Re: hdparm and DMA and raid
[...]
> > As I don't know your mobo I might be wrong, but the general opinion
> > seems to be "stay away from the built in raid and go for linux software
> > raid". Usually the cheap so called raid chip/cards are just new bios
> > instructions to do software raid anyway, something that the kernel could
> > probably do better. raid0 can be done with either od MD, LVM or EVMS.
> > I've used LVM before and found it both quite simple and well
> > functioning, and is testing EVMS currently.
>
> I didn't know that. I always thought that the hardware Raid would be
> better. My mobo is an ASUS A7V8X with VIA VT8235 chip and support for
> ATA133 and the new serial IDE drives. The drive currently is connected
> with a 80pin cable. It supports Raid0 and Raid1 apparently. The
Don't know anything about it, the statement was meant in general. It
_may_ be that the mobo have REAL hardware raid, but I doubt. (I briefly
looked at asus web page for the mb, usually it's a lot more info about
the raid if it's real raid.)
> processor is an AMD 1.67Ghz and 512Mb DDR-266 Ram. The Drive is WD400JB
> (40GB, 7200RPM, ATA133). I expected the hard drive to "kick ass"
> relative to my older machine on an older mobo with normal 40pin connector,
> WD400BB (40GB, 7200RPM, ATA100?). But in fact the data transfer speeds
> from the new harddisk is only a fraction of the older one's.
Since the dma doesn't work. When you got it working I think it might be
faster, but not _that_ much faster than your old drive. The ata-XX just
say what the interface in theory could give (might even be specified as
burst rate i.e. not continuos speed), the actual physics of the disk
seems very similar (same brand, size and rotational speed)..
[...]
> I will do that. One further question though: If I do get another drive
> and implement the software RAID as you suggest, it shouldn't matter if
> the drives are different in size or type, right?
Size do matter, at least to some point. The size of the partitions used
in a raid set must match each other, or the larger one won't be fully
used (say A=100xB, B=110xB, then you "waste" (max(A,B)-min(A,B)= 10xB
from B).
If you make the raid partitions of equal since, you can use the spare
space of the larger as a simple partition (non raid), but you will
suffer performance when you use that AND the raid simultaneously (as you
will have to access to the same disk).
Performance differences also matter a bit, more (I think) for raid1 than
for raid0. (Since for write you will always wait for the slowest disk,
for read the fastest will usually respond first and be used.) Using
raid0 you should at least get a performance boost compared to using only
the slower drive. (Unless you plan to mix some really old 42MB disk
with a shiny new 20krpm disk I'm pretty sure the total performance will
be higher than only the faster disk to...)
There should be plenty of pages to read about raid and stuff laying
around on the net, probably more than useful. You should make sure you
understand the basics of the different modes though.
Regards,
Emil
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