On Fri, Jan 7, 2011 at 3:16 AM, Matthew D. Fuller <fulle...@over-yonder.net> wrote: > On Thu, Jan 06, 2011 at 03:45:04PM +0200 I heard the voice of > Daniel Kalchev, and lo! it spake thus: >> >> You should also know that having large L2ARC requires that you also >> have larger ARC, because there are data pointers in the ARC that >> point to the L2ARC data. Someone will do good to the community to >> publish some reasonable estimates of the memory needs, so that >> people do not end up with large but unusable L2ARC setups. > > Estimates I've read in the past are that L2ARC consumes ARC space at > around 1-2%.
Each record in L2ARC takes about 250 bytes in ARC. If I understand it correctly, not all records are 128K which is default record size on ZFS. If you end up with a lot of small records (for instance, if you have a lot of small files or due to a lot of synchronous writes or if record size is set to a lower value) then you could potentially end up with much higher ARC requirements. So, 1-2% seems to be a reasonable estimate assuming that ZFS deals with ~10K-20K records most of the time. If you mostly store large files your ratio would probably be much better. One way to get specific ratio for *your* pool would be to collect record size statistics from your pool using "zdb -L -b <pool>" and then calculate L2ARC:ARC ratio based on average record size. I'm not sure, though whether L2ARC stores records in compressed or uncompressed form. --Artem _______________________________________________ freebsd-stable@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"
