From: Calvin Walton <calvin.walton@kepstin.ca>
To: Hugo Mills <hugo@carfax.org.uk>
Cc: "Matwey V. Kornilov" <matwey.kornilov@gmail.com>,
Austin S Hemmelgarn <ahferroin7@gmail.com>,
linux-btrfs@vger.kernel.org
Subject: Re: btrfs: obtain block checksums from user space
Date: Mon, 28 Sep 2015 19:25:45 -0400 [thread overview]
Message-ID: <1443482745.12614.5.camel@kepstin.ca> (raw)
In-Reply-To: <20150928231612.GB25907@carfax.org.uk>
On Mon, 2015-09-28 at 23:16 +0000, Hugo Mills wrote:
> On Mon, Sep 28, 2015 at 07:11:51PM -0400, Calvin Walton wrote:
> >
> > The problem with trying to use btrfs checksums to compare two
> > different
> > files is that the blocks might not match up, if only due to
> > fragmentation. E.g., the same 1gb file might be stored like this on
> > one
> > machine:
> >
> > [ 256MB ][ 512 MB ][ 256MB ]
> >
> > And like this on the other:
> > [ 512MB ][ 512MB ]
> >
> > Since the checksums are per block, and the blocks can be different
> > arrangements on different machines, they're not really all that
> > useful
> > for doing comparisons like you want.
>
> No, those are extents, not blocks, and the FS doesn't checksum in
> extents. :) Blocks are 4 KiB in size.
Oh, wow, btrfs is storing a lot more checksums than I thought it was...
Thanks for clearing up my misconception.
--
Calvin Walton <calvin.walton@kepstin.ca>
next prev parent reply other threads:[~2015-09-28 23:25 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-24 18:06 btrfs: obtain block checksums from user space Matwey V. Kornilov
2015-09-24 18:35 ` Austin S Hemmelgarn
2015-09-24 18:48 ` Matwey V. Kornilov
2015-09-24 19:47 ` Austin S Hemmelgarn
2015-09-28 23:11 ` Calvin Walton
2015-09-28 23:16 ` Hugo Mills
2015-09-28 23:25 ` Calvin Walton [this message]
2015-10-01 16:59 ` David Sterba
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