Thanks, it really explain well.
"Howard J. Rogers" <dba_at_hjrdba.com> wrote in message
news:aglk04$61j$1_at_lust.ihug.co.nz...
> There are 4 levels of RAID to consider.
>
> RAID 0 is the best for performance on both reads and writes... data is
> striped across many physical disks, so you are effectively parallelizing
> reads and writes. The degree of parallelism is effectively the number of
> physical drives you are striping across. Trouble is, as the name implies,
> RAID 0 has no actual redundancy at all: lose one hard disk, and the entire
> set it broken, and all data it loses is gone.
>
> RAID 5 is similar, in that data is striped across lots of hard disks.
> However, for each write, a piece of "parity" information is calculated
that
> allows you to work out the value of what was written on the other disks.
For
> example, if I told you that A+B = C, and on disk C you saw the value 3,
and
> on disk A you saw the value 1, you could work out that disk B must have
had
> the value 2. So, in RAID 5, whilst you've got the striping of RAID 0, you
> *do* have a high level of redundancy, because the parity information can
be
> used to "replace" the data lost when one of RAID disks decides to die.
> However, RAID 5 is NOT good for performance: rather than just stripe your
> data across multiple drives, you've got to take time out to calculate the
> parity information. Note that RAID 5's performance penalty is entirely on
> the write side of things: for reads, it's just as parallelized as RAID 0,
> and just as fast as a result. But in a write-intensive database (and most
> database involve lots of writes, though not all), the write penalty is
> sufficient to warrant avoiding RAID 5.
>
> It gets more complicated than that, because with battery-backed caches and
> all sorts of other hardware goodies which are entirely vendor-dependent,
> RAID5 can be made pretty attractive even for writes.
>
> RAID 1 is mirroring. You write your document to drive C, and it gets
> automatically written to drive D as well. There's no performance gain here
> at all... purely data redundancy. If Drive C bites the dust, your document
> is safe and sound on drive D.
>
> So they're the three main forms of RAID, and you might reasonably wonder
> what the fourth is I mentioned at the start. Well, it's just RAID 1+0,
> mirroring AND striping. You have five mirror sets, A, B, C, D and E, each
> comprising (say) 2 disks. A1 is mirrored with A2, B1 with B2 and so on:
>
> A1 B1 C1 D1 E1
> A2 B2 C2 D2 E2
>
> You write 100K of data: 20K gets written to A1, and mirrored to A2;
another
> 20K is written to B1, mirrored to B2 and so on. Notice there are no
> cross-letter dependencies: I mean, that A is not dependent on B, nor B on
C.
> You of course need to be able to read *something* from all 5 arrays if you
> want your 100K back, but whether you read from A1 or A2, it makes no
> difference -you've read something from the A array. Therefore, if disk C1
> blows up, you just replace it, and resilver it from C2. Meanwhile, A2, B2,
> C2, D2, and E2 have all been readable, and you've also been able to read
> from A1, B1, D1 and E1. Most of the entire array is therefore still
useable.
>
> So, if it were me, and the money someone else's I'd go for RAID 1+0 every
> time. Unfortunately, 1+0 is the most expensive option, because it requires
a
> large number of disks -the number for the stripe times at least two
> (depending on how many mirrors you want). As a reasonable compromise, RAID
5
> is a good bet, provided it is a *good* implementation of RAID 5 (things
like
> those battery backed caches) which minimise the write penalty. But it *is*
a
> compromise. Hold out for 1+0 if you aren't writing the checks.
>
> Incidentally, you can also do RAID 0+1 -stripe then mirror. Your data is
> striped across A1, A2, A3, A4, A5. The whole lot then gets mirrored to B1,
> B2, B3, B4 and B5. That sounds rather like 1+0, but it isn't: if disk A2
> dies, then the entire A-array is dead, and of no use to anyone. Your
> database would be relying entirely on the B array -and one more failure
> there would mean no database.
>
> Hope that helps
> Regards
> HJR
>
>
> "JohnWood" <jwood_at_microsoft.com> wrote in message
> news:KDrX8.6566$kt5.247951_at_news1.telusplanet.net...
> > I need to get some advice here about what RAID to use for Oracle 9i
> > database. I am a novice Oracle user.
> > I am building a database for about 600G. I am planning to use 2 logical
> > drives, one for storing files for data tablespace and one for index
> > tablespace. I am confused what to use for each logical drive. Should I
> use
> > RAID 1 or RAID 5 for the logical drive ?
> > I was told I should use RAID 1 for Database as it will have better
> > performance and redundancy even though it takes more physical drives to
> > form. But when I read books, it said the RAID 5 will have better
> > performance as the data will spread across different physical drives.
> > Please advise or point me to some site that I can find answer in a
> practical
> > sense.
> > Thanks.
> >
> >
>
>
Received on Fri Jul 12 2002 - 09:23:52 CDT
