Re: [BackupPC-users] Btrfs, hardlinks, and BackupPC...

I to am far from a BackupPC expert.

I do not know if BackupPC test for the file system's hard link upper limit, but do know that it tests for the lower limit (e.g. hard linking greater than 1, i.e. file system support hard links). However I do know that BackupPC does support a configuration to set a hard link upper limit:

$Conf(HardLinkMax) = 31999;

Maximum number of hardlinks supported by the $TopDir file system that BackupPC uses. Most linux or unix file systems should support at least 32000 hardlinks per file, or 64000 in other cases. If a pool file already has this number of hardlinks, a new pool file is created so that new hardlinks can be accommodated. This limit will only be hit if an identical file appears at least this number of times across all the backups.

Mark Potter
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On Mon, Aug 2, 2010 at 9:34 AM, Ken D'Ambrosio <ken AT jots DOT org> wrote:

Hey, all. I just did a quick search, and haven't noticed any hits on
"btrfs" in the subject lines of any e-mail here. Which makes me wonder if
anyone has noticed it. Btrfs only supports 256 (or is it 512?) hardlinks
per directory; that's a hard-coded value that is unlikely to change, as it
would require an on-disk format change. I'm fairly certain that BackupPC
will blow past that for a directory with lots of static entries, no? If
so, should BackupPC
- Figure out if it's on a btrfs filesystem, and, if so,
- switch from hardlinks to COW "clones"?

COW clones are a different mechanism entirely than a hardlink; however,
insofar as BackupPC is concerned, I think they might well be considered
identical. It's my understanding that BackupPC only hardlinks static
files that won't change; in this scenario, I believe a COW clone would
work just the same.

If there's a flaw in my logic, I'd deeply appreciate someone pointing it
out -- I'm not claiming to be an expert in either BackupPC or btrfs, but
believe I've outlined (correctly, even) the issues involved.

If nobody else is interested, I'm willing to take a stab at this, though
my spare cycles for the next month or two are asymptotically approaching
0.

Thanks,

-Ken

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