On 02/07/2017 11:28 PM, Kai Krakow wrote:

Am Thu, 19 Jan 2017 15:02:14 -0500
schrieb "Austin S. Hemmelgarn" [off-list ref]:

quoted

On 2017-01-19 13:23, Roman Mamedov wrote:

quoted

On Thu, 19 Jan 2017 17:39:37 +0100
[...]
And the DUP mode is still useful on SSDs, for cases when one copy
of the DUP gets corrupted in-flight due to a bad controller or RAM
or cable, you could then restore that block from its good-CRC DUP
copy.  

The only window of time during which bad RAM could result in only one 
copy of a block being bad is after the first copy is written but
before the second is, which is usually an insanely small amount of
time.  As far as the cabling, the window for errors resulting in a
single bad copy of a block is pretty much the same as for RAM, and if
they're persistently bad, you're more likely to lose data for other
reasons.

It depends on the design of the software. You're true if this memory
block is simply a single block throughout its lifetime in RAM before
written to storage. But if it is already handled as duplicate block in
memory, odds are different. I hope btrfs is doing this right... ;-)

In memory, it's just one copy, happily sitting around, getting corrupted
by cosmic rays and other stuff done to it by aliens, after which a valid
checksum is calculated for the corrupt data, after which it goes on its
way to disk, twice. Yay.

quoted

That said, I do still feel that DUP mode has value on SSD's.  The 
primary arguments against it are:
1. It wears out the SSD faster.

I don't think this is a huge factor, even more when looking at TBW
capabilities of modern SSDs. And prices are low enough to better swap
early than waiting for the disaster hitting you. Instead, you can still
use the old SSD for archival storage (but this has drawbacks, don't
leave them without power for months or years!) or as a shock resistent
USB mobile drive on the go.

quoted

2. The blocks are likely to end up in the same erase block, and 
therefore there will be no benefit.

Oh, this is probably a point to really think about... Would ssd_spread
help here?

I think there was another one, SSD firmware deduplicating writes,
converting the DUP into single again, giving a false idea of it being DUP.

This is one that can be solved by e.g. using disk encryption, which
causes same writes to show up as different data on disk.

-- 
Hans van Kranenburg