Mailinglist Archive: opensuse (729 mails)

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Re: [opensuse] raid use case
On 05/28/2015 12:57 PM, Carlos E. R. wrote:
(I can also make an hourly rsync crom job if raid is overkill)
Actually, IMO, an rsync job is safer than raid, it covers more failure
modes, but not as fast. Like deleting a file by mistake, or a filesystem
corruption.

I disagree -- only for this reason. The rsync job is not 'atomic'. (in a bastardized sense of the word, -- meaning writes will have occurred between the last rsync and the disk failure you are protecting against).

mdraid is by far the most flexible and robust and reliable and portable raid solution for your typical desktop or server. (it does not have a chip dependency of dmraid/fake raid [it's a good solution too, just know the limits]). But the purpose of raid is to provide real-time fault tolerance. Protection against a disk failing 'now' while preserving all incremental writes since the last rsync shot.

That is why rsync is no substitute for raid.

(it is also equally true)

That raid is no substitute for rsync. (to separate media)

As a general point of interest, both mdraid/dmraid can exist side-by-side in the same install (e.g.):

$ cat /proc/partitions
major minor #blocks name

<snip>
9 0 204608 md0
9 1 52396032 md1
9 2 922944192 md2
9 3 1047552 md3
253 0 244198583 dm-0
253 1 128457 dm-1
253 2 24410736 dm-2
253 3 1951866 dm-3
253 4 217704816 dm-4

So you are not precluded from adding arrays of either type to an existing
install.

Having run both for the past 15 years, I would lean toward mdraid for one primary reason -- rebuilding after a disk failure. Simply install a new drive, boot, and the rebuild is automatic. Boot as normal, an check progress with 'cat /proc/mdstat'. dmraid rebuilds (if available at all), usually involve booting to the dark side. (which is the real-world means shutting down the system, booting gparted (or using cfdisk or dd or rsync) to recreate partitions and then duplicating the remaining good drive to a new drive)

--
David C. Rankin, J.D.,P.E.
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