Messages in this thread | | | Date | Wed, 1 May 2002 18:35:53 +0200 | From | Jakob Østergaard <> | Subject | Re: raid1 performance |
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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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