[dm-devel] How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs

[Neil Brown]

On Wed, 4 Aug 2010 13:53:03 +0200 (CEST)
Mikael Abrahamsson <swmike at swm.pp.se> wrote:

> On Wed, 4 Aug 2010, Christoph Hellwig wrote:
> 
> > The good news is that you have it tracked down, the bad news is that I 
> > know very little about dm-crypt.  Maybe the issue is the single threaded 
> > decryption in dm-crypt?  Can you check how much CPU time the dm crypt 
> > kernel thread uses?
> 
> I'm not sure it's that. I have a Core i5 with AES-NI and that didn't 
> significantly increase my overall performance, as it's not there the 
> bottleneck is (at least in my system).
> 
> I earlier sent out an email wondering if someone could shed some light on 
> how scheduling, block caching and read-ahead works together when one does 
> disks->md->crypto->lvm->fs, becase that's a lot of layers and potentially 
> a lot of unneeded buffering, readahead and scheduling magic?
> 

Both page-cache and read-ahead work at the filesystem level, so only the
device in the stack that the filesystem mounts from is relevant for these.
Any read-ahead setting on other devices are ignored.
Other levels only have a cache if they explicitly need one.  e.g. raid5 has a
stripe-cache to allow parity calculations across all blocks in a stripe.

Scheduling can potentially happen at every layer, but it takes very different
forms.  Crypto, lvm, raid0 etc don't do any scheduling - it is just
first-in-first-out.
RAID5 does some scheduling for writes (but not reads) to try to gather full
stripes.  If you write 2 of 3 blocks in a stripe, then 3 of 3 in another
stripe, the 3 of 3 will be processed immediately while the 2 of 3 might be
delayed a little in the hope that the third will arrive.

The sys/block/XXX/queue/scheduler setting only applies at the bottom of the
stack (though when you have dm-multipath it is actually one step above the
bottom).

Hope that helps,
NeilBrown