Quick question about the interaction of ZFS filesystem compression and the filesystem cache. We have an Opensolaris (actually Nexenta alpha-6) box running RRD collection. These files seem to be quite compressible. A test filesystem containing about 3,000 of these files shows a compressratio of 12.5x.
My question is about how the filesystem cache works with compressed files. Does the fscache keep a copy of the compressed data, or the uncompressed blocks? To update one of these RRD files, I believe the whole contents are read into memory, modified, and then written back out. If the filesystem cache maintained a copy of the compressed data, a lot more, maybe more than 10x more, of these files could be maintained in the cache. That would mean we could have a lot more data files without ever needing to do a physical read. Looking at the source code overview, it looks like the compression happens "underneath" the ARC layer, so by that I am assuming the uncompressed blocks are cached, but I wanted to ask to be sure. Thanks! -Andy This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
