On 04/07/10 01:13, Craig Ringer wrote:
> You appear to be assuming that "disk backup" == "single disk backup" or
> "set of simple disks".
No, not at all. (Though in many smaller installations, it is.) I was
just noting that a single-purpose disk backup tool in which elimination
of interleaving and minimization of fragmentation are desired has some
significantly different needs than a general-purpose filesystem.
> Most practical disk backup setups will involve large RAID-5, RAID-6 or
> RAID-10 arrays. These tend to be striped across the spindles anyway, and
> the file system is rarely properly aware of how this striping occurs.
*nod* Indeed. Nor should it care, since for general purposes, it
shouldn't matter. It's not the filesystem's problem - except of course
in advanced filesystems like ZFS where the filesystem IS the RAID
implementation.
> For what it's worth, a quick check on my volumes does reveal significant
> (25% or so) fragmentation. I'm going to see if I can extend the sd with
> posix_fallocate(...) support and see if I can reduce that.
That would be an interesting and probably welcome proposal.
> Interleaved disk volumes complicate management of retention periods and
> lifetimes. They also make it harder to see what's using what space,
> where. That's why I want to avoid them, not for performance reasons.
Ah, that's very true.
Have you looked into using migration to de-interleave jobs after backup?
Bang it all onto disk as fast as possible during your backup window,
sort it out at leisure later?
> I'm wondering why I should worry too much about fragmentation, actually.
> The array performs quite well when significantly fragmented; I haven't
> noticed any significant write performance drops over time.
>
> It may slow restores a little, but again with a many-spindle array I'm
> not sure how much practical effect it'll have. Is fragmentation
> avoidance worth all this complexity?
Probably not. :)
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