Andrew Miller wrote:
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
Yup, your assumption is correct. We currently do compression below the
ARC. We have contemplated caching data in compressed form, but have not
really explored the idea fully yet.
-Mark
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