On 16 October, 2008 - Ross sent me these 1,1K bytes: > I might be misunderstanding here, but I don't see how you're going to > improve on "zfs set primarycache=metadata". > > You complain that ZFS throws away 96kb of data if you're only reading > 32kb at a time, but then also complain that you are IO/s bound and > that this is restricting your maximum transfer rate. If it's io/s > that is limiting you it makes no difference that ZFS is throwing away > 96kb of data, you're going to get the same iops and same throughput at > your application whether you're using 32k or 128k zfs record sizes.
But with 1Gb FC, if I'm reading 100MB/s it matters if 100MB or 25MB/s of those are actually used for something.. > Also, you're asking on one hand for each disk to get larger IO blocks, > and on the other you are complaining that with large block sizes a lot > of data is wasted. .. if I turn off data caching (and only leave metadata caching on). > That looks like a contradictory argument to me as you can't you're > asking have both of these. You just need to pick whichever one is > more suited to your needs. > > Like I said, I may be misunderstanding, but I think you might be > looking for something that you don't actually need. Ok. ZFS prefetch can help, but I don't want it to use up all my ram for data cache.. Using it for small temporary buffers while reading stuff from disk is good, but once data has been read from disk and used once (delivered over NFS), there is a very low probability that I will need it again before it has been flushed (because 4TB > 8GB). With default tuning, ZFS will keep stacking up these "use once" data blocks in the cache, pushing out metadata cache which actually has a good chance of being used again (metadata for all our files can fit in the 8GB of ram, but 4TB of data can't). So if I could tell ZFS, "Here you have 512M (or whatever) of ARC space that you can use for prefetch etc. Leave the other 7.5GB of ram for metadata cache." /Tomas -- Tomas Ögren, [EMAIL PROTECTED], http://www.acc.umu.se/~stric/ |- Student at Computing Science, University of Umeå `- Sysadmin at {cs,acc}.umu.se _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
