu, Jul 1, 2010 at 9:12 PM, Peter Taps <ptrtap at yahoo.com> wrote:
> Folks,
>
> While going through a quick tutorial on zfs, I came across a way to create
> zfs filesystem within a filesystem. For example:
>
> # zfs create mytest/peter
>
> where mytest is a zpool filesystem.
>
> When does this way, the new filesystem has the mount point as
> /mytest/peter.
>
> When does it make sense to create such a filesystem versus just creating a
> directory?
>
> # mkdir mytest/peter
>
> Thank you in advance for your help.
>
> Regards,
> Peter
> --
> This message posted from opensolaris.org
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
>
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On 07/ 2/10 04:12 PM, Peter Taps wrote:
> Folks,
>
> While going through a quick tutorial on zfs, I came across a way to create
zfs filesystem within a filesystem. For example:
>
> # zfs create mytest/peter
>
> where mytest is a zpool filesystem.
>
> When does this way, the new filesystem has the mount point as
/mytest/peter.
>
>    
When you want different attributes (including delegation) for a 
directory, or you want finer grain snapshots
> When does it make sense to create such a filesystem versus just creating a
directory?
>
>    
When you don''t want any of the above.

-- 
Ian.

> From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-
> bounces at opensolaris.org] On Behalf Of Peter Taps
> 
> # zfs create mytest/peter
> 
> where mytest is a zpool filesystem.
> 
> When does it make sense to create such a filesystem versus just
> creating a directory?
This is a thorny bush, with flowers and fruit hidden in it.  ;-)

For most people, in most cases, it doesn''t make sense to make subdirs
be
separate zfs filesystems, just because you have to know what you''re
doing in
order to avoid unexpected gotchas.  But it''s common in some situations,
like
when you''re hosting a lot of different peoples'' home dirs and
stuff.

If you have nested zfs filesystems, you can use "df" instead of
"du" to
check how much space is used by that thing.  Almost instant result, instead
of walking the tree.

With nesting, your snapshots are managed separately.  This could be a pro or
a con for you.

With nesting, you cannot access a subdir''s snapshots by accessing the
snapshots of the parent.  The snapshots of a given file exist *only* in the
root of that filesystem, under the ".zfs" hidden directory.

When you "zfs send" you cannot exclude subdirectories.  But a nested
filesystem isn''t a subdirectory, and therefore isn''t included
in the
parent''s zfs send.

If you export a filesystem via "share" I believe its nested children
filesystems are not shared.  But if you export using the "sharenfs"
property
and allow property inheritance, then the children are exported too.  If you
have a solaris nfs client, it somehow automatically knows about nested
shares and it''s able to access the nfs nested filesystems via nfs, but
if
you have linux ... it doesn''t know.  So if you want access to the
nested nfs
mounts via linux client, you have to configure them.  (So I believe.)

What else ...  Basically, all the properties and operations that you can do
on zfs filesystems, you can suddenly do with finer granularity, by using
nested filesystems.  That''s a pro.  But generally speaking, you
suddenly
*have to* do with finer granularity by using nested filesystems. 
That''s a
con.

If you have a large number of nested filesystems, your boot time can be very
long.  I think the filesystem mount time scales linearly with the number of
filesystems ... I am guessing from the top of my head ... 0.5 seconds per
filesystem.

If you have one big filesystem, "mv /tank/somedir/somefile
/tank/someotherdir/somefile" is instant.  If you have separate filesystems,
"mv /tank/somefilesystem/somefile /tank/anotherfs/somefile" will need
to
read and write all the bytes of the file.

Thank you all, especially Edward, for the enlightenment.

Regards,
Peter
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