Steve Bergman wrote:
On Tue, 2006-09-12 at 15:25 -0500, Les Mikesell wrote:
If the disks are scsi the writes happen pretty much in parallel and
you only read one copy when you read. It should only slow you down
if you are using 2 ide drives on the same controller for the mirrors
in which case you have to wait for one write to complete before
the other starts.
They are SATA. My original thinking today was that performance would be
better if I had 2 swap partitions: /dev/sda2, /dev/sdb2 set at the same
priority so that *different* pages could be swapped simultaneously
to/from the 2 drives. But as Matt pointed out, that would be less
robust.
The best performance is if you don't swap at all. Avoid swapping, don't
relay on "fast swap". There's no such thing as "fast swap" ;-)
Anyhow, theoretically when you read from RAID1, you read different pages
from different drives in parallel. That's why RAID1 theoretically has
two times faster reads than single drive. Writing to RAID1 is
theoretically the same speed as writing to single drive. Of course, as
Les pointed out, it all depends on the actual hardware. If hardware
isn't capable of doing read/writes in parallel (for example two IDE
drives on the same controller), you get slower performance.
Historically, SCSI with its command queuing was very good in doing
things in parallel to several devices (even though it was chain-type of
bus), hence its popularity in high performance applications. Some (not
all) SATA devices come with support for queuing (they call it "native
command queuing"), however I've no idea if Linux supports it or not.
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