Messages in this thread | | | Date | Sat, 15 Jul 2006 23:17:35 -0400 | From | Chuck Ebbert <> | Subject | Re: raid io requests not parallel? |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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