Disk 100% busy

[Brad Knowles]

At 2:35 PM -0500 2005-11-03, Francisco Reyes wrote:

>>  If you're using maildir, that is one of the situations which works
>>  pretty well with RAID-5, although RAID-10 is also (always? :-) a good
>>  choice.
>
>  How about for database? In particular postgresql.
>  How bad would RAID 5 be for it?

	RAID-5 is usually about the worst case for most database 
applications, since you have to read and write entire blocks across 
all disks just to change a single bit.  You have to wait for all 
disks to read or write their respective data, before you can return. 
Some RAID arrays will have intelligent controllers that allow you to 
cache writes and return before the data is actually written, but then 
intelligent controllers can be used with all RAID types.

	The only thing that would be worse would be RAID-3, where you'd 
have a hot parity disk, whereas RAID-5 spreads the parity bits around 
the array of disks.

-- 
Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.