Re: [Xen-users] Disk i/o on Dom0 suddenly too slow

[Micky <mickylmartin@xxxxxxxxx>]

> I've had similar issues, in fact, for the life of the LVM snapshot,
> performance seems to severely degrade. Usually a single snapshot is ok, but
> I wanted to have three snapshots, and each day delete the oldest and create
> a new one.
>

What a coincidence! I am doing exactly the same.

> I've found two "solutions":
> 1) Make your storage backend perform like a god so that after you take the
> snapshots performance is like a stroll down the road. (ie, I've upgraded to
> SSD based storage which can get approx 1.5TB/s write and 2.5TB/s read) ....
> 2) Only keep a single snapshot, and if possible, remove it as soon as your
> backup is completed.... and/or keep writes to a minimum while the snapshot
> is active.

That's what the script I wrote, is doing. Check
http://github.com/bassu/xen-scripts/
As for SSDs, I didn't find them stable as in long-term production environments!

> My plan is to do something like this:
> 1) Have two storage backend machines
> 2) Use DRBD to sync the two of them (primary sits on RAID device, secondary
> sits on LVM on RAID device)
> 3) Use LVM on top of the DRBD to create LV's for each domU
> 5) Take a snapshot using the underlying LVM (below DRBD) on the secondary
> 6) Run your backup processes on the snapshot of the DRBD
> 7) Delete the snapshot

Sounds a lot complicated. Block level snapshots under grouped block
level devices -- seems like a lot of overhead!
Gluster may be a lot more useful in this case -- just a slight guess.

> I haven't yet got that far in the process, so if you do something it would
> be helpful to hear about it.
>
> Also any other people who can share what they do and what works well/doesn't
> work would be nice to see.

I am experimenting with a few tricks. I will share the outcome like
the script I just shared :)

> Finally, the other problem I have with LVM on Debian (stable) is that every
> week or two, it will freeze on lvremove, and other lvs or LV related
> commands will freeze. The only solution seems to be a reboot. (Using kernel
> 3.2.0-4-686-pae #1 SMP Debian 3.2.41-2 i686). I haven't tracked this down or
> reported it yet, but it is frustrating to have to reboot the dom0 so often.

LVM is slow as heck when it comes to snapshots. And everywhere I look,
people talk about the "copy on write" magic,
but no one tells you that you are gonna bite your tongue!

> Regards,
Cheers.

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