[CentOS] xfs not getting it right?

[Fred Smith]

On Wed, Sep 20, 2017 at 06:12:39PM +0200, hw wrote:
> Chris Adams wrote:
> >Once upon a time, hw <hw at gc-24.de> said:
> >>xfs is supposed to detect the layout of a md-RAID devices when creating the
> >>file system, but it doesn´t seem to do that:
> >>
> >>
> >># cat /proc/mdstat
> >>Personalities : [raid1]
> >>md10 : active raid1 sde[1] sdd[0]
> >>      499976512 blocks super 1.2 [2/2] [UU]
> >>      bitmap: 0/4 pages [0KB], 65536KB chunk
> >
> >RAID 1 has no "layout" (for RAID, that usually refers to striping in
> >RAID levels 0/5/6), so there's nothing for a filesystem to detect or
> >optimize for.
> 
> Are you saying there is no difference between a RAID1 and a non-raid
> device as far as xfs is concerned?

RAID1 is simply two or more drives with the same stuff on each drive.
its a simple form of redundancy. striping would put some of it on one
drive and more on another, and so forth for as many drives as you use
with some kind of redundancy or checksumming. RAID1 is much simpler:
simply make all the drives carry the same data.


> 
> What if you use hardware RAID?
> 
> When you look at [1], it tells you to specify su and sw with hardware
> RAID and says it detects everything automatically with md-RAID.  It doesn´t
> have an example with RAID1 but one with RAID10 --- however, why would that
> make a difference?  Aren´t there stripes in a RAID1?  If you read from both
> disks in a RAID1 simultaneously, you have to wait out the latency of both
> disks before you get the data at full speed, and it might be better to use

RAID1 wouldn't have that problem, necessarily. since all drives carry
the same data, it is necessary to read from only one of them. it is during
a write  operation that all drives are written, and even then they may not
be written at exactly the same time... one can be written and the data for the
other in buffer-cache until the system gets a chance to write it.

I don't know details, per the above, for Linux software raid, but I also
have a USB-attached HW raid box (jmicron chip) and I can watch the lights
on the drives and see it doing exactly that. I would find it hard to believe
that software raid in Linux is significantly different (in fact, i rather
suspect that the external raid box is probably running some older version
of Linux)
> stripes with them as well and read multiple parts of the data at the same
> time.
> 
> [1]: http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance
> 
> 
> > The chunk size above is for the md-RAID write-intent bitmap; that's not
> > exposed information (for any RAID system that I'm aware of, software or
> > hardware) or something that filesystems can optimize for.
> 
> Oh, ok.  How do you know what stripe size was picked by mdadm?  It seemd a
> good idea to go with defaults as far as possible.
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-- 
---- Fred Smith -- fredex at fcshome.stoneham.ma.us -----------------------------
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