On Mon, May 13, 2013 at 8:59 AM, Edward Ned Harvey (lopser)
<lop...@nedharvey.com> wrote:
>> From: tech-boun...@lists.lopsa.org [mailto:tech-boun...@lists.lopsa.org]
>> On Behalf Of Skylar Thompson
>>
>> I think checksumming has a place in backup/archive systems, but I'm not sure
>> that end-to-end checksumming will allow sufficient scalability, at least with
>> current filesystem technology. At $WORK, if we had to checksum each file on
>> each filesystem we backup, I doubt we could complete our backups in our
>> window,
>
> Right on. If you have block-level or filesystem-implemented data integrity,
> then you can rely on the filesystem. But without it, the only way you can
> check is to run a huge intensive scan. You definitely DON'T want to do that
> on every send, but you definitely WANT or NEED to do it sometimes.
>
> With a checksumming filesystem (at least, with btrfs & zfs) you're not likely
> to do your backups with tar or rsync or anything else. You're virtually
> certain to use zfs send, or btrfs send. It has also the benefit of not
> needing to walk the filesystem searching for changed stuff.
>
> But without those filesystems, it's very useful to be able to keep linearly
> rolling incremental backups, including a command to validate all your data,
> and perform validation upon restores as well.
>
> I do believe btrfs will obsolete ext someday. But I don't believe ext is
> going extinct any time soon.
>
>
>> What I think /could/ work, though, is if checksumming filesystems like ZFS
>> could expose the checksum data to user applications (like backup clients),
>
> The reason that's not possible is because the ZFS checksums don't relate to
> the files. They relate to data blocks, which may be file fragments, or
> contain multiple files, and always include various forms of filesystem
> metadata. So you'll always have to utilize your ZFS checksums via zfs
> internal commands. You can scrub your whole pool... There might be a fringe
> use case where it's useful to just validate the blocks that are related to
> certain files, without doing the whole pool... But I can't think of such a
> use case.
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