List-id: Xen user discussion <xen-users.lists.xensource.com>
On 02/27/2009 04:03 PM, DOGUET Emmanuel wrote:
I have receive my Battery pack and 512MB upgrade, now it's fine :
Hardware RAID 5
512M cache
Write Cache (25/75)
8 x 146G
dom0 (1024MB, 1 cpu) 213 MB/s
domU ( 512MB, 1 cpu 192 MB/s
domU (4096MB, 2 cpu) 249 MB/s
A lot of hardware card tends not to use write cache when there is no
battery because they decide that's it's unsafe (which is not wrong ...
to my mind).
There is also a problems with filesystems ie xfs vs ext3 the first one
use barrier (if the underlining device support it which means not lvm at
least), if you do a bench against xfs it will be light year away from
ext3, remounting xfs -o nobarrier gives you back performance (in short
using barrier insure you that your metadata are *really* written to the
disk before doing something to real data which seems to be an hypothesis
of journalised FS)
Last year there was some papers in LWN about that and the fact because
some wants to turn on by default on ext3.
As a rule of thumbs: always buy battery if you intend to use cache in
your raid controller.
Matthieu.
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