On Mon, Sep 26, 2016 at 9:31 AM, Wido den Hollander <wido@xxxxxxxx> wrote:
Hi,
This has been discussed on the ML before [0], but I would like to bring this up again with the outlook towards BlueStore.
Bcache [1] allows for block device level caching in Linux. This can be read/write(back) and vastly improves read and write performance to a block device.
With the current layout of Ceph with FileStore you can already use bcache, but not with ceph-disk.
The reason is that bcache currently does not support creating partitions on those devices. There are patches [2] out there, but they are not upstream.
I haven't tested it yet, but it looks like BlueStore can still benefit quite good from Bcache and it would be a lot easier if the patches [2] were merged upstream.
This way you would have:
- bcache0p1: XFS/EXT4 OSD metadata
- bcache0p2: RocksDB
- bcache0p3: RocksDB WAL
- bcache0p4: BlueStore DATA
With bcache you could create multiple bcache devices by creating partitions on the backing disk and creating bcache devices for all of them, but that's a lot of work and not easy to automate with ceph-disk.
So what I'm trying to find is the best route to get this upstream in the Linux kernel. That way next year when BlueStore becomes the default in L (luminous) users can use bcache underneath BlueStore easily.
Does anybody know the proper route we need to take to get this fixed upstream? Has any contacts with the bcache developers?
Kent is pretty heavy into developing bcachefs at the moment. But you can hit him up on IRC at OFTC #bcache . I've talked ot him about this before and he is 100% willing to accept any patch to solves this issue in the standard way the kernel typically allocs major/minors for disks. The blog post you listed from me does _not_ solve this in an upstream way, though the final result is pretty accurate from my understanding.
I will look into a more better way to patch this upstream since there is renew interested in this.
Also, checkout bcachefs if you like bcache. It's up and coming, but it is pretty sweet. My goal is to use bcachefs with bluestore in the future.
Thanks!
Wido
[0]: http://www.spinics.net/lists/ceph-devel/msg29550.html
[1]: https://bcache.evilpiepirate.org/
[2]: https://yaple.net/2016/03/31/bcache-partitions-and-dkms/
SamYaple
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