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Re: What's the best hardver for PostgreSQL 8.1?

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Ron <rjpeace(at)earthlink(dot)net>
Cc: Michael Stone <mstone+postgres(at)mathom(dot)us>, pgsql-performance(at)postgresql(dot)org
Subject: Re: What's the best hardver for PostgreSQL 8.1?
Date: 2005-12-27 17:51:58
Message-ID: 200512271751.jBRHpw307927@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Historically, I have heard that RAID5 is only faster than RAID10 if
there are six or more drives.

---------------------------------------------------------------------------

Ron wrote:
> At 08:35 AM 12/27/2005, Michael Stone wrote:
> >On Mon, Dec 26, 2005 at 10:11:00AM -0800, David Lang wrote:
> >>what slows down raid 5 is that to modify a block you have to read 
> >>blocks from all your drives to re-calculate the parity. this 
> >>interleaving of reads and writes when all you are logicly doing is 
> >>writes can really hurt. (this is why I asked the question that got 
> >>us off on this tangent, when doing new writes to an array you don't 
> >>have to read the blocks as they are blank, assuming your cacheing 
> >>is enough so that you can write blocksize*n before the system 
> >>starts actually writing the data)
> >
> >Correct; there's no reason for the controller to read anything back 
> >if your write will fill a complete stripe. That's why I said that 
> >there isn't a "RAID 5 penalty" assuming you've got a reasonably fast 
> >controller and you're doing large sequential writes (or have enough 
> >cache that random writes can be batched as large sequential writes).
> 
> Sorry.  A decade+ RWE in production with RAID 5 using controllers as 
> bad as Adaptec and as good as Mylex, Chaparral, LSI Logic (including 
> their Engino stuff), and Xyratex under 5 different OS's (Sun, Linux, 
> M$, DEC, HP) on each of Oracle, SQL Server, DB2, mySQL, and pg shows 
> that RAID 5 writes are slower than RAID 5 reads
> 
> With the one notable exception of the Mylex controller that was so 
> good IBM bought Mylex to put them out of business.
> 
> Enough IO load, random or sequential, will cause the effect no matter 
> how much cache you have or how fast the controller is.
> 
> The even bigger problem that everyone is ignoring here is that large 
> RAID 5's spend increasingly larger percentages of their time with 1 
> failed HD in them.  The math of having that many HDs operating 
> simultaneously 24x7 makes it inevitable.
> 
> This means you are operating in degraded mode an increasingly larger 
> percentage of the time under exactly the circumstance you least want 
> to be.  In addition, you are =one= HD failure from data loss on that 
> array an increasingly larger percentage of the time under exactly the 
> least circumstances you want to be.
> 
> RAID 5 is not a silver bullet.
> 
> 
> >  On Mon, Dec 26, 2005 at 06:04:40PM -0500, Alex Turner wrote:
> >>Yes, but those blocks in RAID 10 are largely irrelevant as they are 
> >>to independant disks.  In RAID 5 you have to write parity to an 
> >>'active' drive that is part of the stripe.
> >
> >Once again, this doesn't make any sense. Can you explain which parts of
> >a RAID 10 array are inactive?
> >
> >>I agree totally that the read+parity-calc+write in the worst case 
> >>is totaly bad, which is why I alway recommend people should _never 
> >>ever_ use RAID 5.   In this day and age of large capacity chassis, 
> >>and large capacity SATA drives, RAID 5 is totally inapropriate IMHO 
> >>for _any_ application least of all databases.
> I vote with Michael here.  This is an extreme position to take that 
> can't be followed under many circumstances ITRW.
> 
> 
> >So I've got a 14 drive chassis full of 300G SATA disks and need at 
> >least 3.5TB of data storage. In your mind the only possible solution 
> >is to buy another 14 drive chassis? Must be nice to never have a budget.
> 
> I think you mean an infinite budget.  That's even assuming it's 
> possible to get the HD's you need.  I've had arrays that used all the 
> space I could give them in 160 HD cabinets.  Two 160 HD cabinets was 
> neither within the budget nor going to perform well.  I =had= to use 
> RAID 5.  RAID 10 was just not usage efficient enough.
> 
> 
> >Must be a hard sell if you've bought decent enough hardware that 
> >your benchmarks can't demonstrate a difference between a RAID 5 and 
> >a RAID 10 configuration on that chassis except in degraded mode (and 
> >the customer doesn't want to pay double for degraded mode performance)
> 
> I have =never= had this situation.  RAID 10 latency is better than 
> RAID 5 latency.  RAID 10 write speed under heavy enough load, of any 
> type, is faster than RAID 5 write speed under the same 
> circumstances.  RAID 10 robustness is better as well.
> 
> Problem is that sometimes budget limits or number of HDs needed 
> limits mean you can't use RAID 10.
> 
> 
> >>In reality I have yet to benchmark a system where RAID 5 on the 
> >>same number of drives with 8 drives or less in a single array beat 
> >>a RAID 10 with the same number of drives.
> >
> >Well, those are frankly little arrays, probably on lousy controllers...
> Nah.  Regardless of controller I can take any RAID 5 and any RAID 10 
> built on the same HW under the same OS running the same DBMS and 
> =guarantee= there is an IO load above which it can be shown that the 
> RAID 10 will do writes faster than the RAID 5.  The only exception in 
> my career thus far has been the aforementioned Mylex controller.
> 
> OTOH, sometimes you have no choice but to "take the hit" and use RAID 5.
> 
> 
> cheers,
> Ron
> 
> 
> 
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-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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