Re: Poor write performance with write-intent bitmap?

[John Robinson <john.robinson@anonymous.org.uk>]

On 22/04/2009 10:16, Goswin von Brederlow wrote:
> John Robinson <john.robinson@anonymous.org.uk> writes:
>> Can't do that, my root filesystem is on the RAID-5, and part of the
>> reason for wanting the bitmap is because the md can't be stopped while
>> shutting down, so it was always wanting to resync at startup, which is
>> rather tedious.
> 
> Normal shutdown should put the raid in read-only mode as last step. At
> least Debian does that. That way even a mounted raid will be clean
> after reboot.

Yes, I would have thought it should as well. But I've just looked at 
CentOS 5's /etc/rc.d/halt and as far as I can see it doesn't try to 
switch md devices to read-only. Of course the root filesystem has gone 
read-only but as we know that doesn't mean the device underneath it gets 
told that. In particular we know that ext3 normally opens its device 
read-write even when you're mounting the filesystem read-only (iirc it's 
so it can replay the journal).

Another issue might be the LVM layer; does that need to be stopped or 
switched to read-only too?

> I would also suggest restructuring your system like this:
> 
> sdX1 1GB  raid1  / (+/boot)
> sdX2 rest raid5  lvm with /usr, /var, /home, ...
> 
> Both / and /usr can usualy be read-only preventing any filesystem
> corruption and raid resyncs in that part of the raid.

I did do this multiple partition/LV thing once upon a time, but I got 
fed up with having to resize things when one partition was full and 
others empty. The machine is primarily a fileserver and Xen host, so the 
dom0 only has 40GB of its own, and I couldn't be bothered splitting that 
up. Having said all this, your suggestion is a good one, it's just my 
preference to have it otherwise :-)

Cheers,

John.