On Mar 25, 2010, at 6:13 AM, Damon Atkins wrote: > In the "Thoughts on ZFS Pool Backup Strategies thread" it was stated that zfs > send, sends uncompress data and uses the ARC. > > If "zfs send" sends uncompress data which has already been compress this is > not very efficient, and it would be *nice* to see it send the original > compress data. (or an option to do it) > > I thought I would ask a true or false type questions mainly for curiosity > sake. > > If "zfs send" uses standard ARC cache (when something is not already in the > ARC) I would expect this to hurt (to some degree??) the performance of the > system. (ie I assume it has the effect of replacing current/useful data in > the cache with not very useful/old data depending on how large the ZFS send > is)
If you restrict answers to "true/false" then the answer is false :-) Actually, the answer is mostly false. The ARC is divided into a most frequently used cache and a most recently used cache. The send data should stick to the most recently used side. > If above true, zfs send and “zfs backup” (if it the cmd existed to backup > and restore a file or set of files with all ZFS attributes) would improve the > performance of normal read/write by avoiding the ARC cache (or if easier to > implement having its own private ARC cache). The zio pipeline can, in theory, be tapped between the checksum and decompression side, but I think you will find that this defeats both piped compression and receive compression. > > Or does it use the same sort of code, as setting “primarycache=none” on a > file system. > > Has anyone monitored ARC hit rates while doing a large zfs send? Yes. I see very good ARC hit rates when I send from a high transaction system. This is a good thing because recently written data is likely to be in the ARC. -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
