Some comments on ZFS

Robert Watson rwatson at
Tue Apr 17 09:29:32 UTC 2007

On Mon, 16 Apr 2007, Maxim Sobolev wrote:

> Joerg t wrote:
>>> There is something weird - Just ran same test again to see whether it was 
>>> a fluke - it still take 1h12 - however I saw the oppersite of Joerg. The 
>>> "systat -vm" Freemem was permently on 2,2GB - this on a 4GB system that 
>>> normally shows a 1,2GB - or less after a buildworld.
>>> hmm. Pawel are there tests that can help?
>> also how can we determine the real memory consumption by zfs,arc and his 
>> vfs cache share ?
>> how can we precisely instrument zfs ?
> The "vmstat -m" will give you precise breakdown of kernel memory usage.

You'll also want to use "vmstat -z".  "-m" captures memory allocated by 
kernel malloc(9), which is used for variable-sized objects, or objects that 
are irregularly allocated.  "-z" captures memory allocated by the kernel zone 
allocator, used for regularly sized and frequently allocated objects.  Small 
allocations in malloc(9) are actually allocated via fixed size memory buckets 
in the zone allocator, so if looking at "-m" as well, ignore zones who's names 
consist solely of a number (i.e., "16", which is the 16-byte bucket).

Robert N M Watson
Computer Laboratory
University of Cambridge

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