On Sun, 18 Apr 1999, Dave Cinege wrote:
> "m. allan noah" wrote:
> >
> > ok dave, look at it this way.
> >
> > 1 in a million chance that one drive will fail.
> > 1 in a million chance that you other drive will fail.
> >
> > 2 disk raid0 setup, either disk can destroy your filesystem.
> > 2 in a million chance of md device failure.
>
> I can reverse that and quantify it exponentially that the chance of full
> or partial file recovery is greater if the data is spread across multiple
> drives.
i may be wrong, but i dont think this is worth the risk. i lost one disk
out
of a 3 disk raid0 array. the small size of the individual stripes, but
relatively large size of some files meant that i lost 1/3 of the body of
each of those files. in the areas where i had many small files, some files
were gone entirely, others where half missing, etc. yes, the chance of
recovery is there, but most of the files were so screwed up that it was
not worth the effort.
>
> But that doesn't matter because important data gets put to multiple back-up
> tapes, and their is NO drive combination to substitute that.
>
> > two disks doubles your chances of railure in raid0. three disks triple the
> > chance. and do not think you can recover any of the data, cause the
> > striping makes that a bitch. i know, i have done it.
>
> Sector data is stripped (in HW raid anyway), leaving you a very good chance
> many peices of data are contigous across fewer then X amount of drives in the
> RAID.
> (As a result of the filesystem algorith)
>
i am not familiar with the algorithm used be hw raid controllers, but most
stuff i have heard of from a theoretical standpoint is to use the
individual disks in a round-robin fashion. so yes, you may keep several
contiguous blocks, but the file will still be trash if any of it is on
the other disk. this will still require restoration from tape most likely.
raid 0 increases the chances that i will have to restore from tape, cause
it is often difficult to deal with the disks directly after they have
been striped. somehow increasing the chance that you will need to restore
your / fs from tape is a bad idea.
perhaps this comes down to a personal time versus money choice. the time
spent restoring busted fs on a raid0 array is more expensive than a new
bigger disk, that is more likely to last longer than your little disk
anyway.
allan
