Re: raid1 performance

[Jakob Østergaard]

On Tue, Apr 30, 2002 at 10:21:48AM -0400, Kent Borg wrote:
> On Tue, Apr 30, 2002 at 01:38:16PM +0100, Arjan van de Ven wrote, very
> roughly: 
> [that RAID 1 is only as fast in reading as the fastest disk because of
> seeking over alternate blocks, and ]
> 
> > The only way to get the "1 thread sequential read" case faster is by
> > modifying the disk layout to be
> > 
> > Disk 1: ACEGIKBDFHJ
> > Disk 2: ACEGIKBDFHJ
> > 
> > where disk 1 again reads block A, and disk 2 reads block B.  To read
> > block C, disk 1 doesn't have to move it's head or read a dummy block
> > away, it can read block C sequention, and disk 2 can read block D
> > that way.
> >
> > That way the disks actually each only read the relevant blocks in a
> > sequential way and you get (in theory) 2x the performance of 1 disk.
> 
> I am confused.  
> 
> Assuming a big enough read is requested to allow a parallelizing to
> two disks, why can't the second disk be told not to read alternate
> blocks but to start reading sequential blocks starting half way up the
> request?

This is *not* as simple as it sounds.  Believe me, I spent a week trying...

However, with ext2 (and other filesystems as well), a large sequential file
read is *not* sequential on the disk.  You should actually see better performance
on RAID-1 than on a single disk for very large reads, becuase some of the lookups
needed (block indirection or whatever) will be run by the "best" disk in the given
situation.

> 
> Also, why does hdparm give me significantly faster read numbers on
> /dev/md<whatever> than it does on /dev/hd<whatever>?  I had assumed
> there was parallelizing going on.  Does this mean I would get a speed
> improvement if I ran my single disk notebook as a single disk RAID 1
> because there is some bigger or better buffering going on in that code
> even without parallelizing?

hdparm is not a good benchmark for this.

Use bonnie, bonnie++, tiotest, or even 'dd' with *huge* files.

-- 
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