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Raid 10 or Raid 5 on Dell PowerEdge

From: "Dave Dutcher" <dave(at)tridecap(dot)com>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Raid 10 or Raid 5 on Dell PowerEdge
Date: 2007-01-17 15:57:16
Message-ID: 000401c73a50$29fd4f90$8300a8c0@tridecap.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi.  I sent this email to the list last week, but for some reason I never
saw it show up on the list.  I apologize if it appears twice now.
 
We recently bought two Dell PowerEdge 2900 servers each with a 2.3 Ghz 5140
Xeon, 4 Gigs of RAM, 8 15k SAS drives, and a PERC 5/i raid controller with
256 megs of battery backed cache.  Our database is more of an OLTP type, and
everything I've read says that 10 would be better, but I thought I would
test both 10 and 5.  The other guys here wanted to run the Raid 5 with a hot
spare, so the RAID 10 uses 8 disks and RAID 5 uses 7 plus the hot spare.  We
are running CentOS 4.4.  I started testing with bonnie++ 1.03a (bonnie++ -s
16g -x 3 -q ) and I got these numbers.  These tests were done with a RAID
stripe size of 64KB, no RAID controller read ahead, and write back caching
enabled.
 
 
RAID 10
name file_size  putc putc_cpu put_block put_block_cpu rewrite rewrite_cpu 
Raid10     16G 50546       94    149590            34   81799          17 
Raid10     16G 50722       95    139080            31   82987          17 
Raid10     16G 50526       94    148433            33   82278          17 

 getc getc_cpu get_block get_block_cpu seeks seeks_cpu num_files 
50678       84    236858            30 602.7         1        16 
50845       85    240024            31 594.8         1        16 
50921       85    240238            31 547.5         1        16 

 
Raid 5               
name file_size  putc putc_cpu put_block put_block_cpu rewrite rewrite_cpu  
Raid5      16G 51253       95    176743            40   87493          19 
Raid5      16G 51349       96    182828            41   89990          19 
Raid5      16G 51627       96    183772            42   91088          20 

 getc getc_cpu get_block get_block_cpu seeks seeks_cpu num_files 
50750       83    232967            29 378.5         0        16 
51387       84    237049            31 385.0         0        16 
51241       84    236493            30 391.8         0        16 

 
I was somewhat surprised that the RAID 5 was equal or better on almost
everything.  I assume this must be because it has 6 data disks as opposed to
4 data disks.  The one number that I find strange is the seeks/sec.  Is
there any reason why a RAID 5 would not be able to seek as quickly as a RAID
10?  Or are the numbers from the Raid 10 bogus?  I've also done some testing
with Postgres 8.2.1 and real world queries, and the two machines are
basically performing the same, but those seek numbers kinda bug me.


Thanks,


Dave Dutcher
 


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Subject: Re: PG8.2.1 choosing slow seqscan over idx scan
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Subject: Re: PG8.2.1 choosing slow seqscan over idx scan

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