Re: UDB RAID

[Joerg Bruehe <joerg sql de>]

Hi ! 

"Christopher W. Curtis" schrieb:
> 
> Here's one you probably don't see much:
> 
> I have a UDB with _a_lot_ of disk space.  Something like 30GB.
> I want to put about 10GB of it as RAID, ~3GB on 3 different disks.
> I'm not really sure whether I want to do 0+1 or 5 though.  I'm
> concerned about the access speed for RAID 5, but want to minimize
> space loss.  

The information you give is insufficient. 

You can not have all three: speed, security, and space economy. 
So you have to make up your mind which is the most important, 
and that probably depends on the intended usage and access pattern.  Examples: 

- For a database log, security and (writing) speed are topmost 
  so you would use (software or hardware) mirroring; 
  space is less important because the logs are not that large. 

- For database data (with multi-user access), space economy 
  increases in importance relative to speed, so RAID 5 might 
  be considered. 

- For plain file systems, the Unix/Linux kernel buffering 
  ensures speed so again mirroring may be too costly. 

- For paging devices, it depends upon the answer to 
  "Can I accept a crash which does not cause data losses ?" 
  Assuming your data are in a DBMS and protected (by dual logging), 
  you might decide not to mirror your (two or more) page devices 
  and have the machine crash in case of a disk error there, 
  reboot, let the DBMS work its redo/undo recovery, and proceed. 
  For an online system like flight bookings, I would not take 
  that course and mirror the paging devices. 
  I would never have paging on RAID-5 due to speed reasons. 

- The more processes access (different parts of) the data, the more 
  you profit from caching and parallelism and the less you are 
  blocked by I/O delays. That is a reason why speed is more critical 
  for DBMS logs than for DBMS data. 

So I claim you yourself have to decide your criteria, and others 
can only provide arguments for fixed scenarios which may not 
fit your situation. 

In all these discussions do not forget that disks are not the only 
item that may fail, for full security you might use mirroring on 
disks connected to independent controllers. 
Also, to protect against total losses due to fire, sabotage, and 
total system misbehaviour (formatting all disks etc.), you cannot 
do without (tape or CD) backups removed from the system and 
stored in other locations. 

HTH 
Joerg Bruehe 

-- 
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
     (speaking only for himself)
mailto: joerg@sql.de