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    SubjectRe: [PATCH v1 00/16] ext4: Add metadata checksumming
    On Sun, Sep 04, 2011 at 07:41:03AM -0400, Martin K. Petersen wrote:
    > >>>>> "Darrick" == Darrick J Wong <djwong@us.ibm.com> writes:
    >
    > Darrick,
    >
    > Darrick> Furthermore, the nice thing about the in-filesystem checksum is
    > Darrick> that we bake in other things like the FS UUID and the inode
    > Darrick> number, which gives you a somewhat better assurance that the
    > Darrick> data block belongs to the fs and the file that the code think
    > Darrick> it belongs to.
    >
    > Yeah, I view DIF/DIX mostly as in-flight protection for writes. Whereas
    > FS metadata checksumming is great for problem detection at read time.
    >
    > Another problem with using the DIF app tag to store filesystem metadata
    > is that many array vendors use it internally and thus only disk drives
    > are likely to provide the app tag space.
    >
    >
    > Darrick> The DIX interface allows for a 32-bit block number and a 16-bit
    > Darrick> application tag ... which is unfortunately small given 64-bit
    > Darrick> block numbers and 32-bit inode numbers.
    >
    > I never understood the 32-bit ref tag. Seems silly to have a check that
    > wraps at the exact boundary where problems are most likely to occur.
    >
    > I advocated for a DIF Type with 16-bit guard tag and 48-bit ref tag but
    > that never went anywhere. Too bad - would have been easy for the storage
    > vendors to implement.

    >
    >
    > Darrick> As a side note, the crc-t10dif implementation is quite slow --
    > Darrick> the hardware accelerated crc32c is 15x faster, and the sw
    > Darrick> implementation is usually 3-6x faster. I suspect somebody will
    > Darrick> want to fix that before DIF becomes more widespread...
    >
    > The CRC32C op on Nehalem and beyond is really, really fast. It's
    > essentially free except for pulling the data through the cache. So it's
    > not entirely fair to use that as baseline for a pure software
    > implementation. What is the faster sw implementation are you referring
    > to, btw.?

    I have some benchmarking data for various crc algorithms here:
    https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums#Benchmarking

    The "faster sw implementation" that I was talking about is the slice-by-8
    algorithm that I sent to the crypto list a few days ago that's based off of Bob
    Pearson's slice-by-8 crc32 patch.

    In the huge table, "crc32c-by8-le" is crc32c slice-by-8.

    > lib/crc-t10dif is a regular 256-entry table-based CRC implementation. It
    > is done pretty much like all our other software CRCs. I seem to recall
    > attempting a bigger table but that yielded worse real life results due
    > to cache pollution.

    Yes, the only downside to the slice-by-8 method is that it eats 8K of data
    cache for the table. Not a huge issue on recent Intel and POWER where the L1D
    is 32K, but I imagine it could be painful elsewhere.

    Do you know of any faster crc16 algorithms? I guess it wouldn't be hard to
    make a family of crcs, each with different cache/speed characteristics.

    > On Westmere and beyond it is possible to accelerate generic CRC
    > calculation using the PCLMULQDQ operation. There are many of our CRC
    > functions that could benefit from this. However, so far intel have not
    > been willing to contribute the relevant code to Linux.
    >
    >
    > Darrick> The good news is that if you're really worried about integrity,
    > Darrick> metadata_csum and DIF/DIX aren't mutually exclusive features.
    > Darrick> Rejecting corrupted write commands at write time seems like a
    > Darrick> useful feature. :)
    >
    > Yup!
    >
    > --
    > Martin K. Petersen Oracle Linux Engineering
    > --
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