Re: [SLE] Raid 0 speed question
From: Carlos E. R. (robin1.listas_at_tiscali.es)
Date: 10/08/03
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Date: Wed, 8 Oct 2003 20:10:48 +0200 (CEST) To: suse-linux-e@suse.com
The 03.10.04 at 17:10, H du Plooy wrote:
> I'm sorry, I confused two things when writing my question. I meant
> (hope I have the number right!) Raid-0 i.e. two partitions on two
> seperate discs as one of double the size - and hopefully double the
> speed.
Ah! That's different. From the howto:
· RAID-0
· Also called ``stripe'' mode. The devices should (but need not) have
the same size. Operations on the array will be split on the
devices; for example, a large write could be split up as 4 kB to
disk 0, 4 kB to disk 1, 4 kB to disk 2, then 4 kB to disk 0 again,
and so on. If one device is much larger than the other devices,
that extra space is still utilized in the RAID device, but you will
be accessing this larger disk alone, during writes in the high end
of your RAID device. This of course hurts performance.
· Like linear, there is no redundancy in this level either. Unlike
linear mode, you will not be able to rescue any data if a drive
fails. If you remove a drive from a RAID-0 set, the RAID device
will not just miss one consecutive block of data, it will be filled
with small holes all over the device. e2fsck or other filesystem
recovery tools will probably not be able to recover much from such
a device.
· The read and write performance will increase, because reads and
writes are done in parallel on the devices. This is usually the
main reason for running RAID-0. If the busses to the disks are fast
enough, you can get very close to N*P MB/sec.
> Thing is I'm not thinking of buying an PCI Raid controller - for the
The performance is similar to a software raid; they are not really hardware
only raid systems, from what I read on this list some time ago.
> price difference I could just as well get a 7200rpm 8mb cache drive.
> What I would really like to know is how stiping (sp?) the 5400rpm discs
> with kernel based software raid would perform compared to a new 7200rpm
> disc. The ones I've built into new machines lately - mostly the
> standard Seagate 7200rpm 2mb cache models - are really much faster than
> the drive I have.
Probably the 7200 rpm disk alone would be faster even without raid :-)
> Thanks for the suggestions. Distributing partitions over discs is
> another option, but finding a good solution will take time. Which
> filesystems will get used more? I assume with local mail delivery +
> spamassasin, /var would get used heavily, and I use a fair bit of swap.
If you use swap, my guess is that having it on a completely different HD,
or one seldom used for other things, would make the system faster. Or, you
can distribute one swap partition on each HD, and have them all with the
same priority: the kernel will distribute operations on them all. From the
howto:
2.5. Swapping on RAID
There's no reason to use RAID for swap performance reasons. The kernel
itself can stripe swapping on several devices, if you just give them
the same priority in the fstab file.
By the way, read that howto: Software-RAID-HOWTO.gz
The Software-RAID HOWTO
Jakob Østergaard (jakob@unthought.net)
v0.90.8, 2002-08-05
> Put them on seperate discs, or both on the faster of the two drives? Or
> shall I use the slower disc for the filesystems that get read more often
> and the faster disc for the filesystems that get written to more often?
Er... I'm getting lost O:-)
> I have a feeling the time I would spend figuring out the best partition
> sceme wouldn't be worth the money I save. It is definitely something
> that I'd like to get to know inside out, but I can't afford the time for
> that just now.
There is a section on this on one of the SuSE manuals. I like to separate
the '/opt' partition to another HD: gnome, kde, and a few more things,
like OO, are in there, while Xfree and the rest is mostly under '/usr'.
Thus, when starting up kde, somethings will be read from '/opt' and some
others from '/usr': the speedup gain is noticiable.
Of course, it seems that raid 0 would be faster for everything, but at the
same time, it needs some processing time overhead, and it is more
vulnerable.
--
Cheers,
Carlos Robinson
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