On Jun 11, 2011, at 5:46 AM, Edward Ned Harvey wrote:

>> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
>> boun...@opensolaris.org] On Behalf Of Jim Klimov
>> 
>> See FEC suggestion from another poster ;)
> 
> Well, of course, all storage mediums have built-in hardware FEC.  At least 
> disk & tape for sure.  But naturally you can't always trust it blindly...
> 
> If you simply want to layer on some more FEC, there must be some standard 
> generic FEC utilities out there, right?
>       zfs send | fec > /dev/...
> Of course this will inflate the size of the data stream somewhat, but 
> improves the reliability...

The problem is that many FEC algorithms are good at correcting a few bits. For 
example, disk 
drives tend to correct somewhere on the order of 8 bytes per block. Tapes can 
correct more bytes
per block. I've collected a large number of error reports showing the bitwise 
analysis of data
corruption we've seen in ZFS and there is only one case where a stuck bit was 
detected. Most of
the corruptions I see are multiple bytes and many are zero-filled.

In other words, if you are expecting to use FEC and FEC only corrects a few 
bits, you might be
disappointed.
 -- richard

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