Apart from the dates, this sounds highly plausible :-) If the hashing is done before the compression and the compression is done for isolated blocks, then this could even work!
Any takers? ;-) For a performance enhancement, keep a hash tree in memory for the "n" most recently used/seen blocks?... A good writeup! Thanks for a good giggle. :-) Regards, Martin On 01/04/13 15:44, Harald Glatt wrote: > On Mon, Apr 1, 2013 at 2:50 PM, Josef Bacik <jba...@fusionio.com> wrote: >> Hello, >> >> I was bored this weekend so I hacked up online dedup for Btrfs. It's working >> quite well so I think it can be more widely tested. There are two ways to >> use >> it >> >> 1) Compatible mode - this is a bit slower but will handle being used by older >> kernels. We use the csum tree to find duplicate blocks. Since it is >> relatively >> easy to have crc32c collisions this also involves reading the block from disk >> and doing a memcmp with the block we want to write to verify it has the same >> data. This is way slow but hey, no incompat flag! >> >> 2) Incompatible mode - so this is the way you probably want to use it if you >> don't care about being able to go back to older kernels. You select your >> hashing function (at the momement I only support sha1 but there is room in >> the >> format to have different functions). This creates a btree indexed by the >> hash >> and the bytenr. Then we lookup the hash and just link the extent in if it >> matches the hash. You can use -o paranoid-dedup if you are paranoid about >> hash >> collisions and this will force it to do the memcmp() dance to make sure that >> the >> extent we are deduping really matches the extent. >> >> So performance wise obviously the compat mode sucks. It's about 50% slower >> on >> disk and about 20% slower on my Fusion card. We get pretty good space >> savings, >> about 10% in my horrible test (just copy a git tree onto the fs), but IMHO >> not >> worth the performance hit. >> >> The incompat mode is a bit better, only 15% drop on disk and about 10% on my >> fusion card. Closer to the crc numbers if we have -o paranoid-dedup. The >> space >> savings is better since it uses the original extent sizes, we get about 15% >> space savings. Please feel free to pull and try it, you can get it here >> >> git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next.git dedup >> >> Thanks! >> >> Josef >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >> the body of a message to majord...@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > Hey Josef, > > that's really cool! Can this be used together with lzo compression for > example? How high (roughly) is the impact of something like > force-compress=lzo compared to the 15% hit from this dedup? > > Thanks! > Harald -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
