DM Writecache Target Merged For Linux 4.18, Helping Databases & More

[phoronix]

Phoronix: DM Writecache Target Merged For Linux 4.18, Helping Databases & More

There is at least one interesting feature with the just-merged Device Mapper (DM) changes for the Linux 4.18 kernel...

http://www.phoronix.com/scan.php?page=news_item&px=Linux-4.18-DM-Writecache

[nh2_]

This is fantastic! The write speed and latency of SSDs, the size of HDDs.

I have recently switched some of my servers to use the ext4 external journal feature on SSDs, and it gives me 100x faster fsync performance (great for databases and other services that need to guarantee durable data and thus call fsync()).

This is basically a generalised version of that which works for any file system.

[nh2_]

Michael Could we have a benchmark of this vs ext4 external journals?

(Especially measuring fsync performance, e.g. using `dd if=/dev/zero of=testfile bs=512 count=100 oflag=sync` and postgres/sqlite tests?)

[pegasus]

If I remember correctly, ext4 external journal uses some KB of kernel memory for every MB of its size, so it cannot be scaled indefinitely. DM implementation likely doesn't have this limitation.

I've looked at the patch and it seems it's already written with pmem and dax in mind. I'm sure RedHat is targeting "Intel Optane Memory" with it. Looking forward to get some to play with.

[nh2_]

pegasus If you find a reference to the ext4 external journal thing, I would be very interested in that!

[pegasus]

It was a discussion on linux-kernel with Tweedy or Tytso some 15 years ago I think ...

[SteveW]

Michael Could we have a benchmark of this... postgres/sqlite tests?)

Yes, please. MySQL too, if possible.