Linked by Thom Holwerda on Mon 2nd Nov 2009 23:20 UTC
ZFS has received built-in deduplication. "Deduplication is the process of eliminating duplicate copies of data. Dedup is generally either file-level, block-level, or byte-level. Chunks of data - files, blocks, or byte ranges - are checksummed using some hash function that uniquely identifies data with very high probability. Chunks of data are remembered in a table of some sort that maps the data's checksum to its storage location and reference count. When you store another copy of existing data, instead of allocating new space on disk, the dedup code just increments the reference count on the existing data. When data is highly replicated, which is typical of backup servers, virtual machine images, and source code repositories, deduplication can reduce space consumption not just by percentages, but by multiples."
Thread beginning with comment 392785
To view parent comment, click here.
To read all comments associated with this story, please click here.
To view parent comment, click here.
To read all comments associated with this story, please click here.
RE[2]: Clarification requested
by Kebabbert on Wed 4th Nov 2009 15:57
in reply to "RE: Clarification requested"
Member since:2007-07-27
Standard chksum algorithm is SHA256. Incidentally, Niagara SPARC computes SHA256 in chip hardware, achieving 41GB/sec.
You can also choose to use fletcher4, which is very fast but not cryptographically strong. Which means that there is a very low probability of yielding a collision.
With SHA256, the chance of a collision is 2^(-256) which is extremely extremely low probability. Maybe it is like 10^(-71) or so for two differing blocks to collide.
But, you can request that if there is a hash collision, ZFS must compare bit for bit. This makes dedupe totally safe against collisions.
Recent Original Stories
Recent Comments
Headlines
Random Comments
Random Stories
Random OS Link
Member since:
2009-07-10
I would *expect* to be the same as with compression= and checksum= options.
For example if you switch your checksumming algorithm from A to B, the old files are using A and new files B. Or, if you enable compression in a dataset that already has uncompressed files, they remain uncompressed. Only newly created files are affected.