Re: Performance question

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Wed, Jan 21, 2009 at 08:14:52PM +0100, Piergiorgio Sartor wrote:
> Hi again,
> 
> [--detail vs. --examine]
> > --detail looks at the running arrays, while --examine most
> > likely (depending on mdadm.conf) looks at all partitions
> > on the system. 
> > 
> > Given that the arrays are just created in the installation process, and
> > the active running arrays are most likely the ones you want your system
> > to know of, I think --detail is the better. --examine does on two of my
> > systems generate info that are in conflict and not suitable for a
> > mdadm.conf file, such as two /dev/md1 with different UUIDs.
> 
> yes, but I noticed that with "--detail" and an
> array (RAID-1) resyincing, it reports "spares=1"
> too, while when the array is in sync, it prints
> the correct geometry.
> So, I was wondering, since I also noticed that
> "--examine" produces the arrays with /dev/md/"name",
> so if two arrays have same name, it ends up with
> the same device.
> Is this maybe a bug of mdadm?

I leave this to others to answer this one.
I think it is strange for --detail to report "spares=1"
if it is syncing.

> [metadata position]
> > To me it does not matter that much, except for the booting device.
> > Each partition in the booting device must look like a normal (ext3)
> > partition, as grub and lilo does not know of raids, and just treats
> > a booting partition as a standalone partition. So here you should use
> > 0.90 metadata, which is put at the end of the array.
> 
> Well, I was a bit mixing up things with this question.
> In the back of my head the question was:
> 
> What about performances, RAID-10 f2, bitmap (important)
> and metadata 1.0 vs. 1.1?
> 
> This could be a further test for performances. It would
> be interesting to know if it is better to have the
> metadata at the beginning or at the end of a RAID-10 f2,
> with two HDs, having the bitmap enabled.
> Or if it does not matter at all.
> 
> Reading around I found different "opinions" about bitmap
> and performances, but I did not find a "convincing" test.

I have not tested it. So yes, I think this is something to do a performance test
on.  I think it should not matter much whether it is in the beginning or in
the end. However, if you make a test, then you most likely will do it on
a newly created raid, and then files would tend to be allocated in the
beginning of the file system, thus favouring a metadata block in the
beginning of the raid. In real operation this will tend to even out.
Another issue is that the sectors in the beginning of a disk are much 
faster, a factor of two perhaps, than the sectors in the end of the
drive.

> Thanks again.
> 
> Different item of the wiki, I run into it today.
> Maybe the "initrd" description could be updated, since
> it uses "mdassemble", while the "initrd" I have uses
> directly "mdadm -As --auto=yes ..." (I do not remember
> the full line).

mdasseble is specifically made for initrd, so why not use it here?

Best regards
keld