Facebook Has Been Baking A New Space Cache System For Btrfs

[phoronix]

Phoronix: Facebook Has Been Baking A New Space Cache System For Btrfs

The Btrfs file-system updates were mailed in Sunday evening for the Linux 4.5 kernel. There is one big but experimental feature with this pull...

http://www.phoronix.com/scan.php?page=news_item&px=Btrfs-Pull-Linux-4.5

[wodencafe]

So what is the impact for somebody who uses Btrfs for normal day-to-day file system things? Speed improvements?

[jacob]

So what is the impact for somebody who uses Btrfs for normal day-to-day file system things? Speed improvements?

According to the article you will see speed improvements for 30+ TB volumes. Arguably not a common day-to-day scenario, then.

[FastCode]

So what is the impact for somebody who uses Btrfs for normal day-to-day file system things? Speed improvements?

It depends on the implementation, normally it means slower code for very small sizes.
From my experience the point where trees become faster than linear searches in practice are 2^10..2^16 items depending on the implementation. Of course I have no idea what is actually in the tree or how often it is accessed and/or updated(linear updates take considerably more and the number is in double digits instead of 4/5).
If someone really cares enough to read the code. he can write a minimal test and benchmark it for everyone.

[SystemCrasher]

were reported for file-systems of around 30+ Terabytes.

Nice to see they're flying high and testing at scale

[sarkyscouser]

I'm not normally one for reactivating 3 month old threads, but this update to the btrfs space_cache has made a noticeable improvement on my modest setup.

I have a NAS running Debian Testing (Stretch) that's recently just received the 4.5 kernel so I thought I'd give it a shot. The NAS has 2xWD Red Pro 4TB hard drives in RAID1 using btrfs, accessed by a couple of Windows 10 PC on the same gigabit LAN via Samba. However when copying large (500MB-2TB) files to the NAS there's always been a few seconds of disk thrashing before the transfer starts (5-10 secs). I've played around with various Samba settings but it's always been there, even on an Arch setup previously with the same hardware.

Switching to space_cache=v2 instantly improves this with no noticeable disk thrashing or delay at the start of the file transfer so it's possibly not necessary to have a 30TB storage system to potentially see the benefit of the new space_cache setting. And as the space_cache can safely be rebuilt it's a relatively safe direction to move in.