On Thu, Feb 03, 2000 at 01:08:20PM -0000, Jakma, Paul wrote:
> > Sorry, hadn't considered the possibility that somebody might do raid5
> > without writeback enabled. *Shudder*
>> well.. writeback is very dangerous. Especially with any kind of journalling
> fs, in which case you should not use writeback without proper protection, eg
> your controller should have a battery backup module.
> then the controller is parity-bound. RAID5 should be faster at writing than
> RAID1 if the controller can do the parity calculations fast enough.
Inclined to disagree here. No matter how fast the raid5 implementation,
it still has to do two reads (data and parity blocks), a parity calculation
and two writes (data and parity blocks) for every individual write. A raid1
only has to do two writes, and no parity calculation. Quite apart from
the extra work, the raid5 scenario has a serialisation issue - it has to
read twice, *then* calculate parity, *then* write twice. The raid1
scenario only involves two writes which can Just Happen.
> Mylex is pretty decent iirc. At least 10MB/s block output from bonnie i
> think (4mb cache writeback, old DAC960-PDU). I'll try find a box to run some
> tests on.
10MB/s is fairly respectable. Nice.
> > As for the disk space issue, disks are cheap. Raid5 is great
> > where you
>> disks are cheap.. true. But i'd rather have 27GB of disk space with RAID5
> than 18GB with RAID1. And RAID5 is much faster for reads, and should be
> decent for writes if the controller is up to it. Why waste 16% drive space,
> (25% with 4 drives!!). Rack space isn't cheap either.
Fair enough. Me, if I was pretty sure I was only going to use 18GB, I'd
go with the 18GB raid1 over the 27GB raid5. If I needed 27GB, I'd prob
just get more disks and raid1 or raid1+0 them. :-)
As you say, rack space isn't cheap. And when there are *lots* of disks
to be raid-ed, raid5 looks more and more attractive. Especially if the
data is pretty much static.
I don't accept that raid5 is necessarily faster at reading either, though.
Any raid1 controller worth its salt will interleave requests onto both
disks in a pair. Any raid1 controller worth its salt will also trivially
do striping (1+0/0+1). (*)
> And if money isn't a problem, then why not spend it on a beefier controller?
> Then you can have everything...
Other reasons I like raid1 (and its striped friends) are:
o In theory, Up to half of the disks in an array can fail and still have the
array accessible with no performance degradation. You can even get a
performance improvement. :-)
o Beyond a certain point, cost of backup starts to make the cost of disk
*drives* look trivial and the cost of (filled) disk *space* look more
and more expensive. Raid1 treats the cost of disks drives with contempt
and disk *space* with reverence.
o If I want redundancy and performance, raid1 is my friend. If I just want
performance, I'll go with raid0. If I just want redundancy, I'll go
with raid5. It's been a while since I wanted redundancy without
(*) Instead of classical raid1 over multiple pairs - where "once one pair is
full, it starts on the second". Eugh.
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