[dm-devel] [PATCH] dm writecache: fix performance degradation in ssd mode

[J. Bruce Fields]

Sorry for the delay testing:

On Sat, Jan 23, 2021 at 09:19:56AM -0500, Mikulas Patocka wrote:
> On Fri, 15 Jan 2021, J. Bruce Fields wrote:
> 
> > Any recommendations?  I spent some time with blktrace/blkparse/btt and
> > can't make head or tails of them, I'm afraid.
> > 
> > --b.
> 
> Hi
> 
> Try this patch:

Looks good!

I applied this to Fedora's 5.10.7-200.fc33.x86_64 and timed an untar of
linux-5.9.tar.gz onto an NFS export of my dm-writecache setup and got
5m14s, where without the patch it took nearly 2 hours.

For comparison, the same untar took about 100 minutes to an export of
the backend hard drives, and 3m47s to an export of the caching device
(an Optane MEMPEK1W016GA).

I also looked again at a trace of an untar of the smaller nfs-utils
source, and saw that knfsd's vfs_sync_range()s were mainly returning in
a few hundred microseconds with the occasional latency of up to 3ms,
where before they frequently taking about 15ms.

(This is all nice because it means a $20 piece of hardware plus any old
hard drives can get me acceptable NFS server performance, whereas once
upon a time it would have required a drive array with battery-backed RAM
cache, or more recently an enterprise SSD with power loss protection).

Thanks for the persistence!  (In both senses.)

--b.

> From: Mikulas Patocka <mpatocka at redhat.com>
> 
> Fix a thinko in ssd_commit_superblock. region.count is in sectors, not
> bytes. This bug doesn't corrupt data, but it causes performance
> degradation.
> 
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> Fixes: dc8a01ae1dbd ("dm writecache: optimize superblock write")
> Cc: stable at vger.kernel.org	# v5.7
> Reported-by: J. Bruce Fields <bfields at redhat.com>
> 
> ---
>  drivers/md/dm-writecache.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6/drivers/md/dm-writecache.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-writecache.c	2021-01-23 10:51:31.000000000 +0100
> +++ linux-2.6/drivers/md/dm-writecache.c	2021-01-23 15:16:53.000000000 +0100
> @@ -523,7 +523,7 @@ static void ssd_commit_superblock(struct
>  
>  	region.bdev = wc->ssd_dev->bdev;
>  	region.sector = 0;
> -	region.count = PAGE_SIZE;
> +	region.count = PAGE_SIZE >> SECTOR_SHIFT;
>  
>  	if (unlikely(region.sector + region.count > wc->metadata_sectors))
>  		region.count = wc->metadata_sectors - region.sector;