Quick ZFS mirroring question for non-mirrored pool

[Kaya Saman]

Many thanks guys for providing so much valuable input and knowledge!!!

I really appreciate all your advice and knowledge.

Please excuse my naivety but the statement below:

On 05/16/2010 03:51 AM, Bob Friesenhahn wrote:
> As long as the pool is not the boot pool, zfs makes such testing quite 
> easy. 

I was under the impression that one needed a UFS2 filesystem in order to 
be able to boot FreeBSD as that is the only FS available upon 
install..... unlike Solaris10/OpenSolaris which creates the ZFS 
filesystem upon install.

The plan I originally conceived was to use a 40GB solid state disk as 
the / (root) directory comprising of all descending file systems, eg. 
/usr, /proc, /lib etc... using the UFS2 FS

....and then use ZFS for the storage portion of my server using 2TB 
Western Digital RE4 Enterprise SATA drives.

Since it's a simple home based server and not a massive enterprise grade 
environment performance is not too much of an issue. However, system 
backups are and without funding for a spare system or DAS or SAN 
solution the only real option I have is to use a RAID0 esq based setup 
so if one or both the primary drives go offline then at least I have all 
my data backed up and still available.

Regards,

Kaya




On Sat, May 15, 2010 at 07:51:17PM -0500, Bob Friesenhahn wrote:

> >  On Sat, 15 May 2010, Jeremy Chadwick wrote:
>    
>> >  >What you have here is the equivalent of RAID-10.  It might be more
>> >  >helpful to look at the above as a "stripe of mirrors".
>> >  >
>> >  >In this situation, you might be better off with raidz1 (RAID-5 in
>> >  >concept).  You should get better actual I/O performance due to ZFS
>> >  >distributing the I/O workload across 4 disks rather than 2.  At least
>> >  >that's how I understand it.
>>      
> >  
> >  That would be a reasonable assumption but actual evidence suggests
> >  otherwise.  For sequential I/O, mirrors and raidz1 seem to offer
> >  roughly similar performance, except that mirrors win for reads and
> >  raidz1 often win for writes.  The mirror configuration definitely
> >  wins as soon as there are many seeks or multi-user activity.
> >  
> >  The reason why mirrors still do well for sequential I/O is that
> >  there is still load-sharing across the vdevs (smart "striping") but
> >  in full 128K blocks whereas the raidz1 config needs to break the
> >  128K blocks into smaller blocks which are striped across the disks
> >  in the vdev. Breaking the data into smaller chunks for raidz
> >  multiplies the disk IOPS required.  Disk seeks are slow.
> >  
> >  The main reason to choose raidz1 is for better space efficiency but
> >  mirrors offer more performance.
> >  
> >  For an interesting set of results, see the results summary of "Bob's
> >  method" at"http://www.nedharvey.com/".
> >  
> >  The only way to be sure for your own system is to create various
> >  pool configurations and test with something which represents your
> >  expected work load.  As long as the pool is not the boot pool, zfs
> >  makes such testing quite easy.
>    
Thanks Bob.  You're absolutely right.

I'd never seen/read said data results before, nor had I read the below
material until now; quite interesting and educational.

http://blogs.sun.com/roch/entry/when_to_and_not_to

-- | Jeremy Chadwick jdc at parodius.com | | Parodius Networking 
http://www.parodius.com/ | | UNIX Systems Administrator Mountain View, 
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