Re: Readahead with softraid1

[Greg Freemyer <greg.freemyer@gmail.com>]

On 7/8/05, Danny Cox wrote:
> Erik,
> 
> On Fri, 2005-07-08 at 14:00 +0200, Erik Slagter wrote:
> > I am using softraid 1 on two sata disks and I'm trying to get the best
> > possible performance. IMHO read actions (if properly addressed) should
> > be split over the two drivers and performed independently. However, I
> > don't notice anything to back this up. The read performance (with the
> > dreaded hdparm) shows read performance on sda,sdb and md0 exactly the
> > same.
> ...
> > What am I doing wrong here???
> 
>         Nothing.  I'll take a shot at answering this one instead of lurking
> this time.  Then, I'll crawl back under my rock.
> 
>         The raid1 driver keeps a "last visited block" for each drive.  This is
> the block number that was most recently read or written by that drive.
> When a read request arrives, the driver examines each drive for the
> nearest last visited block to the one requested.  Guess what?  If the
> read starts with drive sda, then it will *always* be the one chosen to
> service the read in the future, because the last visited block number is
> only one off.  This would only change if there are multiple processes
> performing I/O on the md device.  Then, it may switch to another drive.
> In any case, it will *tend* to stick with the same drive.
> 
>         Did I explain that well, or only muddy the waters?
> 
> --
> Daniel S. Cox
> Internet Commerce Corporation
> 

Interesting.  Unfortunately, I do a lot of sequential reading with
little or no other computer activity and had wondered about the "slow"
speed of RAID 1 on read.

Does anyone know if that a common implementation on Hardware Raid
controllers too?

I have actually been working mostly with 3ware 7000 series cards, so
the md implementation does not affect me, but if that is a common
design then the 3ware card may have a similar algorithm.

Greg
-- 
Greg Freemyer
The Norcross Group
Forensics for the 21st Century