Spun off from
viewtopic.php?p=4236#p4236royfactorial wrote:… how much RAM does the UBC normally take up on a system that isn't under memory pressure? For example, I have 32GB, with only about 10GB being used at the moment. Is the 'free' RAM listed in Activity Monitor being used for UBC behind the scenes?
To the Activity Monitor question: no.
My understanding:
- free memory is truly free – not used for the unified buffer cache or any other purpose
- the memory used for UBC is classified as inactive.
royfactorial wrote:… how much?
Sorry, I can't recall whether there's an easy way to measure the amount or percentage of memory that's used for UBC.
royfactorial wrote:My concern with ZFS was that, since it likes to use … RAM for ARC, that it might be caching a significantly greater quantity of data vs HFS+ and the UBC. …
In the opening post here –
generally, ZFS needs lots of memory.
Why might an operating system with ZFS use more memory than if the same data was stored with HFS Plus?There are probably good technical explanations beyond this forum.
Less technical, echoed from
viewtopic.php?p=3812#p3812 –
grahamperrin wrote:With HFS Plus, there's the catalog file and so on. Ready made, if you like.
As I understand it … with ZFS there's nothing like that catalogue. And so, more need to traverse things. …
– and I guess that less equity (in output from zstat) may be seen when, for example, there's less traversal.
LinksI/O Kit Fundamentals: Managing Data: More on Memory Cursors: DMA and System Memory,
https://developer.apple.com/library/mac ... aMgmt.html#//apple_ref/doc/uid/TP0000019-BCIFHFJJ
> … The Unified Buffer Cache (UBC) is a kernel optimization
> that combines the file-system cache and the virtual-memory
> (VM) cache. The UBC eliminates the situation where the same
> page is duplicated in both caches. Instead, there is just
> one image in memory …
Activity Monitor: View system memory usage (2012-05-08)
Using Activity Monitor to read System Memory and determine how much RAM is being used (2012-05-03)
- includes a link to another part of the Mac Developer Library, although that part doesn't mention UBC.
Last but not least, there's
discussion of ARC and UBC under
Large number of fseventsd errors during send.