[linux-lvm] Q: LVM over RAID, or plain disks? A:"Yes" = best of both worlds?

Nataraj incoming-redhat at rjl.com
Mon Nov 29 18:57:36 UTC 2010

hansbkk at gmail.com wrote:
>  - - - - - - My abject apologies to all for improper addressing in my
> previous messages (thanks to all those who set me straight :)
> Hope you're all still willing to consider my request for feedback.
> Start with a bit of context:
> - SAN/NAS (call it FILER-A) hosting say a dozen TB and servicing a few
> dozen client machines and servers, mostly virtual hosts. Another,
> larger (FILER-B - still just tens of TB) host's drives are used for
> storing backup sets, via not only Amanda, but also filesystems
> comprising gazillions of hard-linked archive sets created by (eg)
> rdiff-backup, rsnapshot and BackupPC. We're on a very limited budget,
> therefore no tape storage for backups.
> - I plan to run LVM over RAID (likely RAID1 or RAID10) for IMO an
> ideal combination of fault tolerance, performance and flexibility.
> - I am not at this point overly concerned about performance issues -
> reliability/redundancy and ease of recovery are my main priorities.
> Problem:
> For off-site data rotation, the hard-linked filesystems on FILER-B
> require full filesystem cloning with block-level tools rather than
> file-level copying or sync'ing. My current plan is to swap out disks
> mirrored via RAID, marking them as "failed" and then rebuilding using
> the (re-initialized) incoming rotation set.
> HOWEVER - the use of LVM (and possibly RAID10) adds complexity to the
> filesystems, which makes disaster recovery from the detached disk sets
> much more difficult than regular partitions on physical disks.
> Theoretical solution:
> Use RAID1 on the "top layer" to mirror the data stored in an LVM (set
> of) disk(s) on the one hand (call it TopRAID1) to ***regular
> partitions*** on actual physical disks on the other (call this the
> TopRAID2 side).
Your proposed solution is a bit confusing to understand, however raid1 
works for doing backups in the manner that you describe .  I use it 
myself and I have, over time read about others doing so as well.  Be 
sure to create your volumes with --bitmap=internal, that way when you 
swap in a drive, it won't need to replicate the entire drive, only the 
part that is changed.

If your not going to manage the drives yourself, you will need an 
operations staff that has a pretty good understanding of how raid works 
and/or possibly write a robust set of scripts to manage the drives, 
ensure that the correct volume is mounted, etc.  Also, I don't 
personally feel that disks are a suitable media for long term data 
archival, so if that is really your purpose, as opposed to a quick way 
to recover from a disk failure, then you might consider doing periodic 
tape or optical media backups as well.


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