Hi folks On Tuesday 17 April 2007 11:49, Darren J Moffat wrote: > Leonardo Francalanci wrote: > > Hi, > > > > regarding ZFS compression method: what happens when a compressed file is > > udpated/appended? Is it ALL un-compressed first, updated/appended and > > then re-compressed? Or only the affected blocks are uncompressed and > > then recompressed? > > ZFS does NOT compress files. It compresses the individual blocks that > are written to comprise the file. > > What this means is if you have a 10G file and you need to change one > small part in the middle you seek(2) to that point, read(2) the data > make your change to your in memory copy and write(2) it back out again. > ZFS will update (and recompress) only those blocks that actual need to > get changed.
How can this work? With compressed data, its hard to predict its final size before compression. How is the seek(2) accurate? How can the seek know where the uncompressed block (say) 345 is after the data is compressed? It will be physically on a different actual block...??. or I probably dont understand how compression with ZFS works.... ;-) Or - does it compress an 8k block into a smaller 6 or 4K block, but with the same block number, and so the seek to a block will still work, but less disk space is taken up ? Regards Jerome -- Jerome Haynes-Smith Sun TSC Storage Backline ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NOTICE: This email message is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply email and destroy all copies of the original message. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
