On Sat, 20 Jun 2009 23:29:26 +0200, Dan Naumov <dan.nau...@gmail.com>
wrote:
I have the following setup:
A single consumer-grade 2tb SATA disk: Western Digital Green (model
WDC WD20EADS-00R6B0). This disk is setup like this:
16gb root partition with UFS2 + softupdates, containing mostly static
things:
/bin /boot /etc /root /sbin /usr /var and such
a 1,9tb non-redundant zfs pool on top of a slice, it hosts things like:
/DATA, /home, /usr/local, /var/log and such.
What should I do to ensure (as much as possible) filesystem
consistency of the root filesystem in the case of the power loss? I
know there have been a lot of discussions on the subject of
consumer-level disks literally lying about the state of files in
transit (disks telling the system that files have been written to disk
while in reality they are still in disk's write cache), in turn
throwing softupdates off balance (since softupdates assumes the disks
don't lie about such things), in turn sometimes resulting in severe
data losses in the case of a system power loss during heavy disk IO.
One of the solutions that was often brought up in the mailing lists is
disabling the actual disk write cache via adding hw.ata.wc=0 to
/boot/loader.conf, FreeBSD 4.3 actually even had this setting by
default, but this was apparently reverted back because some people
have reported a write performance regression on the tune of becoming
4-6 times slower. So what should I do in my case? Should I disable
disk write cache via the hw.ata.wc tunable? As far as I know, ZFS has
a write cache of it's own and since the ufs2 root filesystem in my
case is mostly static data, I am guessing I "shouldn't" notice that
big of a performance hit. Or am I completely in the wrong here and
setting hw.ata.wc=0 is going to adversely affect the write performance
on both the root partition AND the zfs pool despite zfs using it's own
write cache?
Another thing I have been pondering is: I do have 2gb of space left
unused on the system (currently being used as swap, I have 2 swap
slices, one 1gb at the very beginning of the disk, the other being 2gb
at the end), which I could turn into a GJOURNAL for the root
filesystem...
Using gjournal is a very trusted way for a good balance in consistency and
speed. I don't know about any performance impact of having the journal at
the other 'end' of the disk than where the fs is. You can try, because
switching back is possible.
Ronald.
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