[CentOS] Re: RAID5 or RAID50 for database?

Sun May 25 15:55:46 UTC 2008
Ross S. W. Walker <rwalker at medallion.com>

Christopher Chan wrote:

> William Warren wrote:
> > I'm not a fan of RAID 5 at all since it can only tolerate one failure at 
> > all.  Go with raid 10 or something like that which is able to handle 
> > more than one failure.  Intermittent, uncorrectable sector failures 
> > during rebuilds are becoming an increasing problem with today's drives.
> > 
> Is that raid10 or raid 1+0 or raid 0+1? :D
> At least for the latter two, their handling more than one failure 
> depends on which disks blow. Not sure how the raid10 module 
> handles things.

Whoever implements RAID10 will want the RAID1+0 which is a stripe set
of mirrors, rather then the RAID0+1 which is a mirror of stripe sets.

The problem being two fold, 1) in a RAID0+1 a single drive failure
on either side of the mirror will put the whole array into total
failure jeopardy, a failure on both sides is a total loss, 2) the
pathway for simultaneous operations is cut down from (say X is an
even number of disks) X reads, X/2 writes, to 2 reads, 1 write.

On a RAID5/6 array you are limited to a pathway of 1 read and 1
write at a time and all writes must write across the entire stripe,
so if you do choose RAID5/6 then it is highly recommended to use a
hardware RAID controller with a BBU write-back and read-ahead cache
which can minimize the impact of this by caching a whole stripe set
to write at once and to have a stripe set of reads waiting for io

For database log files and other applications that do a lot of
random io it is recommended to use fast RPM drives in a RAID10
which has the multiple pathways for reads and writes which will
maximize the total number of random IOPS (ios per second).

Typically most vendors recommend a two-prong approach, keep the
database data files on a RAID5/RAID6 type array and keep the
log files on a RAID10 array.


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