[CentOS-virt] Slightly OT: FakeRaid or Software Raid

Luke S Crawford

lsc at prgmr.com
Thu Dec 3 02:01:49 UTC 2009

Grant McWilliams <grantmasterflash at gmail.com> writes:

> Interesting thoughts on raid5 although I doubt many would agree. I don't see
> how the drive
> type has ANYTHING to do with the RAID level. 

raid5 tends to suck on small random writes;   SATA sucks on small
random anything, so your worst-case (and with my use case,  and most
'virtualization' use cases, I you spend almost all your time in the 
worst-case)  is much worse than raid5 on SAS, which has reasonable 
random performance. 

The other reason why I think SATA vs SAS matters when thinking about 
your raid card is that the port-cost on a good raid card is so 
high that you could almost double your spindles at sata prices,
and without battery backed cache, the suckage of RAID5 is magnified,
so soft-raid5 is generally a bad idea. 

> There are different RAID levels
> for different situations
> I guess but a RAID 10  (or 0+1) will never reach the write or read
> performance<http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html>of
> a RAID-5.

I understand raid5 is great for sequential, but I think
my initial OS install is the last sequential write any of my servers
ever see.  My use-case, you see, is putting 32GiB worth of VPSs on one mirror
(or a stripe of two mirrors)  It is all very random, just 'cause you have
30+ VMs on a box.  

  If you do lots of sequential stuff, you will see very different
results, but the vitalization use case is generally pretty random, because
multiple VMs, even if they are each writing or reading sequentially, 
make for random disk access. 

(now, why do my customers tolerate this slow sata disk?  because it's 
cheap.  Also, I understand my competitors use similar configurations.)  

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