Re: raid io requests not parallel?

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <e0e4cb3e0607151704o479371afpc9332a08fb84ba09@mail.gmail.com>

On Sat, 15 Jul 2006 17:04:57 -0700, Jonathan Baccash wrote:

> As expected, the multi-threaded reads are 2x as fast as single-threaded
> reads. But I would have expected (assuming the write to both disks can
> occur in parallel) that the random writes are about the same speed (10
> seconds) as the single-threaded random reads, for both the
> single-threaded and multi-threaded write cases. The fact that the
> multi-threaded reads were
> twice as fast indicates to me that read requests can occur in parallel.
> 
> So.... why doesn't the raid issue the writes in parallel? Thanks in
> advance for any help.

But it does issue writes in parallel.  The problem is in the way RAID1
works.  When you do a read from the RAID1, it issues one read request
to one of the underlying disks.  When you do a write, it issues one write
request to _each_ of the underlying disks.  So with two disks in the
mirror set, every write issued by your program causes two disk writes.

-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush