Some comments on ZFS

Gary Jennejohn garyj at
Mon Apr 16 10:28:55 UTC 2007

I've now moved to using ZFS on my -current machine.

Very nice not having to worry about the size of a FS.

My box has 2GB of memory and an AMD64 X2 4200+.

However, I've noticed some things which I find rather strange.

I. My old setup looked like this:
a) 2 36GB SCSI disks on AHC UFS/soft-updates - 80MB/s max. transfer rate.
   /usr/src was on one disk and /usr/obj on the other.
b) 2 SATA1 drives hooked to AHC using SATA -> SCSI adapters GJOURNAL on top
   of UFS/soft-updates - also 80MB/s max. transfer. These were also mounted

II. The new setup:
a) 1 36GB SCSI drive hooked to AHC - UFS/soft-updates. /usr/src is on this
b) 2 500GB SATA1 drives ZFS-mirrored. One is hooked up to the MB and the
   other is hooked up to the AHC using a SATA -> SCSI adapter.
c) 1 300GB and 1 160GB SATA drive ZFS-concatenated. One is hooked up to the
   MB and the other is hooked up to the AHC using a SATA -> SCSI adapter.
   /usr/obj is on this one. These are the same disks as in I(b).

I've turned off atime and compression on all ZFS file systems.

With the old setup I saw:
a) ``make -j2 buildworld'' usually pegged the CPU at 97%.
b) transferring data over NFS from a Linux client to this one pretty
   much always saturated the 100Mb/s link.

With the new setup:
a) ``make -j2 buildworld'' only reaches about 70% CPU load. Is ZFS stealing
   so much CPU?
b) while transferring data from I(b) to II(b) I saw several hard hangs of
   the machine without any error messages or warnings (I was working on the
   console). The only recourse was a hard reset. It turned out that some
   directories were somehow pathological and the box always hung while
   trying to copy them (I made several attempts). The only solution was
   to delete these (fortunately unimportant) directories.
c) transferring data over NFS from the same Linux client to this one (target
   II(c)) pretty much never saturates the 100Mb/s link. The CPU load wasn't
   all that high - maybe 3% or 4%.

Is there any ZFS setting analogous to async which I could use to speed up

Gary Jennejohn / garyjATjennejohnDOTorg gjATfreebsdDOTorg garyjATdenxDOTde

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