Opinions on new Fedora Core 2 install with LVM 2 and snapshots?

[Bill Rugolsky Jr.]

On Mon, Jul 26, 2004 at 06:41:52PM -0400, Bryan J. Smith wrote:
> At $325 for a 4-channel 3Ware Escalade 9500S-4LP, I really see no reason
> not to put one in when you have 4 drives.  You get a powerful 64-bit
> ASIC, plus SRAM for 0 wait state transfers/queuing, plus 128MB
> (expandible upto 1GB) of NVRAM backed SDRAM for buffered I/O (RAID-5
> writes as well as general read buffer).
 
I've used earlier 3ware products, and they are quite good.  Now that there
is a CLI configuration tool, even better.  (Though the fact that it is
closed source is rather silly.)

There are some corruption issues with 3ware 9500's on Opteron boards.
The IOMMU flush workaround is ugly.  I'll wait to see where and how
it gets resolved.  For now, I have 4 (somewhat slower) onboard SATA ports.

> What is RAID-6?  Is that 2 parity stripes so you can lose 2 disks?
 
Yes.  H. Peter Anvin implemented it a few months ago, and it is in the
mainline 2.6 kernel.

> But if people feel XFS is better for LVM2, then I'd use it.  I didn't
> see SGI releasing XFS for Fedora releases (unlike RHL before it), so I
> figured there was little reason to go that direction.  But I had to
> ask.
 
XFS is in the mainline kernel, and available in the Fedora 2.6 kernels.
xfsprogs is in FC2.

> > I like the 3ware controllers, but until their meta-data is supported
> > by dmraid or the like, I'll pass.
> 
> Why?  Is there some bonus for dmraid?  I rather like the fact that the
> OS has no idea what is underneath.

I don't like the idea of my data stored in a proprietary format.
When the controller or machine dies (perhaps several years from now) or I
have a simultaneous two-drive partial failure (which happened three years
ago), I want to be able to attach the drives to standard ports and use
"dd" and Device Mapper or MD to recover my data.

> > Because every kernel has bugs, and hardware can be flakey.
> > Corruption can occur irrespective of journaling.
> 
> Really?  I haven't run into this with Ext3 or XFS (other than the one
> XFS 1.0 bug that took out my /var on one system).  Is LVM2 flaky?
> 
> I'd rather not run it if so.

It seems that many of the "ext3 filesystem is corrupt" reports on
ext3-users and elsewhere have been IDE drives that don't flush writes on
powerdown, bad RAM, bad cables, overheating chipsets, and buggy DMA engines.
(Witness the 3ware/Opteron problem.)  Add to that occasional driver,
VM, and filesystem bugs (typically race conditions), as well as operator
error when doing things like fscking one partition of a RAID1 pair (i.e.,
sda1 instead of md1), or using GRUB "savedefault".

Journaling prevents none of this.  You've been lucky; better to be
vigilant.  We compare md5sums longitudinally (inter-day) and latitudinally
(across different hosts) every day via cron.  Snapshots allow the meta-data
to be sanity checked too.

> Hey, if you have any notes, I'm more than willing to put them into
> a HOWTO.

Probably not for a few months.

Thanx dude!
 
> P.S.  Do I need to do anything beyond loading a LVM2 kernel with "device
> mapper" to use "pvcreate" to do snapshots?
 
Something like:

modprobe dm-snapshot
lvm lvcreate -L 1G -s -n lv00snap vg00/lv00

Regards,

	Bill Rugolsky