Note in-line
--
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html
Optimising Oracle Seminar - schedule updated Sept 19th
"Frank van Bortel" <fvanbortel_at_netscape.net> wrote in message
news:co760g$kne$1_at_news5.zwoll1.ov.home.nl...
> Howard J. Rogers wrote:
>>
>> The real concern with RAID 5 is what happens when you pull one of the
>> disks out. A single physical I/O now requires a read to be made against
>> every surviving member of the array, so that the missing data can be
>> deduced. RAID5's I/O performance under failure conditions is woeful. And
>> you suffer it for the duration of the array rebuild when you plug the
>> disk back in.
>>
If you consider a single stripe across the traditional 5-disc raid, then
either
(a) the dead disk is the parity disk (one time in 5) - in which case
reads
don't do any extra reads, and you no longer have a write overhead
on the other four.
or
(b) the dead disk is one of the data disks (4 chances in 5) - in which
case
three of the possibly reads won't have to do any extra reads, and
only
one read in 4 will have to read the other three disks and the parity
disk
to infer re-create the fourth set of data.
>> Regards
>> HJR
>
> In addition to that:
> - pull out a second one on RAID-5: nothing goes.
> - pull out a second one on RAID 1+0: who cares?
> (unless you happen to pull the one disk, that
> had no mirror anymore)
>
> Basic message: RAID10 (or 1+0) is *safer* than RAID5,
> because you can *never* loose 2 disks in RAID5 without
> serious disruption (involving restoring of backups).
But since people tend to create multiple RAID-5
sets with a small number of discs in each set, it
IS still possible to lose several discs from the cabinet
before your worst case scenario happens.
It's still the standard triangle of cost, benefit and risk
with different answers appropriate to different circumstances.
> --
>
> Regards,
> Frank van Bortel
Received on Fri Nov 26 2004 - 07:15:53 CST
