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From:Jim McAtee Date:August 23 2003 2:37am
Subject:Re: RAID or not?
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Alec.Cawley@stripped wrote:


> > I say go with RAID 5, on a controller card.
>
> > Mirroring just gives you backup, and you lose half your diskspace. It
> offers
> > no performance benefit, and actually the computer might have to work
> harder
> > to make sure the drives are in sync.
>
> I do not think it is true that mirroring gives no performance benefit (on a
> well implemented controller). For reads, the raid controller can read
> either copy of the data, so that effectively two reads can be in progress
> at the same time, doubling read performance.

Yes, doubling read performance (best case) when compared with a non-RAID disk
system. But with RAID5, you have more disks and can therefore read more than
just two disks simultaneously.  The more spindles you have, the better the
performance.

> On the write side, for small
> writes a raid 5 has to read the overwritten data (in order to remove it
> from the parity) then do a read/modify/write on the parity.

I don't quite understand the need to read data before any write.  Why wouldn't
it just calculate the parity of whatever is being written and just write it to
disk?  Wouldn't there be slack space, as with any disk system?  Write a 1 byte
file and it uses an N byte block on one disk, plus an N byte parity block on
another.

> Performance
> again should be doubled (two writes on the mirrored system, read/overwrite
> and read/modify/write on the Raid5 system, with the two halves of each
> operation requiring a full rotation between them). For large writes, the
> Raid5 system catches up, because the parity can be entirely calculated from
> the data sent, so it does not need to do the reads.

>
> I would therefore expect a mirrored system to approach, but not reach,
> twice the performance of a Raid5 system

A mirrored system when writing should, at best, be no faster than a
non-mirrored system.  Unlike reading, it has to write a complete copy of the
data to both disks, so there are no performance gains to be had.  RAID5 should
be a bit slower than RAID1, due to the need for calculating parity.


Thread
RAID or not?Jackson Miller21 Aug
  • Re: RAID or not?Jon Drukman21 Aug
    • Re: RAID or not?Jackson Miller21 Aug
      • RE: RAID or not?Steven) (Lefevre21 Aug
        • Re: RAID or not?David Griffiths21 Aug
        • Re: RAID or not?Per Andreas Buer21 Aug
        • Re: RAID or not?Jon Drukman22 Aug
          • Re: RAID or not?Dan Nelson22 Aug
          • RE: RAID or not?Steven) (Lefevre22 Aug
            • RE: RAID or not?Michael Loftis2 Sep
  • Re: RAID or not?Colbey21 Aug
  • Re: RAID or not?Per Andreas Buer21 Aug
RE: RAID or not?Alec.Cawley22 Aug
  • Re: RAID or not?Jim McAtee23 Aug
    • Re: RAID or not?Michael Loftis2 Sep
RE: RAID or not?Alec.Cawley22 Aug
Re: RAID or not?Alec.Cawley26 Aug