Adam Goryachev wrote:
>
> Is it really worthwhile considering a 3 drive RAID1 system, or even a 4
> drive RAID1 system (one hot spare). Of course, worthwhile depends on the
> cost of not having access to the data, but from a "best practice" point
> of view. ie, Looking at any of the large "online backup" companies, or
> gmail backend, etc... what level of redundancy is considered acceptable.
> (Somewhat surprising actually that google/hotmail/yahoo/etc have ever
> lost any data...)
I use a 3-member software RAID1 with one of the members created as
'missing', then periodically connect an equal-sized drive, add it to the
array and let it sync, then fail and remove it and rotate offsite. I
used to use external firewire drives as the rotated media, then switched
to SATA in a trayless hot-swap enclosure. This leaves a pair of
internal drives running as mirrors all the time (and yes, I have had
these fail, including a time when I only had one internal drive and had
to copy back from a week-old version). The advantage of this approach
is that the mirroring to the rotating drives can happen with the system
still running and the partition mounted. I only stop backuppc and
unmount it momentarily while failing the drive - and even if I didn't,
it would recover like it would if the system had crashed at that point.
Realistically, though, the disk is pretty busy during the sync and it
would not work well to have much other activity happening. It takes
about 2 hours to sync a 750 Gig partition if there are no backups or
restores running. If you didn't have the raid set up this way you could
simply stop backuppc, unmount the archive partition and use dd to
image-copy to an equal sized drive or partition.
I also have a USB cable adapter for the drive so I can mount the offsite
copies in a laptop or elsewhere in case of emergencies.
> So, using 4 x 100G drives provides 133G usable storage... we can lose
> any two drives without any data loss. However, from my calculations
> (which might be wrong), RAID6 would be more efficient. On a 4 drive 100G
> system you get 200G available storage, and can lose any two drives
> without data loss.
The real advantage of RAID1 is that you can access the data on any
single drive. The disadvantage is that you are limited to the size of a
single drive (probably 2TB now) and the speed it can work.
>> ZFS also is able to put metadata on a different volume and even have a
>> cache on a different volume which can spread out the chance of a loss.
>> very complicated schemes can be developed to minimize data loss.
>
> In my experience, if it is too complicated:
> 1) Very few people use it because they don't understand it
Well, mostly they don't use it because Linux doesn't include it...
> 2) Some people who use it, use it in-correctly, and then don't
> understand why they lose data (see the discussion of people who use RAID
> controller cards but don't know enough to read the logfile on the RAID
> card when recovering from failed drives).
Again, it is hard to beat software RAID1 for data recovery. Take any
drive that still works, plug into any computer that still works, mount
it and go.
> Last time I heard of someone using ZFS for their backuppc pool under
> linux, they didn't seem to consider it ready for production use due to
> the significant failures. Is this still true, or did I mis-read something?
ZFS doesn't work with Linux. The freebsd version might work well enough
but it would make more sense to use some flavor of opensolaris if you
want ZFS now. It should be possible to use the ZFS incremental
send/receive function to keep a remote system in sync but I don't think
anyone has done a serious test with backuppc.
> Personally, I used reiserfs for years, and once or twice had some
> problems with it (actually due to RAID hardware problems). I have
> somewhat moved to ext3 now due to the 'stigma' that seems to be attached
> to reiserfs. I don't want to move to another FS before it is very stable...
I used reiserfs for a while with no problems too. Before ext3 it was the
best way to avoid long fscks. However I'm not sure I'd expect an fsck
to be reliable if you ever do need recovery.
>> On-line rebuilds and
>> filesystems aware of the disk systems are becoming more and more relevant.
>
> I actually thought it would be better to disable these since it:
> 1) increases wear 'n' tear on the drives
> 2) what happens if you have a drive failure in the middle of the rebuild?
>
> Mainly the 2nd one scared me the most.
I've had that with a 2-drive RAID1 and you end up with both drives bad.
Which is why I now use a 3-drive mirror and 4 drives, one of which is
always offsite.
--
Les Mikesell
lesmikesell AT gmail DOT com
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