RAID without TLER

[Lennart Sorensen]

On Wed, Feb 20, 2013 at 02:31:49PM -0500, D. Hugh Redelmeier wrote:
> Sale price for 3T Red: 140; 3T Seagate $90; both 7200RPM; Seagate has no 
> ERC.

I wouldn't put any data of mine on a Seagate drive.  No comparison at all.
Compare it against something reliable.

> Just what does that mean?  Drives don't know what time of day they are
> on.  Which spec realistically reflects this, MTBF?  Or is this another
> market segmentation trick.

Many drives sold for desktop use are only expected to run about 8 hours
a day 5 days a week.  They expect they will be off the rest of the time.

They claim the MTBF of the Red is 35% higher than the standard desktop
drive (I would think that means the Blue model).

Of course my experience is that leaving a drive always on makes it
last longer.

WD did put some of the WD green features in too, to help control heat
in small NAS enclosures, but without the annoying constant park that
the green drives like to do.

> Limiting error recovery time costs WD nothing.  Reliability might.
> Hence I'm willing to pay for that if it is demonstrable.  Neither
> anecdotes nor marketing constitute demonstrations.

Firmware wise it costs them nothing.

They offer free 24/7 tech support for red drives apparently.  No idea
what that's useful for.

> Doesn't matter too much: if you need an absolute bound on latency,
> maybe.  This has little effect on average latency since these errors
> are very rare (or something is very wrong and needs to be fixed).

What seagate calls ERC, western digital calls TLER, and it's not something
a RAID uses, it is something it requires.  If a disk decides to spend 2
minutes trying to complete a read that is failing in the hopes that it
just might eventually read it and then be able to remap it right away,
then the RAID will usually drop that disk as being dead.  That's not good.
Since you have raid (raid0 isn't raid), you would rather have the read
fail quickly, keep the disk in the raid with one unreadable area, have
the raid controller rewrite the bad area which lets the disk remap it.
No RAID rebuild needed and no slowdown.

> This terminology is a mess.  TLER is a WD marketing term (a good one,
> and apparently not trademarked).  In the interest of being neutral, I
> switchted to ERC which seems to be the generic term.

ERC is a seagate marketing term and much less accurate than TLER.
ERC sounds like something that the controller needs to talk to.
That isn't the case.  TLER says exactly what it does.

> Well yes, that's what I was saying (with some more details).  And
> ordinary inexpensive drives used to have that capability.  So they are
> giving us back, at a price, what they took away.  Seagate took away ERC 
> too.

That is true.  Why they took it away I don't know, but at least now
cheap drives are again available that have it.  I will certainly be
buying the WD red drives for any raid setup I do.  I think I have been
lucky never to have a problem with the Blacks so far, probably because
none of them have ever had a read error that took long to deal with.

-- 
Len Sorensen
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