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Re: SCSI vs SATA

From: david(at)lang(dot)hm
To: Ron <rjpeace(at)earthlink(dot)net>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: SCSI vs SATA
Date: 2007-04-08 03:13:51
Message-ID: Pine.LNX.4.64.0704071932540.28411@asgard.lang.hm (view raw or flat)
Thread:
Lists: pgsql-performance
On Sat, 7 Apr 2007, Ron wrote:

>> Ron, why is it that you assume that anyone who disagrees with you doesn't 
>> work in an environment where they care about the datacenter environment, 
>> and aren't in fields like financial services? and why do you think that we 
>> are just trying to save a few pennies? (the costs do factor in, but it's 
>> not a matter of pennies, it's a matter of tens of thousands of dollars)
> I don't assume that.  I didn't make any assumptions.  I (rightfully IMHO) 
> criticized everyone jumping on the "See, cheap =is= good!" bandwagon that the 
> Google and CMU studies seem to have ignited w/o thinking critically about 
> them.

Ron, I think that many people aren't saying cheap==good, what we are 
doing is arguing against the idea that expesnsive==good (and it's 
coorelary cheap==bad)

> I've never mentioned or discussed specific financial amounts, so you're 
> making an (erroneous) assumption when you think my concern is over people 
> "trying to save a few pennies".
>
> In fact, "saving pennies" is at the =bottom= of my priority list for the 
> class of applications I've been discussing.  I'm all for economical, but to 
> paraphrase Einstein "Things should be as cheap as possible; but no cheaper."

this I fully agree with, I have no problem spending money if I believe 
that there's a cooresponding benifit.

> My biggest concern is that something I've seen over and over again in my 
> career will happen again:
> People tend to jump at the _slightest_ excuse to believe a story that will 
> save them short term money and resist even _strong_ reasons to pay up front 
> for quality.  Even if paying more up front would lower their lifetime TCO.

on the other hand, it's easy for people to blow $bigbucks with this 
argument with no significant reduction in their maintinance costs.

> The Google and CMU studies are =not= based on data drawn from businesses 
> where the lesser consequences of an outage are losing $10Ks or $100K per 
> minute... ...and where the greater consequences include the chance of loss of 
> human life.
> Nor are they based on businesses that must rely exclusively on highly skilled 
> and therefore expensive labor.

hmm, I didn't see the CMU study document what businesses it used.

> In the case of the CMU study, people are even extrapolating an economic 
> conclusion the original author did not even make or intend!
> Is it any wonder I'm expressing concern regarding inappropriate extrapolation 
> of those studies?

I missed the posts where people were extrapolating economic conclusions, 
what I saw was people stateing that 'you better buy the SCSI drives as 
they are more reliable', and other people pointing out that recent studies 
indicate that there's not a significant difference in drive reliability 
between the two types of drives

>> I actually work in the financial services field, I do have a good 
>> datacenter environment that's well cared for.
>> 
>> while I don't personally maintain machines with hundreds of drives each, I 
>> do maintain hundreds of machines with a small number of drives in each, and 
>> a handful of machines with a few dozens of drives. (the database machines 
>> are maintained by others, I do see their failed drives however)
>> 
>> it's also true that my expericance is only over the last 10 years, so I've 
>> only been working with a few generations of drives, but my experiance is 
>> different from yours.
>> 
>> my experiance is that until the drives get to be 5+ years old the failure 
>> rate seems to be about the same for the 'cheap' drives as for the 'good' 
>> drives. I won't say that they are exactly the same, but they are close 
>> enough that I don't believe that there is a significant difference.
>> 
>> in other words, these studies do seem to match my experiance.
> Fine.  Let's pretend =You= get to build Citibank's or Humana's next mission 
> critical production DBMS using exclusively HDs with 1 year warranties.
> (never would be allowed ITRW)

who is arguing that you should use drives with 1 year warranties? in case 
you blinked consumer drive warranties are backup to 5 years.

> Even if you RAID 6 them, I'll bet you anything that a system with 32+ HDs on 
> it is likely enough to spend a high enough percentage of its time operating 
> in degraded mode that you are likely to be looking for a job as a consequence 
> of such a decision.
> ...and if you actually suffer data loss or, worse, data corruption, that's a 
> Career Killing Move.
> (and it should be given the likely consequences to the public of such a F* 
> up).

so now it's "nobody got fired for buying SCSI?"

>> this is why, when I recently had to create some large capacity arrays, I'm 
>> only ending up with machines with a few dozen drives in them instead of 
>> hundreds. I've got two machines with 6TB of disk, one with 8TB, one with 
>> 10TB, and one with 20TB. I'm building these sytems for ~$1K/TB for the disk 
>> arrays. other departments sho shoose $bigname 'enterprise' disk arrays are 
>> routinely paying 50x that price
>> 
>> I am very sure that they are not getting 50x the reliability, I'm sure that 
>> they aren't getting 2x the reliability.
> ...and I'm very sure they are being gouged mercilessly by vendors who are 
> padding their profit margins exorbitantly at the customer's expense.
> HDs or memory from the likes of EMC, HP, IBM, or Sun has been overpriced for 
> decades.
> Unfortunately, for every one of me who shop around for good vendors there are 
> 20+ corporate buyers who keep on letting themselves get gouged.
> Gouging is not going stop until the gouge prices are unacceptable to enough 
> buyers.

it's also not going to be stopped until people actually look at the 
reliability of what they are getting, rather than assuming that becouse 
it's labled 'enterprise' and costs more that it must be more reliable.

frankly, I think that a lot of the cost comes from the simple fact that 
they use smaller SCSI drives (most of them haven't starting useing 300G 
drives yet), and so they end up needing ~5x more drive bays, power, 
cooling, cableing, ports on the controllers, etc. if you need 5x the 
number of drives and they each cost 3x as much, you are already up to 15x 
price multiplier, going from there to 50x is only adding another 3x 
multiplier (which with the extra complexity of everything is easy to see, 
and almost seems reasonable)

> Now if the issue of price difference is based on =I/O interface= (SAS vs SATA 
> vs FC vs SCSI), that's a different, and orthogonal, issue.
> The simple fact is that optical interconnects are far more expensive than 
> anything else and that SCSI electronics cost significantly more than anything 
> except FC.
> There's gouging here as well, but far more of the pricing is justified.

going back to the post that started this thread. the OP was looking at two 
equivalently priced systems, one with 8x73G SCSI and the other with 
24x300G SATA. I don't buy the argument that the SCSI electronics are 
_that_ expensive (especially since SATA and SAS are designed to be 
compatable enough to plug togeather). yes the SCSI drives spin faster, and 
that does contribute to the cost, but it still should't make one drive 
cost 3x the other.

>> I believe that the biggest cause for data loss from people useing the 
>> 'cheap' drives is due to the fact that one 'cheap' drive holds the capacity 
>> of 5 or so 'expensive' drives, and since people don't realize this they 
>> don't realize that the time to rebuild the failed drive onto a hot-spare is 
>> correspondingly longer.
> Commodity HDs get 1 year warranties for the same reason enterprise HDs get 5+ 
> year warranties: the vendor's confidence that they are not going to lose 
> money honoring the warranty in question.

at least seagate gives 5 year warranties on their consumer drives.

> AFAIK, there is no correlation between capacity of HDs and failure rates or 
> warranties on them.

correct, but the larger drive will take longer to rebuild, so your window 
of vunerability is larger.

> Your point regarding using 2 cheaper systems in parallel instead of 1 gold 
> plated system is in fact an expression of a basic Axiom of Systems Theory 
> with regards to Single Points of Failure.  Once components become cheap 
> enough, it is almost always better to have redundancy rather than all one's 
> eggs in 1 heavily protected basket.

also correct, the question is 'have hard drives reached this point'

the thought that there isn't a big difference in the reliability of the 
drives doesn't mean that the enterprise drives are getting worse, it means 
that the consumer drives are getting better, so much so that they they are 
a valid option.

if I had the money to waste, I would love to see someone open the 
'consumer grade' seagate Barracuda 7200.10 750G drive along with a 
'enterprise grade' seagate Barracuda ES 750G drive (both of which have 5 
year warranties) to see if there is still the same 'dramatic difference' 
between consumer and enterprise drives that there used to be.

it would also be interesting to compare the high-end scsi drives with the 
recent SATA/IDE drives. I'll have to look and see if I can catch some 
dead drives before they get destroyed and open them up.

> Frankly, the only thing that made me feel combative is when someone claimed 
> there's no difference between anecdotal evidence and a professional opinion 
> or advice.
> That's just so utterly unrealistic as to defy belief.
> No one would ever get anything done if every business decision had to wait on 
> properly designed and executed lab studies.

I think the assumption on lists like this is that anything anyone says is 
a professional opinion, until proven otherwise. but a professional 
opinion (no matter who it's from) isn't as good as a formal study

> It's also insulting to everyone who puts in the time and effort to be a 
> professional within a field rather than a lay person.

it's also insulting to assume (or appear to assume) that everyone who 
disagrees with your is a lay person. you may not have meant it (this is 
e-mail after all, with all the problems that come from that), but this is 
what you seem to have been implying, if not outright saying.

> Whether there's a name for it or not, there's definitely an important 
> distinction between each of anecdote, professional opinion, and study result.

the line between an anecdote and a professional opinion is pretty blury, 
and hard to see without wasting a lot of time getting everyone to give 
their credentials, etc. if a professional doesn't spend enough time 
thinking about some of the details (i.e. how many drive failures of each 
type have I seen in the last 5 years as opposed to in the 5 year timeframe 
from 1980-1985) they can end up giving an opinion that's in the range of 
reliability and relavance that anecdotes are.

don't assume malice so quickly.

David Lang

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Subject: Re: SCSI vs SATA
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