Lee H. Marzke on 9 Dec 2011 19:44:49 -0800

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Re: [PLUG] Lost gigabytes?

> From: "Rich Freeman" <r-plug@thefreemanclan.net>
> Actually one of my gripes with ZFS (which could have changed since I
> last checked) was that it didn't permit re-shaping RAID-Z (the COW
> equivalent of RAID5).  Linux software raid does allow online resizing
> of raid5.  Btrfs doesn't support RAID5 at all yet but I saw a patch
> on
> the mailing list - probably doesn't support resizing yet unless it
> just happens to work due to architecture.

Don't know if Btrfs and ZFS are the same or not here.

With ZFS you Normally group your disks
into sets of 2-disk mirrors, or 3-disk RaidZ vDevs and the zpool
is then striping data across all of them.

You then create folders and set quota.  Normally you only have
a single zpool.  So you don't create data inside the RaidZ, you
use many RaidZ mirror sets to form the pool and then allocate from that
so you would never need to resize anything,  it's all done with quota.

> Actually, btrfs's treatment of RAID is somewhat non-traditional - any
> file can be treated differently with regard to raid - you can have 6
> copies of one file and it will spread them out across as many disks
> as
> it can (probably with more than one copy on a disk unless you have a
> big array).  Everything is versioned and checksummed so if it can
> find
> one good copy it can use it. 

ZFS uses "ditto blocks' to save multiple copies of very important
blocks, such as directory meta-data.  This is done even on a single
disk without RAID.    RAIDZ on ZFS uses variable length stripes and
is non-standard that way,  and also has 'birth-times' and checksums
in the meta-data - so that is extremely similar to Btrfs.

"Between subtle shading and the absence of light lies the nuance of iqlusion..." - Kryptos 

Lee Marzke, lee@marzke.net http://marzke.net/lee/ 

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