[TriLUG] New system configuration thoughts

Kevin Jones mrkevinj at yahoo.com
Thu May 3 21:00:39 EDT 2007


I spoke to a friend of mine who works in a large datacenter and asked him their failure rates with SCSI vs. SATA and he (well, his boss actually) said they had one SATA SAN that lost an average of 4 to 5 times as many drives as their SCSI SANs. They are replacing almost one a month on average he said. That seems excessive, but he mentioned that they were supposedly very high tolerance SATA disks. 

I suppose as long as you have a good maintenance contract and the company will replace every drive that breaks and you have sufficient redundancy (as in EVA-level) it probably doesn't make too much difference whether it's SCSI or SAN. Just seems like more of a headache to have to place that many service calls.

Kevin

----- Original Message ----
From: Jim Ray <jim at neuse.net>
To: Triangle Linux Users Group discussion list <trilug at trilug.org>
Sent: Thursday, May 3, 2007 3:17:55 PM
Subject: RE: [TriLUG] New system configuration thoughts

Learn something new every day!  Life is good.

Since SAS (serial attached SCSI) is here, I opted for SAS on my last server.
U320 may be a dying breed.

I have never user SATA RAID and probably never will in any mission critical
environment.

Regards,
 
Jim
 
Jim Ray, President
Neuse River Networks
tel: 919-838-1672 cell: 919-606-1772
http://www.Neuse.Net
 
Connecting You to the World since 1997
 

> -----Original Message-----
> From: trilug-bounces at trilug.org [mailto:trilug-bounces at trilug.org] On
Behalf Of Kevin
> Jones
> Sent: Thursday, May 03, 2007 10:43 AM
> To: Triangle Linux Users Group discussion list
> Subject: Re: [TriLUG] New system configuration thoughts
> 
> RAID 6 is RAID 5 with an extra parity disk so you can actually lose two
entire disks from
> an array without data loss. I would still recommend a hot spare for a RAID
6 array as well
> to allow the rebuild process to begin immediately. There is more overhead,
of course with
> RAID 6, as two entire disks have to be written out, in parity, across the
array; but it prevents
> the possibility of losing the entire array from a two-disk loss. I'm not
sure how often this
> actually occurs, but if I had a lot of very important data on a RAID 5
array, I'd definitely
> keep it backed up on disk or tape at all times just to be sure. Even with
a hot spare in a
> RAID 5 array, there is a 12-24 hour (depending on the size of the disks)
window where you
> could lose the entire array if another drive drops out during the rebuild
process.
> 
> To get RAID 6, you need to have a RAID 6 capable Host Bus Adapter (HBA). I
have set
> them up on HP DL3xx servers that come with a built-in HBA and also on
Adaptec SATA
> HBA's. Most newer RAID cards have RAID 6 capability. SCSI drives are
definitely the
> way to go for enterprise-class storage (particularly with high I/O
activity such as databases)
> as the MTBF (mean time between failure) rate is MUCH better with SCSI than
(S)ATA
> drives (approx. 3 years for SCSI vs. 1 year for SATA on average I think.)
In other words,
> plan on buying three SATA drives for every one U320 SCSI drive. Also, be
very careful
> about the SATA drives you purchase if putting them in a RAID array as
there is a very big
> difference between desktop-grade and enterprise (RAID-level)-grade SATA
drives.
> Unfortunately, the vendors probably don't know and manufacturers don't
make this point
> clear, but SATA is reasonably new to the enterprise and you can get stuck
with really
> crappy drives if you buy the wrong kind.
> 
> Until you do actual benchmarking with a particular drive and card, it's
difficult to know
> what you're real throughput will be. You also need a fast processor and
bus on the
> motherboard or the money spent on faster drives and HBA may be wasted.
It's difficult to
> make recommendations when it comes to RAID as there are so many different
scenarios. If
> you were running a large database you'd want to set up your array in a
very different
> manner from setting up a file server, etc. Just do your research before
putting anything in to
> production.
> 
> Kevin
> 
> 
> ----- Original Message ----
> From: Jim Ray <jim at neuse.net>
> To: Triangle Linux Users Group discussion list <trilug at trilug.org>
> Sent: Thursday, May 3, 2007 5:30:22 AM
> Subject: RE: [TriLUG] New system configuration thoughts
> 
> 
> I RAID6 the same as RAID5 plus a hot spare?  What is your favorite flavor
of
> RAID card and hard disk?
> 
> I'm partial to those serial attached SCSI drives with the 15,000 rpm
speed.
> 
> Regards,
> 
> Jim
> 
> Jim Ray, President
> Neuse River Networks
> tel: 919-838-1672 cell: 919-606-1772
> http://www.Neuse.Net
> 
> Connecting You to the World since 1997
> 
> > -----Original Message-----
> > From: trilug-bounces at trilug.org [mailto:trilug-bounces at trilug.org] On
> Behalf Of Kevin
> > Jones
> > Sent: Wednesday, May 02, 2007 10:54 PM
> > To: Triangle Linux Users Group discussion list
> > Subject: Re: [TriLUG] New system configuration thoughts
> >
> > I'll attest to that as it happened to me. Since that event I have never
> trusted RAID 5 again
> > and have opted for RAID 6 whenever possible. Of course, if you can't do
> RAID 6 always,
> > always have a hot spare online so the rebuild can begin immediately.
> >
> > Kevin
> >
> > ----- Original Message ----
> > From: Brian Henning <Brian.Henning at datadirect.com>
> > To: Triangle Linux Users Group discussion list <trilug at trilug.org>
> > Sent: Wednesday, May 2, 2007 7:53:23 PM
> > Subject: RE: [TriLUG] New system configuration thoughts
> >
> > Kevin Flanagan supposed:
> >
> > > How is RAID mirroring not redundancy?  It's redundancy where you don't
> > have
> > > a system outage in order to replace a disk that has failed.  Odds are
> > in
> > > your favor that you won't have 2 failures at the same time.  Mirror
> > first,
> > > _then_ you replicate the data to another system.
> >
> > Ever heard the suggestion about always replacing your headlights in
> > pairs?  The odds of having two failures (of equally-aged,
> > equally-stressed hardware, as in a RAID mirroring setup) at the same
> > time are fairly high.
> >
> > ~B
> >
> >
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> >
> >
> >
> >
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