Tuesday, September 05, 2006

How much memory does ZFS consume?

When using ZFS standard tools give inaccurate values for free memory as ZFS doesn't use normal page cache and rather allocates directly kernel memory. When low-memory condition occurs ZFS should free its buffer memory. So how to get how much additional memory is possibly free?


bash-3.00# mdb -k
Loading modules: [ unix krtld genunix specfs dtrace cpu.AuthenticAMD.15 ufs md ip sctp usba fcp fctl lofs zfs random nfs crypto fcip cpc logindmux ptm ipc ]
> ::memstat
Page Summary                Pages                MB  %Tot
------------     ----------------  ----------------  ----
Kernel                     859062              3355   41%
Anon                       675625              2639   32%
Exec and libs                7994                31    0%
Page cache                  39319               153    2%
Free (cachelist)           110881               433    5%
Free (freelist)            385592              1506   19%

Total                     2078473              8119
Physical                  2049122              8004
>

bash-3.00# echo "::kmastat"|mdb -k|grep zio_buf|awk 'BEGIN {c=0} {c=c+$5} END {print c}'
2923298816


So kernel consumes about 3.2TB of memory and about 2.7GB is allocated to ZFS buffers and basically should be treated as free memory. Approximately free memory on this host is: Free (cachelist) + Free (freelist) + 2923298816.

I guess small script which do all the calculations automatically would be useful.
at

2 comments:

  1. Anonymous28 May, 2009 02:27

    #!/bin/ksh

    # ZFS I/O buffer (can be treated as free memory)
    ZIO_BUF=`echo "::kmastat"|mdb -k|grep zio_buf|awk 'BEGIN {c=0} {c=c+$5} END {pri
    nt c}'`
    ((ZFS_BUF = $ZIO_BUF / 1024 / 1024 ))

    # Free memory from cachelist and freelist out of memstat
    FREE=`echo "::memstat"|mdb -k|grep Free|awk 'BEGIN {c=0} {c=c+$4} END {print c}'
    `

    ((FREE_MEM = $ZFS_BUF + $FREE ))

    echo "Free memory = $FREE_MEM MB"

    ReplyDelete
  2. BK06 August, 2009 19:21

    A faster (and less resource intensive method) is :

    kstat -p zfs:0:arcstats:size

    So:

    FREE=$(($(pagesize)*$(kstat -p unix:0:system_pages:freemem|awk '{print $NF}')))

    ZIO_BUF=$(kstat -p zfs:0:arcstats:size|awk '{print $NF}')

    FREE_MEM=$((FREE+ZIO_BUF))

    ReplyDelete