On 02/07/2017 11:28 PM, Kai Krakow wrote: > Am Thu, 19 Jan 2017 15:02:14 -0500 > schrieb "Austin S. Hemmelgarn" <ahferro...@gmail.com>: > >> On 2017-01-19 13:23, Roman Mamedov wrote: >>> 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. >> 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. > >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
