RAID without TLER

[D. Hugh Redelmeier]

| From: Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org>

| On Wed, Feb 20, 2013 at 01:41:10AM -0500, D. Hugh Redelmeier wrote:
| > 1) they are quite a bit more expensive.
| 
| No, WD enterprise (yellow label) drives are expensive.
| 
| 2TB Red: $120
| 2TB Green: $100 (but much slower)
| 2TB Black: $170
| 
| 1TB Red: $85
| 1TB Green: $75
| 1TB Blue: $75
| 1TB Black: $95

Sale price for 3T Red: 140; 3T Seagate $90; both 7200RPM; Seagate has no 
ERC.

| So given the Red is rated 24/7 operation

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.

| with time limited error recovery,
| and has similar performance to the Blue, that seems pretty reasonable
| to me.

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.

| > 2) according to the smallnetbuilder article, consumer NAS devices
| >    don't use ERC.  Sounds odd to me.
| 
| Maybe not, but a drive that doesn't take very long to respond with
| 'bad read' is still good for any raid controller.

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).

| The time limited error recovery is what RAIDs have needed.  ERC isn't
| needed, but can be useful for high end RAID controllers perhaps.

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.

| A drive that disappears for 2 minutes trying to read a failing sector
| will cause the drive to drop out of most RAIDs, which is what the WD
| Red is meant to avoid.

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.
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