Re: The Small Company's Guide to Hard Drive Failure and Linux

["Anthony.zboralski" <bcs2005 () bellua com>]

On 19 Nov 2004, at 03:34, Derek Vadala wrote:

> Anthony.zboralski said:
>
> > Make sure you stay away from "hardware RAID" as most of the
> > implementations
> > don't even support RAID5 and the performance is really poor 15
> > meg/second again 100+ with software raid. Plus you're stuck with a
> > vendor with poor support.
>
> Again, what you are calling hardware RAID here is probably firmware 
> RAID.
> All the decent vendors can do RAID-5 in hardware. In good cases, this 
> can
> be better than software RAID-5 due to caching. On-controller batteries
> also help with those pending writes when a power failure strikes. 
> Software
> RAID-5 has good read speeds, like any RAID, but writes can be an 
> issues.
> Imagine you have a five disk RAID-5 with a 5k stripe size (nevermind 
> that
> this is not a valid stripe size). Each time you perform a write, you 
> need
> to pre-read the parallel blocks in the stripe you are writing. This 
> causes
> a lot of performance overhead for writes, and it's why a lot of people 
> who
> can afford to use RAID-10 instead of RAID-5.
>
> > /dev/md0:
> >   Timing buffered disk reads:  168 MB in  3.01 seconds =  55.79 MB/sec
> > root@dis:/home/acz# hdparm -T /dev/md0
> >
> > /dev/md0:
> >   Timing cached reads:   1176 MB in  2.00 seconds = 586.62 MB/sec
> >
> > CPU usage is really minimal on this machine (1.8ghz AMD 2500+, 1gig of
> > DDR ram),
>
> Again, CPU overhead for RAID hasn't been a factor in a really long 
> time.
> This is about I/O, not CPU-- just like nearly every computing 
> bottleneck
> these days. The stats you posted are totally useless too. Even if they 
> are
> accurate, which I doubt, they are for read performance. You're always
> going to get read performance that's near the speed of your real disks.
> I'm guessing your disks have a transfer rate of about 55MB/sec. Writes 
> are
> what is important. Always. Reads are commodity.

True but the performance's still better the "firmware" raid I have.
Reads may be commodity but in my case I use this storage mostly for 
reading
and I don't have imperatives on how long writing will take.

> > RAID5 or RAID6 is really the best way to go in terms of security and
> > performance. RAID5
> > allows 1 drive failure (and will rebuilt its state automatically if 
> > you
> > have a spare) and RAID6 allows 2 drives to fail at the same time. 
> > Using
> > other raid modes for anything is pure waste unless you work with  big
> > temporary files, for which the performance boost of a stripping array
> > will come handy; 1 disk failure on a stripping array and you can say
> > bye to your data.
>
> Okay. This is one of those things that ends up buried in a mailing list
> archive for years, causing confusion among people trying to get good
> information.
>
> Most people don't want RAID-6. Most people don't care about surviving a
> multi-disk failure on a single array. Most people who need to do that 
> will
> probably use a 3-disk RAID-1. That's right, you can create a RAID-1 of 
> as
> many member disks as you like. Three disks means you get three copies 
> of
> the data, four.... five... etc...
>
> RAID-5 and RAID-6 are not the best way to go in terms of security and
> performance. RAID-10 probably is, or perhaps even RAID-1. RAID-5 is 
> great
> when you are poor and don't want to waste disk space, but there are
> drawbacks. RAID-6 inherits all of that and then some.

Thanks for the clarification, I might no expert in RAID storage and I 
just wanted
to share my personal experience.

Best regards,

Anthony

--
Anthony C. Zboralski  <anthony.zboralski () bellua com>
PT Bellua Asia Pacific - http://www.bellua.com
Bumi Daya Plaza 18th Floor, jl. Iman Bonjol No.61
Jakarta 10310 Indonesia. Phone: +62213918330 HP:+628159102495
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