[appologies for being away from my data last week]
David Dyer-Bennet wrote:
The more I look at it the more I think that a second copy on the same
disk doesn't protect against very much real-world risk. Am I wrong
here? Are partial(small) disk corruptions more common than I think?
I don't have a good statistical view of disk failures.
This question was asked many times in this thread. IMHO, it is the
single biggest reason we should implement ditto blocks for data.
We did a study of disk failures in an enterprise RAID array a few
years ago. One failure mode stands heads and shoulders above the
others: non-recoverable reads. A short summary:
2,919 total errors reported
1,926 (66.0%) operations succeeded (eg. write failed, auto reallocated)
961 (32.9%) unrecovered errors (of all types)
32 (1.1%) other (eg. device not ready)
707 (24.2%) non-recoverable reads
In other words, non-recoverable reads represent 73.6% of the non-
recoverable failures that occur, including complete drive failures.
Boo! Did that scare you? Halloween is next month! :-)
Seagate said today that in a few years 3.5" disks will store 2.5 TBytes.
Boo!
While I don't have data on laptop disk failures, I would not be surprised
to see a similar distribution, though with a larger mechanical damage
count. My laptops run hotter inside than my other systems and, as a rule
of thumb, your disk failure rate increases by 2x for every 15C change in
temperature. Is your laptop disk hot?
The case for ditto data is clear to me. Many people are using single-disk
systems, and many more people would really like to use single-disk systems
but they really can't.
Beyond spinning rust systems, there are other forms of non-volatile
storage which would apply here. For example, those people who suggested
that you should backup your presentation to a CD fail to note that a spec
of dust on the CD could lead you to lose one block of data. In my CD/DVD
experience, such losses are blissfully ignored by the system and you may
blame the resulting crash on the cheap hardware you bought from your
brother-in-law. Beyond CDs, I can see this as being a nice enhancement
to limited endurance devices such as flash.
While it is true that I could slice my disk up into multiple vdevs and
mirror them, I'd much rather set a policy at a finer grainularity: my
files are more important than most of the other, mostly read-only and
easily reconstructed, files on my system.
When ditto blocks for metadata was introduced, I took a look at the
code and was pleasantly suprised. The code does an admirable job of
ensuring spatial diversity in the face of multiple policies, even in
the single disk case. IMHO, this is the right way to implement this
and allows you to mix policies with ease.
As a RAS guy, I'm biased to not wanting to lose data via easy-to-use
interfaces. I don't see how this feature has any downside, but lots
of upside.
-- richard
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